In Brief

The assessment can usually begin once current architectural plans show the site boundary, building roofs, driveways, paving, landscaping and the proposed location of any stormwater treatment measures.

The assessor will generally need:

• the project address and development description;
• a current site plan;
• a roof plan;
• the total site or assessment area;
• roof, driveway and other impervious areas;
• landscaped and permeable areas;
• rainwater tank capacities and connected roof catchments;
• proposed rainwater reuse demands;
• details of raingardens, permeable paving or other treatment measures; and
• relevant council correspondence or permit conditions.

The drawings do not always need to be fully resolved before work begins. However, the assessment should be reviewed when later changes affect the measured areas, runoff connections or treatment strategy.

Preparing the Project Information

A STORM assessment translates the physical design shown on the project drawings into a quantitative stormwater treatment model.

To do this accurately, the assessor needs to understand:

• the boundary of the area being assessed;
• which surfaces generate stormwater runoff;
• which surfaces remain permeable;
• where runoff from each catchment is directed;
• which treatment measures receive that runoff;
• how rainwater will be stored and reused; and
• which parts of the site remain untreated.

Not every detail needs to be final at the start. The plans must nevertheless contain enough information to support a realistic treatment strategy.

For a broader explanation of STORM reports and Victorian treatment assessments, visit the Certified Energy STORM Knowledge Hub.

1. Project Address and Development Description

The initial information should identify the property and the type of development proposed.

This commonly includes:

• the full street address;
• the relevant lot or title details where available;
• the local council;
• the proposed development type;
• the number of dwellings or buildings;
• whether the project is new construction, an extension or redevelopment;
• the planning application or permit reference, if known; and
• the project contact or consultant responsible for coordination.

This context helps establish the likely assessment boundary and allows the report to be tied to the correct project and drawing set.

2. Current Site Plan

The site plan is usually the most important drawing for a STORM assessment.

It should show, as far as available:

• the property boundary;
• site dimensions;
• existing and proposed buildings;
• driveways and vehicle access;
• car parking and turning areas;
• paths, patios and courtyards;
• landscaped areas;
• private open space;
• rainwater tank locations;
• raingardens or other treatment areas;
• permeable paving; and
• other major external surfaces.

A dimensioned plan or scaled PDF is preferable because it allows the various surface areas to be measured accurately.

Where only an image or unscaled concept plan is available, additional dimensions or area schedules may be needed.

3. Roof Plan

The roof plan helps establish the roof catchments available for rainwater harvesting.

It can also reveal whether different parts of the roof are likely to drain in separate directions.

Useful roof information includes:

• the outline of each roof section;
• roof levels;
• ridges and valleys;
• gutters and drainage directions where known;
• garage, carport or outbuilding roofs;
• balcony or terrace areas;
• downpipe locations where available; and
• the rainwater tank intended to receive each catchment.

The complete roof area should not automatically be assigned to one tank. Only roof sections that can realistically drain to that tank should be included as connected catchment.

4. Total Site or Assessment Area

The assessment needs a clearly defined total area against which the individual surface types can be checked.

This will commonly be the title area or another project boundary established for the assessment.

The total should reconcile with the sum of:

• roof areas;
• driveways and parking;
• paths and conventional paving;
• permeable paving;
• landscaped areas;
• other pervious surfaces; and
• any existing areas included within the assessment boundary.

Where the sum of the entered surfaces differs materially from the total site area, the assessment may contain an omission, overlap or classification error.

5. Impervious Surface Areas

Impervious areas generate most of the runoff evaluated in a STORM assessment.

The assessor will generally need to identify:

• dwelling or building roofs;
• garage and carport roofs;
• balconies and terraces;
• driveways;
• car parking spaces;
• turning areas;
• pedestrian paths;
• patios and courtyards;
• bin storage pads;
• pool surrounds where relevant; and
• other sealed surfaces.

Small surfaces should not be ignored simply because they are individually minor. Collectively, paths, porches and other paved areas can affect the overall treatment result.

6. Landscaped and Permeable Areas

The assessment also needs to distinguish surfaces that remain pervious or form part of the proposed treatment response.

Relevant information may include:

• garden beds;
• lawns;
• deep-soil areas;
• general landscaped areas;
• permeable paving;
• gravel or other permeable finishes;
• raingardens;
• biofiltration areas; and
• other infiltration or treatment zones.

The classification should reflect the actual proposed construction.

Conventional concrete or tightly jointed paving should not be counted as permeable solely because it is located outdoors.

7. Rainwater Tank Capacity

Where rainwater harvesting forms part of the treatment strategy, the assessor needs the proposed storage capacity of each tank.

The information should identify:

• the capacity of each tank in litres;
• whether the stated capacity is total or usable storage;
• the number of tanks;
• which dwelling or building each tank serves;
• whether tanks operate separately or as a connected system;
• the proposed tank location; and
• any relevant dimensional or access constraints.

If the tank size has not been selected, the assessment can test practical alternatives. The final nominated capacity should then be carried into the architectural and hydraulic documentation.

8. Roof Area Connected to Each Tank

Tank capacity alone is not enough to model rainwater harvesting.

The assessor must also know how much roof runoff reaches each tank.

For a multi-dwelling project, this may require a schedule such as:

• Townhouse 1 roof area connected to Tank 1;
• Townhouse 2 roof area connected to Tank 2;
• Townhouse 3 and garage roof connected to Tank 3; and
• remaining roof area discharging without tank treatment.

Where the exact downpipe arrangement is not yet designed, a reasonable preliminary catchment can be tested. It should still be physically achievable based on the roof form and proposed tank location.

9. Proposed Rainwater Reuse

The assessor needs to understand how stored rainwater will be drawn from the tank.

Possible reuse demands may include:

• toilet flushing;
• landscape irrigation;
• laundry use where relevant and accepted;
• external washdown; or
• another credible non-potable use.

For residential projects, it is also useful to confirm:

• the number of dwellings served;
• the number of toilets connected;
• the landscaped area intended for irrigation;
• whether tanks are individual or shared; and
• whether the reuse commitments are shown on the plans or hydraulic notes.

Reuse assumptions should represent the system that will actually be constructed. They should not be added to the model without corresponding design commitments.

10. Raingarden or Biofiltration Information

Where a raingarden is proposed, the assessment needs enough information to represent its treatment area and connected runoff catchment.

Useful details include:

• the proposed raingarden location;
• the available treatment surface area;
• the roof, driveway or paving that drains to it;
• how runoff enters the treatment area;
• whether an overflow or underdrain is proposed;
• relevant site levels where available; and
• any landscape or construction details already prepared.

Raingardens are designed to filter runoff using vegetation, soil and biological treatment processes. The treatment area used in the model must therefore correspond with a real and buildable part of the site. :contentReference[oaicite:1]{index=1}

11. Permeable Paving Information

If permeable paving contributes to the treatment result, the assessor will need to know:

• the exact area proposed as permeable;
• its location;
• the intended pavement system;
• whether adjoining runoff also drains onto it;
• whether infiltration is proposed;
• any known soil or groundwater constraints; and
• whether the architectural or landscape specification identifies it as permeable.

A general note stating “permeable paving where possible” is usually not enough to establish a measurable treatment area.

12. Runoff Connections and Drainage Intent

The assessment does not necessarily require a complete hydraulic design, but it does need a credible understanding of where runoff goes.

Helpful information may include:

• roof drainage directions;
• downpipe locations;
• driveway falls;
• surface drainage directions;
• the intended path from each catchment to its treatment measure;
• tank overflow locations;
• raingarden overflow arrangements; and
• any available drainage concept plan.

Where these details are not yet designed, the assessor may identify assumptions that need to be confirmed during later hydraulic or civil coordination.

13. Council Correspondence

Council correspondence can be as important as the drawings because it helps clarify why the assessment is being requested.

Relevant documents may include:

• a planning application checklist;
• pre-application advice;
• a request for further information;
• a planning permit condition;
• comments from a council ESD officer;
• drainage or engineering comments;
• a planning officer email;
• a referral authority response; and
• an existing endorsed stormwater condition.

The wording can help determine whether council expects a focused treatment assessment or a broader package that also addresses drainage, detention, discharge or maintenance.

14. Planning and Property Information

Depending on the project, it can also be useful to provide:

• a planning property report;
• the relevant planning controls;
• the development application description;
• an existing planning permit;
• the town planning report;
• a BESS, SDA or SMP request where applicable; and
• other site-specific water-management requirements.

This material helps place the assessment within the correct planning context without turning the STORM report into a broad planning or sustainability document.

15. Landscape Plans

A landscape plan is particularly useful where the treatment strategy includes:

• raingardens;
• biofiltration areas;
• irrigated gardens;
• permeable paving;
• passive irrigation;
• deep-soil areas; or
• other landscaped treatment measures.

The plan helps confirm that sufficient physical space remains available and that the modelled treatment areas do not conflict with access, services, tree planting or private open space.

If the landscape plan has not yet been prepared, the architectural drawings should at least reserve the required treatment area.

16. Available Hydraulic or Civil Information

Detailed hydraulic or civil drawings are not always required before a STORM assessment can begin.

Where available, they may help confirm:

• downpipe and gutter connections;
• tank inlet and overflow arrangements;
• toilet and irrigation reuse connections;
• driveway drainage;
• pit locations;
• the legal point of discharge;
• on-site detention;
• raingarden underdrains; and
• the relationship between treatment and the wider drainage system.

The presence of this information improves coordination but does not make every hydraulic calculation part of the STORM assessment itself.

Can the Assessment Start Before Final Drawings?

Yes. A preliminary assessment can often begin from developed concept plans.

This can be beneficial because it allows the project team to test:

• likely tank capacities;
• available roof connections;
• rainwater reuse options;
• the effect of driveway runoff;
• the approximate raingarden area required;
• the benefit of permeable paving; and
• whether the site has enough space for the proposed strategy.

An early assessment should be treated as preliminary where major design elements remain unresolved.

The result should be reviewed after changes to:

• the building footprint;
• roof geometry;
• driveway or parking layout;
• paving;
• landscape areas;
• tank size or location;
• reuse connections; or
• other treatment measures.

What If Some Information Is Missing?

Missing information does not always prevent the assessment from starting.

Where appropriate, a preliminary model may use clearly stated assumptions for matters such as:

• the proposed tank capacity;
• the portion of roof connected to a tank;
• toilet or irrigation reuse;
• the approximate raingarden area;
• the intended permeable paving area; or
• the routing of driveway runoff.

These assumptions should be identified for the project team and confirmed before the assessment is finalised.

The final report should not quietly rely on unverified measures that are absent from the design documentation.

Common Information Problems

Several recurring issues can delay or weaken an assessment:

• plans without dimensions or usable scale;
• no roof plan;
• unclear site boundaries;
• different tank sizes shown on different drawings;
• no indication of connected roof catchments;
• reuse included without plumbing commitments;
• landscape areas that do not reconcile with the site plan;
• permeable paving without a defined area;
• a raingarden shown without dimensions;
• superseded architectural drawings;
• missing council correspondence; and
• uncertainty about whether a simplified assessment is acceptable.

Providing the current drawing set and the exact council request at the outset usually reduces unnecessary revisions.

What Is Not Usually Needed for the Initial Assessment?

A focused stormwater treatment assessment does not normally require every aspect of the final construction package before modelling can begin.

Depending on the project, the initial assessment may not need:

• final pit and pipe sizing;
• complete civil construction details;
• final structural tank details;
• complete detention calculations;
• final finished surface levels;
• full flood modelling;
• final planting schedules; or
• all supplier product data.

Some of these items may still be required elsewhere in the planning, drainage or construction process.

The STORM assessment needs sufficient information to model treatment performance; it does not replace the separate design of every stormwater component.

STORM, BlueFactor and the Requested Output

Project correspondence may refer to a STORM assessment even where the current accepted output uses a newer assessment platform.

Before finalising the submission, it is useful to confirm:

• which assessment method council accepts;
• whether a calculator certificate is required;
• whether a written report is also expected;
• whether marked-up catchment plans are needed;
• whether the result must be shown on architectural plans;
• whether maintenance notes are required; and
• whether another consultant must address drainage or detention.

Current planning requirements can extend beyond treatment performance to retention, detention, discharge and the wider stormwater management system. The exact submission scope should therefore be read from the applicable planning controls and council request. :contentReference[oaicite:2]{index=2}

Practical Submission Checklist

Before sending a project for assessment, check whether the following items are available:

• project address;
• development description;
• current site plan;
• current roof plan;
• drawing numbers and revision dates;
• total site area;
• roof areas;
• driveway and parking areas;
• paths, patios and other paving;
• landscaped areas;
• permeable paving areas;
• tank capacities;
• roof catchments connected to tanks;
• toilet, laundry or irrigation reuse information;
• raingarden dimensions and catchments;
• available landscape plans;
• available hydraulic or civil plans;
• council requests or permit conditions; and
• any existing stormwater assessment.

Where an item is not yet available, it can be identified as a design input requiring confirmation rather than delaying all preliminary assessment work.

How Certified Energy Can Help

Certified Energy can review the available plans and identify whether there is enough information to begin a stormwater treatment assessment.

The process may include:

• reviewing the council request or permit condition;
• checking the drawing set and revision status;
• measuring roof, driveway, paving and landscape areas;
• identifying treated and untreated catchments;
• reviewing proposed rainwater tanks;
• checking connected roof areas and reuse assumptions;
• reviewing raingardens and permeable surfaces;
• identifying missing project information;
• testing preliminary treatment options where details remain unresolved;
• documenting the final treatment commitments; and
• identifying where separate MUSIC, drainage or civil engineering input may be required.

The assessment can often begin before every construction detail is final, provided the project team understands which assumptions must be retained or confirmed.

Explore the STORM Assessment Knowledge Hub or send through the current plans and council correspondence for an initial project review.

Frequently Asked Questions

What information is needed for a STORM assessment?

The assessment generally needs current plans, the total site area, impervious and landscaped surface areas, proposed stormwater treatment measures, rainwater tank information and relevant council correspondence.

What plans should I provide?

A site plan and roof plan are usually the most important. Floor plans, elevations, landscape plans and available hydraulic or civil plans may also assist.

Do the plans need to be final?

No. A preliminary assessment can often begin from developed concept plans, but the result should be checked again if the design changes.

Is a roof plan essential?

It is highly useful because it helps measure roof catchments and determine which areas can realistically connect to each rainwater tank.

What tank details are required?

The assessor generally needs the tank capacity, connected roof area, uses or dwellings served and proposed reuse demands.

Do I need a hydraulic plan first?

Not always. The assessment may begin before detailed hydraulic design where the proposed catchments, treatment measures and reuse strategy are sufficiently clear.

Should I provide the council request?

Yes. Council correspondence can clarify whether a focused treatment assessment, broader stormwater documentation or a particular modelling method is expected.

What if the tank size has not been selected?

Practical tank options can be tested during a preliminary assessment. The selected capacity should then be incorporated consistently into the project documents.

Can assumptions be used?

Reasonable preliminary assumptions may be used where details remain unresolved, but they should be clearly identified and confirmed before the final report is issued.

Are drainage calculations required?

Not ordinarily for the treatment assessment itself. Drainage capacity, detention, discharge and flood matters may need separate specialist documentation.

Suggested Internal Links

• STORM Assessments and Reports Knowledge Hub
• What Is a STORM Assessment?
• When Is a STORM Assessment Required in Victoria?
• What Does a STORM Report Include?
• How Is a STORM Score Calculated?
• What Does a 100% STORM Rating Mean?
• Can Rainwater Tanks Improve a STORM Rating?
• Common Reasons a Project Fails a STORM Assessment
• What Plans Are Needed for a STORM Report?
• Can a STORM Assessment Be Done Before Final Drawings?

In Brief

A failed STORM assessment usually means that the current combination of impervious areas and stormwater treatment measures does not yet achieve the required treatment benchmark.

Common causes include:

• large untreated driveways or paved areas;
• too little roof area connected to rainwater tanks;
• limited or unrealistic rainwater reuse;
• tanks that are poorly matched to their catchments;
• undersized raingardens or permeable surfaces;
• incorrect site-area calculations;
• omitted impervious surfaces;
• treatment measures that cannot receive the nominated runoff; and
• assessment inputs that no longer match the project drawings.

A low result does not necessarily mean the development cannot proceed. It usually indicates that the treatment strategy or assessment inputs need to be reviewed before the required performance can be demonstrated.

Why STORM Assessments Fall Below the Target

A STORM assessment compares the runoff generated by a development with the treatment performance provided by its proposed stormwater measures.

Roofs, driveways, paths, patios and other impervious surfaces increase the amount of runoff requiring treatment. Rainwater tanks, raingardens, permeable paving and other measures can then be used to manage part of that runoff.

The result falls below the required benchmark when the treatment provided is insufficient relative to the runoff represented in the assessment.

This may happen because the physical design needs further work, but it can also occur because the assessment has been prepared using incorrect, incomplete or outdated information.

For a broader explanation of STORM reports and treatment requirements, visit the Certified Energy STORM Knowledge Hub.

1. Too Much Impervious Area Remains Untreated

One of the most common reasons for a low STORM result is that a large proportion of the site remains impervious and untreated.

Untreated surfaces may include:

• driveways;
• visitor parking spaces;
• turning areas;
• pedestrian paths;
• patios and courtyards;
• balconies;
• small roof sections;
• bin storage areas; and
• other conventional paving.

A project may provide substantial rainwater storage for its roofs but still fall below the target because its driveway and paved areas discharge without treatment.

This is particularly common on townhouse and multi-unit developments where shared accessways occupy a significant proportion of the site.

Potential responses may include directing runoff to a raingarden, introducing genuine permeable paving, reducing unnecessary hardstand or revising the site layout.

2. Insufficient Roof Area Is Connected to the Rainwater Tank

A rainwater tank can only treat runoff from the roof area that actually drains to it.

A project may include a large tank but receive limited modelled benefit where only one small roof plane is connected.

This can occur because:

• the tank is located too far from part of the roof;
• separate gutter systems discharge in different directions;
• roof levels make connection difficult;
• garage or carport roofs have been excluded;
• some downpipes bypass the tank;
• the drawings do not identify the connected catchment; or
• the assessment conservatively includes only the connection that can be verified.

Connecting more roof area can improve the result where the tank has sufficient capacity and reuse demand.

The connected catchment must remain physically credible. The assessment should not assign the entire roof to a tank where the proposed gutter and downpipe arrangement cannot deliver that runoff.

3. Rainwater Reuse Is Too Limited

A rainwater tank provides ongoing stormwater benefit when stored water is used and new storage capacity becomes available before later rainfall.

A tank may contribute less than expected where:

• it is connected only to a garden tap;
• the irrigated landscape area is very small;
• irrigation demand occurs only seasonally;
• toilet reuse has not been included;
• the tank serves too few users;
• the nominated reuse is inconsistent with the plans; or
• no credible reuse demand has been identified.

Garden-only tanks can remain full during wetter periods when irrigation demand is low. Once full, additional roof runoff passes through the overflow.

Regular internal uses such as toilet flushing may provide more consistent drawdown where they are suitable and properly documented.

Melbourne Water notes that rainwater tanks provide the greatest benefit when stored water is used frequently, and that garden-only tanks are generally less effective than tanks connected to toilet flushing. :contentReference[oaicite:1]{index=1}

4. The Tank Is Poorly Matched to Its Catchment and Demand

Tank capacity, connected roof area and reuse demand need to work together.

A poor match can occur where:

• a small tank receives runoff from a very large roof;
• a large tank is connected to a small roof catchment;
• a large tank has very limited reuse demand;
• several dwellings rely on storage that is too small;
• the tank configuration does not reflect the actual dwelling arrangement; or
• the assessment combines catchments and demands that will operate separately.

An undersized tank may overflow frequently, while an oversized tank may provide little extra benefit if it does not receive or release enough water.

Increasing tank capacity is therefore not always the best or most efficient way to improve a failed result.

Read Can Rainwater Tanks Improve a STORM Rating? for a more detailed explanation.

5. The Driveway Has No Treatment

Driveways can be one of the largest untreated catchments on a residential development.

A shared driveway may extend from the street through much of the site and include:

• vehicle access;
• turning areas;
• visitor parking;
• garage aprons;
• pedestrian crossovers; and
• other connected hardstand.

If all of this runoff is entered as untreated, roof tanks may not be sufficient to bring the overall result to the target.

Possible treatment responses may include:

• directing suitable sections to a raingarden;
• using an appropriate permeable pavement system;
• reducing the conventional paved area;
• separating catchments so part of the driveway receives treatment; or
• incorporating another accepted treatment measure.

The proposed treatment must remain compatible with site levels, vehicle access, drainage and maintenance requirements.

6. The Raingarden Is Too Small

A raingarden needs sufficient treatment area for the catchment draining to it.

A low result may occur where:

• a large driveway drains to a very small raingarden;
• the modelled treatment area is smaller than required;
• the connected catchment has increased during design development;
• part of the raingarden has been removed for landscaping or access;
• the treatment configuration is entered incorrectly; or
• runoff bypasses the treatment area.

Increasing the treatment area may improve the result, but the enlarged raingarden must still fit within the development and remain accessible for maintenance.

A theoretical treatment area should not be placed over required accessways, services, private open space or other unavailable parts of the site.

7. Runoff Cannot Physically Reach the Proposed Treatment

A treatment measure only contributes where runoff can be credibly directed to it.

A model may appear successful while the physical design remains unworkable if:

• the treatment area is uphill from the catchment;
• finished levels direct runoff elsewhere;
• kerbs prevent water entering a raingarden;
• downpipes are located on the opposite side of the building;
• the tank cannot receive the nominated roof planes;
• a driveway has several separate low points;
• a balcony has its own drainage outlet; or
• the nominated connection conflicts with underground services.

Council may question a treatment result where the plans do not demonstrate a credible relationship between the catchment and treatment measure.

The assessment should therefore be coordinated with the architectural, landscape, hydraulic and civil information available for the project.

8. Permeable Paving Has Been Overstated

Permeable paving can improve a STORM result, but only where a genuine permeable pavement system is proposed.

Problems may arise where:

• ordinary paving has been entered as permeable;
• the permeable area has been measured incorrectly;
• the paving specification does not support the assessment assumption;
• the sub-base or drainage arrangement is unsuitable;
• the final finishes have changed;
• the area includes buildings, garden beds or conventional concrete; or
• the treatment depends on infiltration that may not be achievable.

The area counted in the assessment should match the drawings and material specification.

Permeable paving should not be used merely as a modelling label for an otherwise conventional sealed surface.

9. Impervious Areas Have Been Omitted

An apparently strong result may fail during review if relevant impervious surfaces have not been included.

Frequently overlooked areas include:

• front paths;
• porches;
• balconies;
• small patios;
• bin pads;
• external stairs and landings;
• garage aprons;
• retaining-wall access strips;
• pool surrounds; and
• roof overhangs or secondary roof structures.

Small omissions may appear insignificant individually, but together they can change the site-area balance and overall treatment result.

All relevant surfaces should be accounted for so the assessment accurately represents the development.

10. The Total Site Area Does Not Reconcile

The sum of the modelled roof, paving, landscape and other surface areas should reconcile with the assessment boundary.

Common area errors include:

• using an incorrect title area;
• measuring only the development footprint;
• double-counting part of the roof or driveway;
• omitting common property;
• including land outside the assessment boundary;
• using approximate areas from an early concept plan;
• failing to update areas after redesign; and
• classifying the same surface as both paving and landscaping.

An unexplained difference between the site area and surface schedule can undermine confidence in the result and may lead to further questions from council.

11. The Assessment Uses Superseded Drawings

A STORM report can become outdated when the architectural or landscape design changes.

Changes that may affect the result include:

• larger building footprints;
• additional roofs or balconies;
• wider driveways;
• new visitor parking;
• reduced garden areas;
• changes to tank capacity;
• relocation of tanks;
• removal of a raingarden;
• replacement of permeable paving; and
• changes to rainwater reuse connections.

The report should identify the drawing numbers and revisions used for the assessment.

Where a material design change occurs, the result should be reviewed rather than assuming the earlier rating remains valid.

12. The Modelled Reuse Is Not Shown on the Plans

A treatment result may rely on toilet flushing, laundry use or landscape irrigation that is not reflected in the project documentation.

This creates a gap between the modelled strategy and the design that will be approved or constructed.

Potential inconsistencies include:

• toilet reuse included in the report but absent from hydraulic notes;
• irrigation demand included without an irrigation connection;
• a shared tank modelled as serving several dwellings but shown as serving one;
• different tank capacities across separate drawings;
• a nominated roof catchment without corresponding downpipe connections; and
• reuse commitments removed during value management.

The report, architectural drawings and hydraulic documentation should describe the same treatment arrangement.

13. The Treatment Measures Are Impractical

A modelled solution may reach the numerical target while remaining unsuitable for the physical site.

Examples include:

• a tank that blocks required access;
• a raingarden occupying private open space;
• treatment located over underground services;
• permeable paving proposed in an unsuitable high-load area;
• a tank with no practical maintenance access;
• a raingarden that cannot receive runoff by gravity;
• a treatment area conflicting with tree planting; and
• shared infrastructure without clear ownership or maintenance responsibility.

Victorian planning decision guidelines specifically consider whether stormwater treatment areas can be effectively maintained. :contentReference[oaicite:2]{index=2}

The objective is not simply to generate a passing score. The treatment strategy should be capable of being built, operated and maintained.

14. The Project Relies on One Treatment Measure

Some developments attempt to achieve the entire result through a single oversized tank or small raingarden.

This may be inefficient where the site contains several different runoff sources.

For example:

• roof runoff may be suited to rainwater harvesting;
• driveway runoff may be better directed to a raingarden;
• courtyard runoff may be reduced through permeable paving; and
• unnecessary hardstand may be replaced with landscaping.

A coordinated combination of measures can produce a more practical and resilient result than relying on one element to compensate for every untreated surface.

15. The Wrong Assessment Method Has Been Used

A simplified STORM-style assessment may be unsuitable where the development contains complex catchments or connected treatment trains.

A more detailed method may be required where the project includes:

• multiple sub-catchments;
• several treatments connected in sequence;
• wetlands or sediment ponds;
• complex stormwater harvesting;
• staged development;
• subdivision-scale drainage;
• significant bypass or overflow arrangements; or
• a specific council or authority requirement for MUSIC modelling.

Melbourne Water describes BlueFactor as the replacement for STORM for suitable small developments, while MUSIC provides more sophisticated modelling of catchments, drainage connections and treatment trains. :contentReference[oaicite:3]{index=3}

A project may therefore fail to satisfy council even with a strong simplified score if the selected method cannot adequately represent the required stormwater system.

16. The Assessment Reaches 100%, but Council Still Raises Concerns

A 100% result does not automatically mean that the full submission will be accepted.

Council may still ask whether:

• the site areas are accurate;
• all impervious surfaces are included;
• the treatment connections are physically achievable;
• the measures match the drawings;
• the proposed reuse will be constructed;
• maintenance is practical;
• the current assessment tool is accepted;
• detention has been addressed;
• the legal point of discharge is resolved; or
• additional drainage documentation is required.

The rating demonstrates modelled stormwater treatment performance. It does not replace council’s review of the broader design and submission.

How to Diagnose a Failed STORM Result

The most useful first step is to identify which parts of the development are producing the largest untreated runoff load.

A practical review may examine:

1. whether the site and surface areas are correct;
2. which impervious catchments remain untreated;
3. how much roof area reaches each tank;
4. whether the tank capacities suit their catchments;
5. whether the nominated reuse demands are credible;
6. whether driveway or paved runoff can receive treatment;
7. whether raingardens or permeable surfaces are correctly sized;
8. whether runoff can physically reach each measure;
9. whether the report matches the latest drawings; and
10. whether the chosen assessment method is appropriate.

This helps distinguish between a genuine design shortfall and an input or coordination problem.

Ways to Improve a Low STORM Rating

Depending on the project, possible improvements may include:

• correcting inaccurate areas;
• connecting more roof area to rainwater tanks;
• introducing regular and documented rainwater reuse;
• revising tank capacities;
• directing driveway runoff to a treatment area;
• adding or enlarging a raingarden;
• introducing genuine permeable paving;
• reducing unnecessary conventional paving;
• retaining more landscaped or pervious area;
• combining several complementary treatment measures; and
• updating the assessment to match the current design.

The most effective change depends on the runoff sources limiting the result.

Read How to Improve a STORM Rating for a dedicated design-response guide.

What Failing a STORM Assessment Does Not Mean

A low result does not necessarily mean:

• the site cannot be developed;
• the planning application will automatically be refused;
• the entire building design must be abandoned;
• the largest available rainwater tank must be installed;
• every impervious surface needs a separate treatment device; or
• a complex civil engineering solution is always required.

It means that the current treatment strategy, as represented in the assessment, does not yet demonstrate the required outcome.

Many projects can improve their result through targeted changes rather than a complete redesign.

What a STORM Assessment Does Not Test

A successful or failed STORM result relates primarily to stormwater treatment performance.

It does not ordinarily determine:

• stormwater pipe capacity;
• pit locations and sizing;
• on-site detention volume;
• peak discharge rates;
• the legal point of discharge;
• finished surface levels;
• overland flow paths;
• flood levels;
• structural design of treatment assets; or
• the complete civil drainage design.

A project may achieve its stormwater treatment target while still requiring separate drainage, detention or flood-related work.

Practical Considerations for Victorian Projects

Use the Latest Plans

Confirm that the assessment references the current architectural, landscape and hydraulic documents.

Measure the Complete Site

Check that every relevant roof, driveway, path, patio and landscaped area is included once and classified correctly.

Review the Largest Untreated Catchments First

Treating a large driveway or roof catchment may provide more benefit than making small adjustments to minor surfaces.

Coordinate Tank Inputs

Tank capacity, roof connection and reuse demand should be reviewed together rather than independently.

Check Physical Drainage Connections

Confirm that runoff can reach the proposed treatment measures based on roof drainage, site levels and the developing hydraulic design.

Retain Space for Treatment

Do not rely on raingardens or tanks that conflict with access, services, landscaping or required open space.

Confirm the Accepted Tool

For new suitable small developments, council may expect BlueFactor or another accepted output rather than a new certificate from the former STORM Calculator.

Separate Treatment From Drainage Design

Check whether council also requires detention calculations, discharge information or civil engineering documentation.

How Certified Energy Can Help

Certified Energy can review a low stormwater treatment result and identify the inputs or design features preventing the project from achieving the required benchmark.

The review may include:

• checking the assessment boundary and total site area;
• remeasuring roofs, driveways, paving and landscaping;
• identifying omitted or untreated catchments;
• reviewing rainwater tank capacities;
• checking connected roof areas and reuse demands;
• reviewing raingarden and permeable paving assumptions;
• testing practical treatment alternatives;
• coordinating the result with current drawings;
• documenting the final treatment commitments; and
• identifying where more detailed MUSIC, drainage or engineering input may be needed.

The objective is to develop a credible treatment response that reaches the required performance without relying on inaccurate areas or impractical measures.

Explore the STORM Assessment Knowledge Hub or send through the current plans, assessment output and council correspondence for an initial project review.

Frequently Asked Questions

Why does a project fail a STORM assessment?

It commonly fails because too much runoff remains untreated or the proposed measures are not properly sized, connected or supported by realistic operating assumptions.

Does a low score mean the development cannot proceed?

Not necessarily. It usually means that the assessment inputs or treatment strategy require further review before the target can be demonstrated.

Can a larger rainwater tank fix a failed result?

It may help, but not always. Tank benefit also depends on connected roof area and regular reuse demand. Another treatment measure may be more effective.

Why is my large rainwater tank providing little benefit?

The connected roof area or reuse demand may be too small, or untreated driveways and paving may be limiting the overall result.

Do untreated driveways affect the score?

Yes. A shared driveway can form a major untreated impervious catchment and may significantly affect the project result.

Can a raingarden be too small?

Yes. Its contribution depends on its treatment area and the size of the catchment draining to it.

Can missing paths or balconies affect the report?

Yes. Omitted impervious areas can make the site schedule and treatment result inaccurate.

Does permeable paving always improve the result?

Only where a genuine permeable pavement system is proposed and the modelled area matches the drawings and specification.

Can council question a 100% result?

Yes. Council may review whether the inputs are accurate, the measures are practical and the report remains consistent with the submitted plans.

How can a failed result be improved?

Possible responses include correcting the inputs, treating more runoff, increasing credible tank reuse, resizing treatment measures, reducing impervious area or combining several treatment approaches.

Suggested Internal Links

• STORM Assessments and Reports Knowledge Hub
• What Is a STORM Assessment?
• What Does a STORM Report Include?
• How Is a STORM Score Calculated?
• What Does a 100% STORM Rating Mean?
• Can Rainwater Tanks Improve a STORM Rating?
• How to Improve a STORM Rating
• Do Rain gardens Improve a STORM Assessment?
• Permeable Paving and STORM Compliance
• What Information Is Needed for a STORM Assessment?

In Brief

Yes. A rainwater tank can improve a STORM rating by intercepting roof runoff and storing it for reuse rather than allowing all of that water to discharge directly into the stormwater system.

Its modelled benefit depends on three connected factors:

• the capacity of the rainwater tank;
• the roof area that drains to it; and
• how regularly the stored water is reused.

A larger tank does not automatically produce a better result. If only a small roof area is connected, the tank may receive limited runoff. If there is little reuse, the tank may remain full and have less capacity available during the next rainfall event.

The strongest result usually comes from coordinating the tank capacity, roof catchment and reuse demand as one stormwater treatment measure.

How Rainwater Tanks Affect a STORM Assessment

A STORM assessment examines runoff from the impervious areas of a proposed development and calculates the treatment performance provided by the nominated measures.

Roof surfaces are a major source of runoff on many residential and small development sites. Where roof water flows directly into the drainage system, it contributes to the project’s untreated runoff.

A rainwater tank changes this pathway by:

• collecting water from the connected roof area;
• temporarily storing that water;
• making the stored water available for non-potable reuse;
• reducing the volume discharged immediately from the site; and
• creating new storage when the water is used between rainfall events.

The assessment tool models this interaction and attributes treatment performance to the tank based on its inputs.

For a broader overview of stormwater treatment assessments, visit the Certified Energy STORM Knowledge Hub.

The Three Inputs That Matter Most

The effect of a rainwater tank cannot be understood from its nominal capacity alone.

Three inputs work together to determine its contribution to the STORM rating.

1. Rainwater Tank Capacity

The tank capacity determines how much water can be stored at a particular time.

A larger tank can potentially capture more runoff before overflowing. However, the additional capacity only helps where enough water reaches the tank and enough stored water is subsequently reused.

2. Connected Roof Catchment

The connected roof catchment determines how much rainfall can enter the tank.

A large tank connected to a small roof section may fill slowly and make limited use of its full capacity. A moderately sized tank connected to a substantial roof catchment may capture runoff more effectively.

3. Regular Rainwater Reuse

Reuse removes water from the tank and creates room for later rainfall.

Without sufficient drawdown, a full tank behaves largely as an overflow point during subsequent rainfall. Regular demand can therefore be as important as storage volume.

Why Regular Reuse Improves Tank Performance

A rainwater tank has the greatest capacity to capture runoff when it is partly empty before rainfall begins.

Consider two identical tanks:

• one supplies toilets throughout the year; and
• the other is used only occasionally for garden watering.

The toilet-connected tank is likely to experience more frequent drawdown. This creates storage space more consistently, including during cooler or wetter periods when garden irrigation demand may be low.

The garden-only tank may remain full for longer periods. Once full, additional roof water passes through the overflow and into the site drainage system.

This is why the STORM result depends on the pattern of reuse rather than simply confirming that a tank is shown on the plans.

Which Rainwater Uses Can Be Included?

The reuse demands included in the assessment must be credible for the proposed development and supported by the project documentation.

Common non-potable uses may include:

• toilet flushing;
• landscape irrigation;
• laundry use where appropriate and accepted;
• external washdown; or
• other documented non-potable demands relevant to the project.

The exact inputs available can depend on the assessment tool and project type.

Reuse should not be added to the model solely to improve the rating if the required plumbing or irrigation connection will not be constructed.

Toilet Flushing and STORM Performance

Toilet flushing can provide a relatively consistent demand because it occurs throughout the year and is not dependent on seasonal irrigation needs.

Where toilet reuse is included in the assessment, the design documentation should identify that the rainwater tank will supply the relevant toilets.

This may require coordination between:

• the architectural plans;
• the hydraulic design;
• the rainwater tank specification;
• the pump and mains-water backup arrangement; and
• the STORM assessment assumptions.

If the toilet connection is later removed, the tank may no longer provide the same modelled performance and the assessment may need to be updated.

Garden Irrigation and STORM Performance

Garden irrigation can also contribute to rainwater reuse, particularly where the development contains a meaningful landscaped area.

The modelled benefit depends on the realistic irrigation demand associated with that area.

Important considerations include:

• the size of the landscaped area;
• whether the area will actually be irrigated;
• whether irrigation is connected to the tank;
• seasonal differences in watering demand;
• the type of landscaping proposed; and
• any council or tool assumptions governing irrigation inputs.

A small garden should not be assigned an unrealistic water demand simply to increase tank drawdown.

Where garden irrigation is the only reuse source, its contribution may be less consistent than regular internal demand.

Does a Larger Tank Always Improve the Rating?

No. Increasing the rainwater tank capacity can improve the result, but the relationship is not unlimited or directly proportional.

A larger tank may provide little additional benefit where:

• the connected roof area is too small;
• the regular reuse demand is limited;
• the existing tank already provides sufficient storage;
• the tank is frequently full;
• other impervious surfaces remain untreated; or
• the rating is being limited by driveway or paved runoff rather than roof runoff.

For example, increasing a tank from 3,000 litres to 5,000 litres may not solve a low rating if most of the roof is not connected or the development has no regular reuse demand.

Before increasing the tank size, it is often more useful to review the complete catchment and reuse arrangement.

How Much Roof Area Should Connect to the Tank?

There is no single roof-catchment percentage that is correct for every project.

The appropriate connected area depends on:

• the total roof configuration;
• the tank location;
• gutter and downpipe arrangements;
• site levels;
• the tank capacity;
• the available reuse demand;
• the treatment target; and
• other measures proposed across the site.

Only roof areas that can credibly drain to the tank should be included.

A report should not assign the complete roof to a tank where parts of the building have separate gutters, incompatible levels or no practical connection.

Can Several Tanks Be Used?

Yes. Multi-dwelling developments often use separate tanks for individual dwellings or groups of roof catchments.

For example, a townhouse project may include:

• one tank serving each dwelling;
• a shared tank serving several dwellings;
• separate tanks for distinct roof catchments; or
• a combination of individual and shared storage.

The assessment should identify:

• the capacity of each tank;
• the roof area connected to each tank;
• the dwellings or uses served;
• the reuse demand assigned to each tank; and
• any roof or paved areas that remain untreated.

The arrangement used in the model should match the physical design rather than combining unrelated tanks and demands into an artificial result.

Can One Shared Tank Improve the Result?

A shared tank may provide a strong and consistent reuse demand where it serves several dwellings or common landscape areas.

It may also reduce the number of separate assets that need to be installed and maintained.

However, shared systems require careful coordination of:

• ownership and access;
• pump arrangements;
• reuse connections;
• maintenance responsibility;
• metering where relevant;
• overflow connections; and
• future body corporate or owners corporation obligations.

The assessment result alone does not establish whether a shared system is the most practical long-term arrangement.

Can a Rainwater Tank Achieve a 100% STORM Rating?

A rainwater tank may be sufficient to achieve the required benchmark for some developments.

This is more likely where:

• roof areas form most of the impervious site area;
• a substantial proportion of the roof drains to the tank;
• there is regular and credible reuse demand;
• the tank is appropriately sized;
• driveway and paved areas are relatively limited; and
• the amount of untreated runoff remains manageable.

Other projects may not reach the target through rainwater harvesting alone.

Where a development contains a large shared driveway, extensive paving or other untreated impervious areas, additional measures such as a raingarden or permeable paving may be needed.

A 100% result should be achieved through a practical treatment response rather than by relying on unrealistic tank inputs.

Why a Tank May Not Improve the Result as Expected

A proposed rainwater tank can produce less improvement than expected for several reasons.

Common causes include:

• too little roof area connected to the tank;
• limited reuse demand;
• garden irrigation being the only demand;
• an oversized tank relative to the catchment;
• a large amount of untreated driveway or paving;
• incorrect catchment allocation;
• incorrect occupancy or demand assumptions;
• the tank already reaching diminishing modelled benefit;
• parts of the roof bypassing the tank; or
• the treatment target being limited by another part of the development.

The most effective improvement is not always a larger tank. It may instead involve increasing the connected roof area, adding consistent reuse or treating another impervious catchment.

Example: A Townhouse Development

Consider a three-townhouse development with three dwelling roofs and a shared driveway.

The original design provides one small tank per dwelling, but only a limited section of each roof is connected and the tanks supply garden taps only.

The STORM assessment may show that the tank contribution is limited because:

• the connected roof catchments are small;
• garden demand is intermittent;
• the shared driveway remains untreated; and
• the tanks remain full for extended periods.

The project team might then investigate:

• connecting more roof area to each tank;
• connecting the tanks to toilet flushing;
• revising the tank capacities;
• directing driveway runoff to a raingarden; or
• using permeable paving within part of the site.

The best response would depend on the available space, building services and site drainage arrangement.

What Happens When the Tank Is Full?

A rainwater tank requires an overflow connection.

Once the tank reaches capacity, additional roof runoff passes through the overflow into the nominated drainage system or another approved part of the stormwater strategy.

This is another reason regular reuse matters. Drawing water from the tank creates available capacity before the next rainfall event.

The STORM assessment does not remove the need to design an appropriate overflow pathway.

The hydraulic or civil documentation may still need to address:

• overflow pipework;
• connection to the legal point of discharge;
• on-site detention interaction;
• backflow protection;
• surface flow paths; and
• other council drainage requirements.

Rainwater Tanks and Other Treatment Measures

A rainwater tank does not need to work alone.

Many developments achieve a stronger and more balanced treatment result by combining rainwater harvesting with other measures.

Rainwater Tank and Raingarden

The tank may treat roof runoff while a raingarden treats runoff from a driveway or paved area.

Rainwater Tank and Permeable Paving

The tank may capture roof water while permeable paving reduces or treats runoff from courtyards, paths or parking areas.

Rainwater Tank and Reduced Impervious Area

Removing unnecessary paving can reduce the overall runoff load and the amount of treatment infrastructure required.

This combined approach can be more practical than installing an oversized tank solely to compensate for untreated hard surfaces elsewhere on the site.

STORM and BlueFactor Terminology

The original Melbourne Water STORM Calculator has been replaced by BlueFactor for suitable small Victorian developments.

Older permit conditions, reports and project discussions may continue to refer to:

• a STORM assessment;
• a STORM report;
• a STORM certificate;
• a STORM rating; or
• a 100% STORM result.

A new application may instead require a BlueFactor assessment or another stormwater treatment method accepted by the responsible authority.

The underlying design principle remains relevant: a rainwater tank performs most effectively when its storage, connected catchment and reuse demand are properly coordinated.

What a Rainwater Tank Does Not Replace

A tank may contribute to the stormwater treatment result, but it does not automatically resolve every stormwater requirement for a project.

It does not ordinarily replace:

• detailed drainage design;
• on-site detention calculations;
• pipe and pit sizing;
• legal point of discharge documentation;
• overland flow analysis;
• flood modelling;
• civil construction drawings;
• structural design or tank support requirements; or
• a broader Stormwater Management Plan where one is required.

The STORM assessment remains focused on the modelled treatment contribution provided by the tank and other nominated measures.

Practical Considerations for Victorian Projects

Confirm the Connected Roof Area

Only include roof sections that can physically drain to the tank through the proposed gutter and downpipe arrangement.

Use Realistic Reuse Assumptions

Toilet, laundry or irrigation uses included in the assessment should be supported by the final design.

Coordinate the Tank Location

The tank must fit within the site without obstructing access, private open space, windows, services or required setbacks.

Check the Tank Specification

The nominated storage capacity should represent usable tank volume and remain consistent across the architectural, hydraulic and assessment documents.

Allow for Pumps and Backup Supply

Internal rainwater uses may require a pump, appropriate controls and mains-water backup. These should be coordinated by the relevant designer.

Show the Overflow Arrangement

The tank overflow must connect to an appropriate drainage pathway and should not be assumed to disappear from the site.

Review Other Impervious Areas

If the rating remains low, check whether driveway and paved runoff requires separate treatment rather than continuing to increase the tank size.

Keep All Documents Consistent

Tank capacity, roof catchment and reuse commitments should match the architectural plans, hydraulic notes and final STORM report.

What Information Is Needed to Model a Rainwater Tank?

The assessor will generally need:

• the proposed tank capacity;
• the location of the tank;
• the roof area connected to it;
• the number of dwellings or users served;
• the proposed toilet connections;
• laundry reuse details where relevant;
• the landscaped area receiving irrigation;
• other proposed reuse demands;
• the current architectural plans;
• the roof plan;
• available hydraulic information; and
• any council condition or request relating to the tank.

Where these details are not final, a preliminary assessment may test reasonable options before the design is fixed.

How Certified Energy Can Help

Certified Energy can assess how proposed rainwater tanks contribute to the stormwater treatment performance of a suitable Victorian development.

The assessment may include:

• measuring the available roof catchments;
• reviewing proposed tank capacities;
• checking the nominated reuse demands;
• testing different roof-to-tank connections;
• identifying whether additional tank capacity is useful;
• reviewing untreated driveway and paved areas;
• testing complementary treatment measures;
• identifying practical ways to improve a low result;
• documenting the final treatment assumptions; and
• coordinating the assessment with the project plans.

Where separate drainage, detention or civil engineering documentation is required, that broader scope should be identified rather than presenting the tank as a complete stormwater solution.

Explore the STORM Assessment Knowledge Hub or send through the current plans and council correspondence for an initial project review.

Frequently Asked Questions

Can a rainwater tank improve a STORM rating?

Yes. It can improve the result by capturing runoff from a connected roof catchment and creating storage capacity through regular reuse.

Does a bigger tank always produce a higher rating?

No. The result also depends on the connected roof area and reuse demand. Additional storage may provide little benefit where either is limited.

Why does reuse matter?

Reuse removes water from the tank and creates space for later rainfall. A tank that remains full has less capacity to capture the next runoff event.

Is toilet flushing better than garden irrigation?

Toilet flushing often provides more regular year-round demand. Garden irrigation can still contribute, but its demand may vary seasonally and depends on the landscaped area.

Does the whole roof need to connect to the tank?

No. However, only roof areas that can realistically drain to the tank should be included as connected catchment.

Can one tank serve several dwellings?

Yes. A shared tank may serve several dwellings or common areas, provided its plumbing, access, reuse and maintenance arrangements are properly coordinated.

Can a tank alone achieve a 100% STORM rating?

It may be possible for some developments. Projects with large driveways or other untreated surfaces may also need raingardens, permeable paving or another treatment response.

Why is my tank providing less improvement than expected?

The connected roof area or reuse demand may be too small, the tank may already be oversized for the available catchment, or other untreated surfaces may be limiting the overall result.

Does tank overflow need to be designed?

Yes. Once a tank is full, additional runoff leaves through its overflow. The overflow must connect to an appropriate drainage arrangement.

Does a rainwater tank replace on-site detention?

Not automatically. Treatment storage and detention storage serve different assessment purposes. Council or the drainage designer should confirm the project’s detention requirements.

Suggested Internal Links

• STORM Assessments and Reports Knowledge Hub
• What Is a STORM Assessment?
• What Does a STORM Report Include?
• How Is a STORM Score Calculated?
• What Does a 100% STORM Rating Mean?
• Common Reasons a Project Fails a STORM Assessment
• How to Improve a STORM Rating
• Do Raingardens Improve a STORM Assessment?
• Permeable Paving and STORM Compliance
• What Information Is Needed for a STORM Assessment?

In Brief

A STORM-style assessment is generally suited to smaller developments with straightforward runoff catchments and relatively simple treatment measures.

MUSIC modelling is generally used where a project contains larger or multiple catchments, connected treatment trains, more complex runoff pathways or subdivision-scale stormwater infrastructure.

The original Melbourne Water STORM Calculator has now been replaced by BlueFactor for suitable small developments. However, the term “STORM assessment” remains common in existing planning permits, council correspondence and project discussions.

The choice should not be based solely on which method is faster or simpler. The accepted assessment depends on the development, the proposed treatment system and the requirements of the relevant council or drainage authority.

Understanding the Two Assessment Methods

STORM and MUSIC occupy the same broad technical territory: both can be used to demonstrate how a development treats stormwater runoff and performs against applicable water-quality objectives.

The main difference is the level of detail each method can represent.

• STORM or BlueFactor provides a simplified assessment for suitable smaller developments.
• MUSIC provides more detailed modelling of runoff, treatment nodes and connected treatment systems.

A simple townhouse development may be capable of being represented using roof catchments, rainwater tanks, a driveway treatment and a small number of untreated areas.

A larger subdivision may involve multiple sub-catchments, roads, sediment ponds, wetlands, raingardens, harvesting systems and staged drainage infrastructure. That type of project is more likely to require MUSIC modelling.

For a broader overview of STORM reports and treatment assessments, visit the Certified Energy STORM Knowledge Hub.

What Is a STORM Assessment?

A STORM assessment is a simplified method of evaluating the stormwater-quality performance of a development proposal.

It commonly considers:

• total site area;
• roof catchments;
• driveways and paved surfaces;
• landscaped or permeable areas;
• rainwater tank capacity;
• roof area connected to tanks;
• rainwater reuse demands;
• raingardens or biofiltration areas;
• permeable paving;
• untreated impervious areas; and
• the overall stormwater treatment result.

The original STORM Calculator produced a relative score indicating how the proposed treatment strategy performed against the relevant best-practice objective.

Victorian planning guidance historically described STORM as most appropriate for small-scale subdivisions, typically below one hectare, while recognising that more sophisticated modelling may be preferable for larger developments.

What Is MUSIC Modelling?

MUSIC stands for Model for Urban Stormwater Improvement Conceptualisation.

It is a more detailed modelling platform used to represent how rainfall, runoff, pollutant loads and stormwater treatment measures interact over time.

A MUSIC model can represent:

• multiple source catchments;
• different land-use and surface characteristics;
• connected drainage pathways;
• sequential treatment measures;
• high-flow bypasses and overflows;
• rainwater or stormwater harvesting;
• raingardens and biofiltration systems;
• sediment ponds;
• constructed wetlands;
• infiltration systems; and
• complex treatment trains.

MUSIC is therefore better suited to projects where the order, connection and performance of several stormwater measures need to be modelled explicitly.

The Main Difference: Simplicity Versus Detailed Modelling

The distinction is not simply that STORM is used for residential projects and MUSIC is used for commercial projects.

The more useful dividing line is stormwater system complexity.

A smaller project may be suitable for a simplified assessment where:

• the assessment boundary is clear;
• the number of runoff catchments is limited;
• the treatment measures are straightforward;
• the runoff connections can be explained simply; and
• the responsible authority accepts a simplified tool.

MUSIC may be more appropriate where:

• the site contains numerous sub-catchments;
• runoff passes through several connected treatments;
• the project is staged;
• different land uses need separate modelling;
• the development contains subdivision-scale drainage assets;
• overflows and bypasses need to be represented;
• stormwater harvesting forms part of the strategy; or
• council or Melbourne Water specifically requires MUSIC.

When a STORM-Style Assessment May Be Suitable

A simplified treatment assessment may suit a relatively small and uncomplicated development.

Examples may include:

• a townhouse development on a conventional residential site;
• a small multi-dwelling development;
• a modest apartment project;
• a residential extension with a specific treatment requirement;
• a small commercial building;
• a small industrial development; or
• another project with a limited number of clearly defined catchments.

The treatment strategy may include a small number of elements such as:

• rainwater tanks connected to dwelling roofs;
• toilet flushing or landscape irrigation reuse;
• a raingarden receiving driveway runoff;
• permeable paving in selected areas; and
• a limited quantity of untreated runoff.

Even where a project appears simple, the relevant council should confirm that the proposed assessment method is acceptable.

When MUSIC Modelling May Be Needed

MUSIC becomes more relevant as the stormwater system grows in scale or complexity.

It may be required for:

• larger residential subdivisions;
• neighbourhood or precinct-scale developments;
• major commercial or industrial sites;
• large car parks and extensive road networks;
• multiple buildings draining to shared treatment assets;
• developments with several linked treatment measures;
• wetlands and sediment ponds;
• complex stormwater harvesting systems;
• staged developments;
• projects referred to Melbourne Water; or
• projects where the responsible authority specifically requests a MUSIC model.

Melbourne Water maintains dedicated MUSIC modelling guidelines for models submitted to it. The current guidance addresses matters such as rainfall data, model inputs, high-flow bypasses, overflows and treatment-node configuration.

How Treatment Trains Affect the Choice

A treatment train is a sequence of stormwater treatment measures through which runoff passes.

For example, runoff may travel through:

1. a gross-pollutant treatment;
2. a sediment pond;
3. a raingarden or wetland; and
4. a harvesting or discharge system.

The order of these measures can affect their operation and treatment performance.

MUSIC can model the connection between individual treatment nodes and show how runoff and pollutant loads move through the system.

A simplified STORM-style assessment is less suitable where the project depends on a detailed sequence of pre-treatment, primary treatment, bypasses, overflows and downstream assets.

Melbourne Water describes a treatment train as a sequence of treatments selected and ordered to maximise performance and protect downstream assets from pollutants such as litter and coarse sediment. :contentReference[oaicite:1]{index=1}

Does Project Area Determine the Required Tool?

Project area is an important indicator, but it is not the only consideration.

Victorian planning guidance has historically described STORM as most appropriate for small subdivisions, typically below one hectare, and MUSIC as suitable for more complex subdivision, neighbourhood or regional-scale projects. :contentReference[oaicite:2]{index=2}

However, a small site can still require MUSIC where it has:

• complex drainage arrangements;
• several connected treatment systems;
• sensitive receiving conditions;
• major harvesting infrastructure;
• significant external catchments; or
• a direct council or authority requirement.

Conversely, the suitability of a simplified method for any larger site should not be assumed merely because the proposed treatments appear simple.

The assessment method should reflect both scale and complexity.

STORM Has Been Replaced by BlueFactor

Melbourne Water replaced its original STORM Calculator with BlueFactor in December 2025.

BlueFactor now occupies the simplified-assessment role for suitable small developments, while MUSIC remains available for projects requiring more detailed modelling. :contentReference[oaicite:3]{index=3}

This means that a new project may be described as requiring:

• a BlueFactor assessment;
• a stormwater treatment assessment;
• a best-practice stormwater result; or
• a MUSIC model.

Older planning permits and council correspondence may still request:

• a STORM report;
• a STORM certificate;
• a STORM score; or
• a 100% STORM rating.

Where the wording refers to the former tool, the project team should confirm whether the responsible authority will accept the current BlueFactor output or requires another assessment method.

Can You Choose STORM Instead of MUSIC?

Not necessarily.

The assessment method may be nominated by:

• the relevant planning scheme;
• a council application checklist;
• a request for further information;
• a planning permit condition;
• a drainage authority;
• Melbourne Water;
• a Stormwater Management Strategy requirement; or
• the technical complexity of the proposed system.

A project should not use a simplified method merely to avoid the additional work associated with MUSIC where the stormwater system cannot be represented adequately.

Likewise, a relatively simple project should not automatically be burdened with detailed MUSIC modelling where an accepted simplified assessment can answer the relevant planning question.

How the Assessment Outputs Differ

A STORM-style report generally provides a concise account of:

• site and impervious areas;
• runoff catchments;
• rainwater tanks and reuse;
• other treatment measures;
• untreated runoff;
• key assumptions; and
• the resulting treatment score or benchmark outcome.

A MUSIC submission may provide substantially more technical detail, including:

• source-node and catchment configuration;
• rainfall and evapotranspiration data;
• runoff parameters;
• pollutant-generation parameters;
• treatment-node properties;
• node-to-node connections;
• bypasses and overflows;
• treatment-train performance;
• annual flow and pollutant results;
• model files;
• input justifications; and
• an auditor report where required.

For Melbourne Water submissions, the model itself and justification of key inputs may be required rather than screenshots or a short summary alone. :contentReference[oaicite:4]{index=4}

Example: A Small Townhouse Development

Consider a three-townhouse proposal with:

• three principal roof catchments;
• three rainwater tanks;
• toilet and garden reuse;
• one shared driveway;
• a small raingarden; and
• several minor untreated paths.

The catchments and treatment measures are limited and can be described clearly.

Subject to council acceptance, this type of project may be suitable for a simplified BlueFactor or STORM-style assessment.

The assessment can test whether the tanks, reuse demands, raingarden and untreated areas collectively achieve the required stormwater treatment result.

Example: A Residential Subdivision

Now consider a larger subdivision containing:

• multiple residential catchments;
• new roads and footpaths;
• public open space;
• several raingardens;
• sediment ponds;
• a constructed wetland;
• a stormwater harvesting system;
• high-flow bypasses;
• staged development; and
• shared downstream drainage infrastructure.

This system depends on the connections between several catchments and treatment assets.

MUSIC is more likely to be appropriate because it can model the treatment train and the movement of runoff and pollutant loads through the connected system.

Do Either of These Assessments Replace Drainage Design?

No. STORM, BlueFactor and MUSIC primarily assess stormwater treatment performance.

They do not automatically provide:

• pit and pipe sizing;
• minor drainage design;
• major overland flow design;
• on-site detention calculations;
• peak discharge assessment;
• legal point of discharge information;
• flood-level assessment;
• finished surface levels;
• structural treatment details; or
• civil construction drawings.

A project may require a stormwater treatment model and separate civil, hydraulic or flood-related documentation.

Victorian planning guidance distinguishes stormwater-quality treatment from the minor and major drainage systems needed to convey runoff and manage larger storm events. :contentReference[oaicite:5]{index=5}

Does MUSIC Automatically Produce a Better Result?

Not necessarily.

MUSIC provides a more sophisticated representation of the system, but the quality of the result still depends on:

• accurate catchment areas;
• appropriate rainfall data;
• justified runoff parameters;
• realistic treatment-node inputs;
• correct treatment connections;
• appropriate bypass and overflow settings;
• coordination with the physical design; and
• compliance with applicable modelling guidelines.

A detailed model using unrealistic inputs is not inherently more reliable than a well-prepared simplified assessment for an appropriately simple project.

Melbourne Water’s MUSIC guidance requires current modelling approaches and input parameters, and it no longer accepts new models based on its superseded 2018 templates or guidance. :contentReference[oaicite:6]{index=6}

Why Choosing the Correct Method Matters

Selecting the wrong assessment method can delay a planning submission or create unnecessary redesign.

A simplified assessment may be inadequate where:

• the treatment train cannot be represented properly;
• external catchments affect the site;
• the authority needs detailed modelling files;
• the development relies on major shared assets; or
• MUSIC has been specifically requested.

Detailed modelling may be unnecessarily complex where:

• the development is small;
• the catchments are straightforward;
• only a few treatment measures are proposed;
• a simplified tool is accepted; and
• the council only needs a concise treatment result.

Early confirmation of the required method helps the project team prepare the correct information and reserve suitable space for the treatment measures.

Practical Considerations for Victorian Projects

Check the Council Request

Determine whether the council has asked for a STORM report, BlueFactor result, MUSIC model, WSUD response or Stormwater Management Strategy.

Review the Project Scale

Consider the overall development area, number of catchments and extent of roads, roofs and other impervious surfaces.

Map the Treatment Connections

Identify whether runoff passes through one treatment or several connected measures.

Identify Shared Infrastructure

Shared wetlands, sediment ponds, harvesting systems and downstream assets often indicate the need for more detailed modelling.

Confirm Authority Requirements

A project referred to Melbourne Water or another drainage authority may need to follow specific modelling guidelines and submission requirements.

Separate Treatment From Drainage

Confirm whether civil drainage plans, detention calculations or flood assessment are required in addition to the water-quality model.

Use the Current Tool and Guidance

New small-development assessments should reflect the transition from STORM to BlueFactor, while MUSIC models should use the applicable current modelling guidance.

Which Assessment Does Your Project Need?

A STORM-style or BlueFactor assessment may be appropriate where:

• the project is relatively small;
• the catchment layout is simple;
• the treatment measures are limited in number;
• the runoff routing is straightforward;
• there is no complex treatment train; and
• the relevant council accepts the simplified method.

MUSIC modelling may be appropriate where:

• the project is larger or subdivision-scale;
• there are multiple source catchments;
• several treatments are connected in sequence;
• the project includes wetlands, sediment ponds or harvesting;
• bypasses and overflows need to be modelled;
• the development is staged;
• Melbourne Water is reviewing the model; or
• the responsible authority specifically requires MUSIC.

Where the requirement is unclear, the available plans and council correspondence should be reviewed before the assessment scope is confirmed.

How Certified Energy Can Help

Certified Energy can review the proposed development and help determine whether a focused STORM-style or BlueFactor assessment is appropriate.

For suitable projects, this may include:

• reviewing the council request or permit condition;
• measuring site and impervious areas;
• identifying runoff catchments;
• reviewing rainwater tanks and reuse demands;
• assessing raingardens and permeable surfaces;
• testing the overall treatment performance;
• documenting the assessment assumptions;
• coordinating treatment measures with the project drawings; and
• identifying when more detailed MUSIC, civil or hydraulic input may be required.

A simplified STORM assessment should not be presented as an alternative to MUSIC where the scale, treatment system or authority requirements call for detailed modelling.

Explore the STORM Assessment Knowledge Hub or send through the current plans and council correspondence for an initial review.

Frequently Asked Questions

What is the main difference between STORM and MUSIC?

STORM is a simplified stormwater treatment assessment for suitable smaller projects. MUSIC provides more detailed modelling of runoff catchments and connected treatment systems.

Is STORM still used in Victoria?

The original STORM Calculator has been replaced by BlueFactor for suitable small developments. The STORM term remains common in existing reports, permits and council requests.

When is a simplified assessment suitable?

It may suit a relatively small project with a limited number of clearly defined catchments and treatment measures, subject to council acceptance.

When is MUSIC required?

MUSIC may be needed for larger developments, multiple catchments, treatment trains, subdivision infrastructure or where the relevant authority specifically requests it.

Is MUSIC only used for subdivisions?

No. It may also be used for complex commercial, industrial, institutional or mixed-use developments where detailed catchment and treatment modelling is needed.

Can a townhouse development require MUSIC?

Yes. Although many townhouse projects are relatively straightforward, MUSIC may be required where the site, drainage arrangement or treatment system is unusually complex or council specifically requests it.

Can a large site use BlueFactor?

Suitability should not be assumed. Larger developments are more likely to require detailed modelling, particularly where they contain multiple catchments or treatment trains.

Does MUSIC replace a Stormwater Management Plan?

No. A MUSIC model may form part of a broader Stormwater Management Plan, but the plan may also address drainage, detention, discharge, maintenance and implementation.

Do STORM and MUSIC assess flooding?

They primarily address stormwater treatment performance. Flooding, drainage capacity and overland flow may require separate assessment.

Who decides which assessment is required?

The appropriate method is influenced by the planning controls, project complexity and the requirements of the relevant council, Melbourne Water or drainage authority.

Suggested Internal Links

• STORM Assessments and Reports Knowledge Hub
• What Is a STORM Assessment?
• When Is a STORM Assessment Required in Victoria?
• What Does a STORM Report Include?
• How Is a STORM Score Calculated?
• What Does a 100% STORM Rating Mean?
• STORM vs WSUD: What Is the Difference?
• STORM Report vs Stormwater Management Plan
• STORM Assessment vs Drainage Design
• What Information Is Needed for a STORM Assessment?

In Brief

WSUD is the broader design approach. It considers how stormwater, water reuse, infiltration, landscaping and drainage can be integrated across a development.

A STORM assessment is a quantitative treatment assessment. It measures how runoff from roofs, driveways, paving and other impervious areas is treated by nominated measures such as rainwater tanks, raingardens and permeable surfaces.

A STORM result may provide evidence that part of a WSUD strategy achieves the required stormwater-quality performance. It does not necessarily describe the complete water-management response for the site.

In practical terms, WSUD shapes the design, while the STORM assessment tests the treatment performance of selected parts of that design.

Understanding the Difference

The terms STORM and WSUD are sometimes used as though they refer to the same report. This can create confusion during project scoping and council submissions.

They occupy different technical territories:

• WSUD is an integrated approach to managing water within urban development.
• STORM is a modelling and reporting process focused on quantitative stormwater treatment performance.

A project team may use WSUD principles to decide where rainwater tanks, raingardens, permeable surfaces and landscaped treatment areas should be located.

The STORM assessment then examines the runoff-generating catchments connected to those measures and calculates whether the proposed combination reaches the required treatment benchmark.

For a wider explanation of STORM reports and treatment assessments, visit the Certified Energy STORM Knowledge Hub.

What Is a STORM Assessment?

A STORM assessment quantifies how effectively a proposed development treats stormwater runoff.

The assessment commonly considers:

• the total site or development area;
• roof catchments;
• driveways and car parking areas;
• paths, patios and other paving;
• landscaped and permeable areas;
• treated and untreated impervious surfaces;
• rainwater tank capacity;
• roof area connected to each tank;
• rainwater reuse demands;
• raingarden or biofiltration areas;
• permeable paving; and
• the combined treatment performance of the design.

The output is used to show whether the proposed treatment measures achieve the applicable stormwater-quality benchmark.

The assessment has a defined and limited purpose. It does not ordinarily provide a complete site-wide water strategy or detailed drainage design.

What Is WSUD?

Water Sensitive Urban Design is an approach to planning and designing development so that water is managed as an integrated part of the site.

Depending on the project, a WSUD response may consider:

• stormwater quality;
• rainwater harvesting and reuse;
• the reduction of unnecessary impervious area;
• infiltration and permeability;
• landscape integration;
• runoff flow paths;
• the relationship between buildings and drainage;
• water conservation;
• treatment trains;
• maintenance of treatment assets; and
• the effect of development on receiving waterways.

WSUD is therefore broader than a single calculator result.

It influences how the site is arranged and how water-management measures work together across architecture, landscape and engineering.

STORM Measures Performance; WSUD Shapes the Strategy

The simplest way to distinguish the two is to consider the question each one answers.

A STORM assessment asks:

Does the modelled combination of catchments and treatment measures achieve the required stormwater treatment performance?

A WSUD strategy asks:

How should water be managed and integrated across the development?

The STORM question is narrower and quantitative.

The WSUD question is broader and design-led. It may involve quantitative modelling, but it also considers site planning, physical integration, water use, landscaping, maintenance and coordination.

How STORM Fits Within a WSUD Response

A STORM assessment can form one part of a broader WSUD response.

For example, the project team may develop a site strategy that:

• collects roof water in rainwater tanks;
• reuses stored water for toilet flushing or irrigation;
• directs driveway runoff to a raingarden;
• uses permeable paving in selected areas;
• retains landscaped infiltration zones;
• reduces unnecessary hardstand; and
• provides accessible maintenance arrangements.

The STORM assessment can test the treatment performance created by those measures.

It may confirm that the strategy reaches the relevant benchmark, or it may identify that additional treatment is needed.

The numerical result supports the design response, but it does not replace the need to show where the measures are located, how runoff reaches them and how they will be maintained.

Can a STORM Report Be Called a WSUD Report?

The terms should not automatically be treated as interchangeable.

A STORM report usually documents:

• site and impervious areas;
• runoff catchments;
• treatment measures;
• modelling assumptions;
• the treatment score or output; and
• the design commitments required to support that result.

A broader WSUD report may also address:

• the overall water-management concept;
• integration with architecture and landscaping;
• water-conservation measures;
• site permeability;
• treatment sequencing;
• maintenance responsibilities;
• construction-stage considerations;
• drainage coordination; and
• other council-specific requirements.

For a straightforward development, council may accept a concise stormwater treatment report and supporting plan information.

For a more complex project, a calculator output alone may not provide enough information to demonstrate the complete WSUD response.

Can a Project Use WSUD Without a STORM Assessment?

Yes. A project can incorporate WSUD principles without necessarily requiring a STORM assessment.

For example, a development may include:

• rainwater reuse;
• permeable landscape areas;
• reduced hard paving;
• passive irrigation;
• vegetated swales;
• raingardens; or
• site-sensitive drainage arrangements.

Whether quantitative treatment modelling is required depends on the planning controls, development type, application pathway and responsible authority.

Some projects may require another form of assessment, such as BlueFactor or MUSIC modelling. Others may demonstrate their water-sensitive response through plans, specifications and a planning report without a separate STORM output.

Can a Project Achieve a STORM Rating Without a Complete WSUD Strategy?

It may be possible for a project to achieve the required numerical result through a limited set of treatment measures.

For example, a large rainwater tank connected to a substantial roof catchment and regular reuse demand may produce a strong treatment result.

However, the calculator result may not answer broader design questions such as:

• whether unnecessary paving has been reduced;
• whether driveway runoff is managed appropriately;
• whether the treatment measures are integrated with landscaping;
• whether the system is accessible for maintenance;
• whether water reuse has been coordinated with building services;
• whether detention or discharge requirements have been resolved; or
• whether the overall water response is suitable for the site.

A successful numerical result should therefore be supported by a credible and coordinated physical design.

Which WSUD Measures Can Improve a STORM Rating?

Several treatment measures associated with WSUD can contribute to the modelled STORM result.

Rainwater Tanks

Rainwater tanks capture runoff from connected roof areas. Their contribution depends on storage capacity, connected catchment and regular reuse demand.

Toilet flushing, laundry use where applicable, and landscape irrigation can help create storage capacity for later rainfall.

Raingardens

Raingardens can filter runoff from roofs, driveways and other impervious catchments.

The modelled benefit depends on the treatment area, catchment size and physical drainage connection.

Biofiltration Systems

Biofiltration systems use vegetation, engineered filter media and drainage components to remove pollutants from runoff.

They may form part of a landscaped treatment response for suitable projects.

Permeable Paving

Permeable paving allows water to pass through the surface and may support filtration, temporary storage or infiltration.

The area must be genuinely designed and specified as a permeable pavement system.

Infiltration Measures

Suitable infiltration measures may reduce runoff and support treatment where site conditions permit.

Soil, groundwater, slope and building constraints may need to be considered before relying on infiltration.

Reduced Impervious Area

Reducing conventional paving or replacing it with landscaping and permeable surfaces can reduce the amount of runoff requiring treatment.

Example: A Townhouse Development

Consider a three-townhouse development with:

• three dwelling roofs;
• a shared driveway;
• pedestrian paths;
• private courtyards;
• small garden areas; and
• three rainwater tanks.

The WSUD strategy may determine that:

• each dwelling roof is connected to its own tank;
• tank water supplies toilet flushing and garden irrigation;
• part of the driveway drains to a raingarden;
• selected courtyard paving is permeable;
• landscape areas receive passive irrigation; and
• the treatment measures remain accessible for maintenance.

The STORM assessment then measures:

• the roof area connected to each tank;
• the tank capacities and reuse demands;
• the driveway catchment connected to the raingarden;
• the permeable pavement area;
• any untreated impervious surfaces; and
• the combined treatment result.

In this example, WSUD provides the integrated site response. STORM tests whether the treatment components achieve the required performance.

STORM, BlueFactor and WSUD

The original Melbourne Water STORM Calculator has been replaced by BlueFactor for suitable small developments in Victoria.

This changes the current assessment tool but not the basic distinction between modelling and design strategy.

• WSUD remains the broader water-sensitive design approach.
• STORM remains the familiar name associated with the earlier quantitative treatment assessment.
• BlueFactor is the current replacement assessment tool for suitable small developments.

Older permit conditions and council correspondence may continue to request a STORM report or STORM score.

For a new submission, the project team should confirm whether council expects BlueFactor, MUSIC or another accepted assessment format.

How STORM and WSUD Differ From MUSIC

MUSIC is another quantitative modelling tool, not a synonym for WSUD.

It can represent more complex catchments, drainage connections and treatment trains than a basic STORM-style assessment.

The relationship can be summarised as follows:

• WSUD is the integrated design approach.
• STORM or BlueFactor can assess treatment performance for suitable smaller and simpler developments.
• MUSIC can model more complex catchments and connected treatment trains.

Both STORM-style assessments and MUSIC models may be used to provide quantitative evidence supporting a WSUD strategy.

Read STORM vs MUSIC: Which Assessment Does Your Project Need? for the dedicated modelling comparison.

How STORM and WSUD Differ From Drainage Design

Neither the term WSUD nor a STORM assessment should automatically be treated as a complete drainage design.

A drainage design may address:

• stormwater pits;
• pipe sizes;
• surface levels;
• gutter and downpipe connections;
• on-site detention;
• peak flow rates;
• overland flow paths;
• the legal point of discharge; and
• civil construction details.

A STORM assessment focuses on treatment performance.

A WSUD strategy may influence drainage design and landscape integration, but detailed hydraulic or civil documentation may still be required separately.

Why the Difference Matters

Understanding the distinction helps the project team commission the correct service and prepare the right documentation for council.

Confusing STORM and WSUD can lead to several problems:

• a calculator result may be submitted where a broader strategy was requested;
• a general WSUD statement may be provided without quantitative treatment evidence;
• treatment measures may not be coordinated with the drawings;
• drainage or detention requirements may be overlooked;
• the project may receive a request for further information; or
• different consultants may assume that another discipline is resolving the stormwater response.

A clear scope should identify whether the project needs:

• a quantitative stormwater treatment assessment;
• a broader WSUD strategy or report;
• BlueFactor or MUSIC modelling;
• a Stormwater Management Plan;
• drainage and detention design; or
• a combination of these services.

Practical Considerations for Victorian Projects

Read the Council Request Carefully

Terms such as STORM report, WSUD response, stormwater treatment assessment and Stormwater Management Plan may imply different deliverables.

Confirm the Accepted Modelling Tool

Establish whether the project should use BlueFactor, MUSIC or another method accepted by the relevant council.

Develop the Strategy Before Finalising the Model

The model should test treatment measures that can physically fit within the architectural and landscape design.

Coordinate the Catchments

Roof, driveway and paved areas should only be assigned to treatment measures where the proposed drainage arrangement makes the connection achievable.

Show the Measures on the Drawings

Rainwater tanks, raingardens and permeable paving used in the assessment should appear consistently in the project documents.

Allow for Maintenance

Treatment assets need practical access and ongoing maintenance. Numerical performance alone does not establish whether a measure will remain effective.

Check for Separate Drainage Requirements

Confirm whether council also requires detention calculations, civil plans, discharge information or flood-related assessment.

Which Service Does Your Project Need?

A focused STORM or BlueFactor assessment may be appropriate where:

• the development is relatively small;
• the catchments are straightforward;
• the treatment measures are uncomplicated;
• council requests a quantitative treatment result; and
• a broader strategy or detailed model is not required.

A broader WSUD response may be appropriate where:

• water management needs to be integrated across the site;
• the design includes several complementary initiatives;
• council requests a written design response;
• landscape and drainage integration is important;
• maintenance commitments need to be documented; or
• the treatment result forms only one part of the submission.

Some developments require both: a WSUD strategy explaining the design and a quantitative assessment demonstrating its treatment performance.

How Certified Energy Can Help

Certified Energy can review the available plans and council correspondence to identify the stormwater treatment assessment required for a Victorian project.

For suitable developments, this may include:

• measuring site and impervious areas;
• identifying runoff catchments;
• reviewing proposed rainwater tanks and reuse demands;
• assessing raingardens and permeable surfaces;
• testing the combined treatment performance;
• identifying practical ways to improve the result;
• documenting the assessment assumptions;
• coordinating treatment commitments with the plans; and
• clarifying where broader WSUD, MUSIC, drainage or engineering input may be required.

The purpose of the assessment is to provide clear evidence of stormwater treatment performance without presenting it as a substitute for a broader water-management or drainage scope.

Explore the STORM Assessment Knowledge Hub or send through the current plans and council request for an initial project review.

Frequently Asked Questions

What is the main difference between STORM and WSUD?

WSUD is the broader approach used to integrate water management into the design of a development. STORM is a quantitative assessment used to measure the treatment performance of selected runoff catchments and measures.

Is STORM part of WSUD?

A STORM assessment can support the stormwater treatment component of a WSUD response. It does not necessarily represent the complete WSUD strategy.

Is a STORM report the same as a WSUD report?

Not always. A STORM report usually focuses on areas, catchments, treatment inputs and the resulting performance score. A WSUD report may cover a broader integrated site response.

Can a project have WSUD without STORM modelling?

Yes. Whether a quantitative assessment is required depends on the project and council requirements. Another modelling method may also be used.

Can a project achieve a STORM rating without good WSUD design?

A project may achieve the numerical benchmark through selected measures, but the result should still be supported by a practical, coordinated and maintainable site design.

Which WSUD measures improve a STORM rating?

Rainwater tanks, raingardens, biofiltration, permeable paving and suitable infiltration measures may contribute where they are properly sized and connected to relevant runoff catchments.

Is BlueFactor the same as WSUD?

No. BlueFactor is an assessment tool for suitable small developments. WSUD is the broader water-sensitive planning and design approach.

Is MUSIC the same as WSUD?

No. MUSIC is a modelling platform that can test more complex catchments and treatment trains. It may be used to support a WSUD strategy.

Does STORM replace drainage design?

No. STORM addresses stormwater treatment performance. Drainage collection, pipe capacity, detention and discharge may require separate civil or hydraulic design.

Can Certified Energy help determine which report is needed?

Yes. The available plans, council request, planning controls and project complexity can be reviewed to identify whether a focused treatment assessment or a broader stormwater scope is appropriate.

Suggested Internal Links

• STORM Assessments and Reports Knowledge Hub
• What Is a STORM Assessment?
• When Is a STORM Assessment Required in Victoria?
• What Does a STORM Report Include?
• How Is a STORM Score Calculated?
• STORM vs MUSIC: Which Assessment Does Your Project Need?
• Can Rainwater Tanks Improve a STORM Rating?
• How WSUD Features Help Improve a STORM Rating
• STORM Report vs Stormwater Management Plan
• STORM Assessment vs Drainage Design

In Brief

A 100% STORM rating indicates that the combined treatment measures entered into the assessment provide enough modelled performance to meet the reference stormwater-quality objectives.

The result may be achieved through a combination of rainwater tanks, connected roof catchments, regular rainwater reuse, raingardens, permeable surfaces and other suitable treatment measures.

It does not mean that 100% of rainfall is retained, 100% of pollutants are removed or every impervious surface is individually treated.

It also does not confirm that drainage design, detention, flood behaviour, discharge arrangements or every planning requirement has been resolved. The rating applies specifically to the modelled stormwater treatment performance.

Understanding the 100% Benchmark

The STORM rating is a relative measure rather than a literal percentage of water or pollution removed from the site.

The assessment compares the proposed treatment performance with established best-practice stormwater-quality objectives. A result of 100% indicates that the modelled design has reached the benchmark represented by the assessment tool.

Those best-practice objectives commonly address reductions in:

• suspended solids;
• total phosphorus;
• total nitrogen; and
• litter.

The treatment measures are assessed together across the proposed development. The resulting rating expresses how the complete strategy performs relative to those objectives.

For a broader explanation of the assessment process, visit the Certified Energy STORM Knowledge Hub.

The Water-Quality Objectives Behind the Rating

Victorian stormwater treatment requirements use quantitative objectives to define best-practice water-quality performance.

These objectives commonly require the development’s treatment strategy to achieve:

• 80% reduction in the mean annual load of suspended solids;
• 45% reduction in the mean annual load of total phosphorus;
• 45% reduction in the mean annual load of total nitrogen; and
• 70% reduction in the mean annual load of litter.

A 100% rating indicates that the proposed combination of catchments and treatment measures achieves the reference level represented by these objectives.

It does not mean the project removes 100% of suspended solids, phosphorus, nitrogen or litter.

The word “100%” describes performance against the benchmark, not total pollutant removal.

How a Project Can Achieve 100%

A development can reach the benchmark through one treatment measure or, more commonly, a coordinated combination of measures.

These may include:

• rainwater tanks connected to suitable roof catchments;
• toilet flushing or other regular rainwater reuse;
• landscape irrigation using captured rainwater;
• raingardens or biofiltration systems;
• permeable paving;
• infiltration areas;
• reduced unnecessary impervious area; and
• careful allocation of runoff between treatment measures.

The result depends on the relationship between the runoff-generating surfaces and the measures that receive and treat that runoff.

Simply including a tank or raingarden on the plans does not guarantee a 100% rating. Its capacity, connected catchment, reuse demand and treatment configuration all influence the result.

Does 100% Mean Every Surface Is Treated?

No. A project may contain some untreated impervious surfaces and still achieve an overall rating of 100%.

For example, a development may include:

• a large roof area connected to an appropriately sized rainwater tank;
• a driveway directed to a raingarden;
• permeable paving within a courtyard; and
• a small entry path that remains untreated.

The assessment considers the combined performance of the development rather than requiring a separate treatment device for every small surface.

However, untreated areas must still be included in the assessment. They should not be omitted to make the result appear stronger.

The final rating should represent the complete proposed development, including both treated and untreated catchments.

Does 100% Mean All Stormwater Remains On Site?

No. A 100% STORM rating is not the same as retaining every litre of rainfall within the property.

Stormwater treatment measures may:

• capture part of the runoff for reuse;
• temporarily store water;
• filter pollutants;
• promote infiltration;
• slow the movement of runoff; and
• discharge treated overflow to the drainage system.

A rainwater tank, for example, may capture roof water until it reaches capacity. Once full, additional runoff may pass through an overflow connection.

A raingarden may filter runoff before excess water leaves through an underdrain or overflow.

The benchmark concerns treatment performance. It does not require the development to operate as a completely closed water system.

Does 100% Mean the Project Is Fully Compliant?

A 100% treatment rating is an important result, but it should not be interpreted as universal approval of the whole stormwater design.

The responsible authority may also need to consider:

• whether the assessment uses the correct site areas;
• whether all impervious surfaces have been included;
• whether the treatment measures can physically fit on the site;
• whether runoff can reach the nominated treatments;
• whether the report matches the architectural and landscape plans;
• whether rainwater reuse connections are documented;
• whether maintenance arrangements are adequate;
• whether detention requirements have been addressed;
• whether the discharge arrangement is acceptable; and
• whether other planning permit requirements have been satisfied.

A 100% rating should therefore be understood as evidence that the modelled water-quality benchmark has been achieved, not as automatic approval of every part of the project.

Can Council Reject a 100% STORM Result?

A council may question or decline to rely on a 100% result where the supporting inputs do not accurately reflect the proposed development.

Potential concerns may include:

• incorrect or outdated site areas;
• omitted driveways, paths or balconies;
• a roof catchment that cannot drain to the nominated tank;
• reuse demands that are not shown in the project documents;
• a raingarden that does not fit within the available landscape area;
• permeable paving that is not supported by the specification;
• double-counting a catchment;
• a treatment measure that conflicts with site levels or services; or
• use of an assessment method that is not accepted for the project.

The quality of the result depends on the quality and credibility of the assessment inputs.

A valid submission should show not only that the numerical benchmark has been reached, but that the treatment strategy can be incorporated into the proposed development.

Can a STORM Rating Be Higher Than 100%?

Some assessment outputs may show treatment performance above the minimum benchmark.

This can occur where the development includes more treatment capacity than is needed to reach the reference objective.

For example, a project may have:

• a high proportion of roof area connected to tanks;
• strong and regular rainwater reuse demand;
• large raingarden treatment areas;
• substantial permeable paving;
• limited untreated impervious area; or
• multiple complementary treatment measures.

A result above 100% can provide additional design resilience, particularly where small changes may occur later.

However, a higher number is not automatically a better design if it depends on oversized, impractical or poorly coordinated treatment measures.

The objective should be a credible and maintainable strategy rather than the highest possible calculator result.

Is It Better to Aim Slightly Above 100%?

In some projects, a modest margin above the benchmark can be useful.

A design that reaches exactly 100% may fall below the target if later revisions:

• increase the roof area;
• add conventional paving;
• reduce landscaped area;
• decrease the connected tank catchment;
• reduce the rainwater tank capacity;
• remove a reuse connection;
• reduce the raingarden area; or
• change permeable paving to an impervious finish.

A small performance margin can make the assessment less vulnerable to minor design changes.

This should not result in unnecessary treatment infrastructure. The margin should remain proportionate to the project and the likelihood of design development.

What Does a Rating Below 100% Mean?

A rating below 100% generally means the current treatment strategy does not yet achieve the benchmark represented by the assessment.

It does not necessarily mean the project is fundamentally unsuitable.

The result may be improved by reviewing:

• the amount of roof connected to rainwater tanks;
• the tank capacities;
• the nominated rainwater reuse demands;
• the treatment of driveway runoff;
• the size of raingardens;
• the use of permeable paving;
• the quantity of untreated impervious area; and
• the overall site layout.

The most effective response depends on which catchments and treatment measures are limiting the result.

A project should not automatically respond by increasing the tank size. Other changes may be more practical or provide a stronger improvement.

How Rainwater Tanks Contribute to 100%

Rainwater tanks can make a significant contribution where they capture runoff from a substantial roof area and serve regular non-potable water demands.

The main factors are:

• the capacity of the tank;
• the roof area connected to it;
• the number of dwellings or users served;
• toilet-flushing demand;
• laundry demand where included;
• irrigation demand; and
• the landscaped area associated with that demand.

Regular reuse creates capacity within the tank for future rainfall.

A large tank with little connected roof area or limited reuse may contribute less than expected. The tank, catchment and demand should be considered as one coordinated treatment system.

Read Can Rainwater Tanks Improve a STORM Rating? for a detailed explanation.

How Other Treatment Measures Contribute

Where a tank alone does not achieve the target, additional treatment may be provided through other site measures.

Raingardens

A raingarden can treat runoff from roofs, driveways or paved areas through filtration and biological processes. Its contribution depends on its treatment area and connected catchment.

Permeable Paving

A genuine permeable pavement system can reduce the amount of untreated impervious area and may provide filtration, temporary storage and infiltration.

Infiltration Areas

Suitable landscaped or engineered infiltration areas may help manage runoff where site conditions support their use.

Reduced Impervious Area

Removing unnecessary paving or replacing sealed surfaces with permeable alternatives can reduce the runoff load that needs to be treated.

The strongest solution is often a coordinated treatment strategy rather than reliance on one oversized measure.

STORM and BlueFactor Terminology

The original STORM Calculator has historically been used for relatively small Victorian developments.

For new suitable projects, current submissions may instead use BlueFactor or another method accepted by the responsible authority.

Older council correspondence and permit conditions may continue to refer to:

• a 100% STORM rating;
• a STORM certificate;
• a STORM report;
• best-practice stormwater treatment; or
• a compliant stormwater treatment score.

Although the software and output format may change, the underlying question remains similar: does the proposed treatment strategy achieve the required water-quality performance?

The accepted tool and submission format should be confirmed for the particular council and project.

What a 100% Rating Does Not Assess

A 100% STORM rating does not provide a complete drainage or flood assessment.

It does not ordinarily confirm:

• stormwater pipe capacity;
• pit locations or dimensions;
• on-site detention volume;
• peak flow rates;
• legal point of discharge requirements;
• finished surface levels;
• overland flow paths;
• flood levels;
• structural design of tanks or raingardens; or
• the complete civil drainage design.

These matters may need to be addressed through separate hydraulic, civil or flood-related documentation.

The STORM rating should not be presented as a substitute for those services.

Why the Rating Matters

The 100% benchmark gives project teams and councils a consistent way to test whether a proposed stormwater treatment response is likely to achieve the required water-quality outcome.

It can help:

• identify whether the treatment strategy is adequate;
• compare different treatment combinations;
• coordinate tanks and reuse requirements;
• allocate sufficient landscape treatment space;
• reduce untreated runoff;
• document planning commitments; and
• support the stormwater treatment component of a council submission.

The real value of the rating is not the number by itself. It is the clear connection between the result and a practical treatment strategy that can be implemented on site.

Practical Considerations for Victorian Projects

Check the Assessment Inputs

Confirm that the site area, roof areas, driveways, paving and landscaping match the current drawings.

Include Untreated Catchments

Small untreated areas should remain in the model so the result represents the complete development.

Verify Treatment Connections

Only assign runoff to a treatment measure where the physical drainage arrangement makes the connection achievable.

Document Rainwater Reuse

Toilet, laundry or irrigation uses included in the assessment should be shown in the appropriate plans or specifications.

Coordinate the Drawings

Tank capacities, raingarden areas and permeable surfaces should remain consistent across the architectural, landscape and hydraulic documentation.

Do Not Treat the Score as the Entire Approval

Confirm whether council also requires drainage plans, detention calculations, maintenance information or a broader Stormwater Management Plan.

How Certified Energy Can Help

Certified Energy can assess the proposed development and identify whether its stormwater treatment strategy reaches the required performance benchmark.

The assessment process may include:

• measuring roof, driveway, paving and landscape areas;
• identifying treated and untreated catchments;
• reviewing rainwater tank capacities;
• checking connected roof areas and reuse demands;
• assessing raingardens and permeable surfaces;
• testing alternative treatment combinations;
• identifying why a result remains below 100%;
• recommending practical improvements;
• documenting the treatment assumptions; and
• coordinating the final result with the project plans.

Where the project requires MUSIC modelling, drainage design, detention calculations or other civil engineering documentation, that wider scope should be identified separately.

Explore the STORM Assessment Knowledge Hub or send through the current plans and council correspondence for an initial project review.

Frequently Asked Questions

What does a 100% STORM rating mean?

It generally means the proposed stormwater treatment strategy reaches the best-practice water-quality benchmark represented by the assessment tool.

Does 100% mean every pollutant is removed?

No. The rating compares the project with defined pollutant-reduction objectives. It does not mean complete removal of suspended solids, phosphorus, nitrogen or litter.

Does every impervious surface need treatment?

Not necessarily. Some small areas may remain untreated where the combined project strategy still reaches the overall benchmark. All relevant areas should nevertheless be included in the assessment.

Does 100% mean no stormwater leaves the site?

No. Treated runoff and overflows may still discharge to the drainage system. The rating relates to water-quality performance, not complete retention of all rainfall.

Does a 100% rating guarantee council approval?

No. Council may also assess the accuracy of the report, consistency with the plans, practicality of the treatment measures and other planning and drainage requirements.

Can a rating exceed 100%?

Some assessment outputs may show performance above the reference benchmark. The measures still need to be practical, maintainable and correctly documented.

Should a project aim above 100%?

A modest margin may be useful where future design changes are likely, but unnecessary or oversized treatment measures should not be added solely to maximise the score.

What does a rating below 100% mean?

It generally means the current treatment strategy does not yet reach the reference benchmark and may need further refinement.

Can a larger tank achieve 100%?

It may help, but tank performance also depends on connected roof area and regular reuse demand. A larger tank alone is not always the most effective solution.

Does the rating assess flooding?

No. Flooding, drainage capacity, detention and discharge may require separate civil, hydraulic or flood assessment.

Suggested Internal Links

• STORM Assessments and Reports Knowledge Hub
• What Is a STORM Assessment?
• What Does a STORM Report Include?
• How Is a STORM Score Calculated?
• Can Rainwater Tanks Improve a STORM Rating?
• Common Reasons a Project Fails a STORM Assessment
• How to Improve a STORM Rating
• Do Raingardens Improve a STORM Assessment?
• Permeable Paving and STORM Compliance
• STORM Assessment vs Drainage Design

In Brief

The calculation begins with the areas of the proposed development. Roofs, driveways, paths, paving and other impervious surfaces are entered as runoff-generating catchments.

Each catchment is then connected to a proposed treatment measure, such as a rainwater tank, raingarden or permeable surface, or identified as untreated.

The assessment tool estimates how effectively the combined treatment strategy reduces stormwater pollutants relative to the applicable best-practice objectives. The resulting percentage is presented as the project’s STORM or stormwater treatment score.

A score of 100% is generally understood to mean that the modelled design meets the relevant best-practice treatment benchmark. It does not mean that all runoff is removed, that no pollutants remain or that the project has satisfied every drainage and planning requirement.

What the STORM Score Represents

The STORM score is a relative measure of stormwater treatment performance.

It compares the treatment achieved by the proposed development with the reference level established by Victoria’s best-practice stormwater-quality objectives.

Those objectives have traditionally included reductions in the typical urban annual loads of:

• suspended solids;
• total phosphorus;
• total nitrogen; and
• litter.

The score is therefore not a simple percentage of rainwater retained on site. It reflects how the modelled combination of catchments and treatment measures performs against the relevant water-quality benchmark.

The exact calculation is performed by the assessment tool. A project team would not normally calculate the final composite score manually.

For an overview of the broader assessment process, visit the Certified Energy STORM Knowledge Hub.

The Best-Practice Objectives Behind the Score

Stormwater treatment assessments in Victoria have historically been tested against established best-practice pollutant-reduction objectives.

These commonly include:

• 80% retention of the typical urban annual suspended solids load;
• 45% retention of the typical urban annual total phosphorus load;
• 45% retention of the typical urban annual total nitrogen load; and
• 70% reduction of the typical urban annual litter load.

The calculator assesses how the proposed treatment strategy performs relative to these objectives.

A 100% result does not mean that 100% of each pollutant is removed. It indicates that the modelled performance reaches the benchmark represented by the tool.

Step 1: Define the Assessment Area

The calculation first needs a clear assessment boundary.

For a typical development, this may be the complete title boundary or the area affected by the proposed buildings and works. The correct boundary depends on the project and the council requirement.

The total area is then divided into surface types, such as:

• building roofs;
• garages and carports;
• balconies or terraces;
• driveways;
• car parking areas;
• paths and patios;
• conventional paving;
• permeable paving;
• landscaped areas; and
• other pervious surfaces.

The areas entered into the model should reconcile with the total site or assessment area.

If impervious surfaces are omitted or underestimated, the score may not accurately represent the proposed development.

Step 2: Identify the Impervious Catchments

Impervious surfaces generate most of the runoff assessed by the tool.

These surfaces prevent rainfall from infiltrating naturally into the ground and instead direct water towards gutters, pits, drains or adjoining areas.

The assessor identifies the area of each catchment and determines where its runoff will go.

For example:

• the main dwelling roof may drain to a rainwater tank;
• a garage roof may discharge without treatment;
• a shared driveway may drain to a raingarden;
• a courtyard may use permeable paving; and
• an entry path may discharge directly to the drainage system.

This allocation is central to the calculation. A treatment measure can only improve the score for the runoff that actually reaches it.

Step 3: Assign Runoff to Treatment Measures

Once the catchments are defined, the assessor links them to the proposed stormwater treatment measures.

Common measures may include:

• rainwater tanks;
• raingardens;
• biofiltration systems;
• permeable paving;
• infiltration systems; and
• other treatment types accepted by the assessment method and council.

The assessment must reflect a credible physical relationship between the catchment and treatment measure.

For example, a roof can only be counted as draining to a tank where suitable gutters and downpipes can connect that roof to the tank. A driveway can only be assigned to a raingarden where the levels and drainage arrangement allow runoff to reach it.

Step 4: Enter the Treatment Details

Each treatment measure requires information describing its size, connection and intended operation.

The inputs vary according to the type of treatment.

Rainwater Tanks

Relevant inputs commonly include:

• tank storage capacity;
• connected roof catchment;
• number of dwellings or occupants served;
• toilet-flushing demand;
• laundry demand, where applicable;
• garden irrigation demand; and
• the landscaped area receiving irrigation.

Raingardens and Biofiltration

Relevant inputs may include:

• treatment surface area;
• connected impervious catchment;
• treatment type;
• infiltration or discharge assumptions; and
• the arrangement of the treatment within the site.

Permeable Paving

The assessment may consider:

• the permeable pavement area;
• the surface itself;
• any adjoining catchment draining onto it;
• the nominated treatment configuration; and
• site conditions affecting its operation.

The score is only as reliable as the assumptions entered into the model.

Step 5: Account for Untreated Runoff

Not every impervious surface will necessarily drain through a treatment measure.

Common untreated areas include:

• small roof sections;
• porches and verandahs;
• balconies with separate outlets;
• parts of a driveway that bypass treatment;
• isolated paths; and
• paved areas draining directly to the site outlet.

These surfaces still need to be entered into the assessment.

A high-quality result should not depend on excluding inconvenient catchments from the model. The calculation should represent the actual proposed development, including both treated and untreated runoff.

Step 6: Calculate the Treatment Performance

The assessment tool uses its internal runoff and pollutant-treatment calculations to estimate the performance of each treatment measure.

It then combines the treated and untreated catchments to determine the overall project result.

In practical terms, the final score is influenced by:

• how much impervious area is created;
• how much of that area receives treatment;
• the type of treatment used;
• the size of each treatment measure;
• the catchment connected to each measure;
• the level of rainwater reuse;
• the pollutant-removal performance attributed to the measures; and
• the amount of runoff that remains untreated.

The output is expressed as a percentage relative to the best-practice benchmark.

What Does a Score Below 100% Mean?

A score below 100% generally indicates that the modelled treatment strategy has not yet reached the reference best-practice objective.

This does not necessarily mean that the overall development has been rejected or that every element of the strategy is unsuitable.

It means that, based on the current inputs, further treatment or design refinement may be needed.

Potential responses can include:

• connecting more roof area to a rainwater tank;
• increasing regular rainwater reuse;
• revising the tank capacity;
• adding or enlarging a raingarden;
• increasing permeable paving;
• redirecting driveway runoff to treatment;
• reducing unnecessary impervious area; or
• combining more than one treatment measure.

The most appropriate change depends on which surfaces and treatment measures are limiting the result.

What Does a 100% Score Mean?

A 100% STORM score generally indicates that the modelled treatment strategy meets the benchmark represented by the assessment tool.

It does not mean:

• that all stormwater remains on the site;
• that 100% of every pollutant is removed;
• that no runoff reaches the drainage system;
• that every impervious surface is individually treated;
• that the drainage design has been approved;
• that detention requirements have been met;
• that flood risk has been assessed; or
• that every planning permit requirement is satisfied.

The score relates specifically to the modelled stormwater treatment outcome.

Read What Does a 100% STORM Rating Mean? for a more detailed explanation.

Can a Score Exceed 100%?

Depending on the assessment output, a treatment strategy may perform above the minimum reference benchmark.

This can occur where the development includes substantial treatment relative to its impervious area.

However, a result above 100% should not be pursued at the expense of a practical and coordinated design.

The treatment measures still need to:

• fit within the site;
• receive the catchments assigned to them;
• be reflected in the drawings;
• operate as assumed;
• remain maintainable; and
• be acceptable to the relevant council.

A mathematically strong result is not useful if the modelled strategy cannot be constructed or maintained.

How Rainwater Tanks Affect the Score

Rainwater tanks can improve a STORM score by capturing roof runoff and creating storage capacity through ongoing reuse.

Three inputs work together:

1. the size of the tank;
2. the roof area connected to it; and
3. the demand that regularly draws water from the tank.

A larger tank does not automatically produce a proportionally higher result.

If only a small roof area is connected, the tank may not receive enough runoff. If there is little reuse, the tank may remain full and have limited capacity to capture the next rainfall event.

The tank, catchment and reuse demand should therefore be considered as one treatment system.

Read Can Rainwater Tanks Improve a STORM Rating? for a focused explanation.

How Raingardens Affect the Score

A raingarden can improve the result by treating runoff through filtration and biological processes before the water is discharged or infiltrated.

Its modelled contribution depends on factors such as:

• the treatment area;
• the impervious catchment draining to it;
• the treatment configuration;
• the selected assumptions; and
• whether runoff can physically reach the system.

A small raingarden connected to a very large driveway may not provide enough treatment. Conversely, a reasonably sized raingarden connected to a defined catchment can make a significant contribution.

How Permeable Paving Affects the Score

Permeable paving can reduce the untreated impervious area and may provide treatment through temporary storage, filtration and infiltration.

Its effect depends on:

• the area genuinely designed as permeable;
• the pavement system selected;
• any additional runoff directed onto it;
• the underlying site conditions; and
• the way it is represented in the assessment.

The modelled area should match the paving specification and drawings. Conventional paving should not be counted as permeable solely to improve the assessment result.

Why Two Similar Projects Can Receive Different Scores

Two developments with similar site areas can produce very different results.

This may be due to differences in:

• total roof area;
• driveway and paving area;
• landscaped area;
• the proportion of runoff receiving treatment;
• tank size;
• connected roof catchment;
• number of dwellings and reuse demand;
• raingarden sizing;
• permeable surface area; and
• untreated runoff.

The score responds to the complete relationship between the development surfaces and its treatment strategy, not merely the presence of one sustainability feature.

Common Input Errors That Affect the Score

Unexpected results are often caused by incomplete or inconsistent inputs rather than the assessment tool itself.

Common issues include:

• omitting part of the driveway or paving;
• entering an incorrect site area;
• assigning an entire roof to a tank that only receives part of it;
• including reuse connections that are not shown on the plans;
• using an outdated tank capacity;
• counting conventional paving as permeable;
• overstating the area of a raingarden;
• failing to include untreated balconies or paths;
• using superseded architectural drawings; and
• double-counting a catchment across two treatments.

The model inputs should be checked against the current architectural, landscape and hydraulic documentation before the report is issued.

STORM and BlueFactor Scores

Melbourne Water’s original STORM Calculator has been replaced by BlueFactor for suitable small-scale Victorian developments.

The two tools belong to the same practical assessment territory: estimating runoff and pollutant generation and testing the treatment measures proposed for a development.

Existing permits and older council correspondence may continue to refer to a STORM score, while a new project may receive a BlueFactor result.

The terminology and exact output format may differ, so the current council requirement should be confirmed before preparing the submission.

What the Score Does Not Calculate

A STORM score is not a complete measure of every stormwater issue affecting a site.

It does not ordinarily determine:

• the required stormwater pipe diameter;
• pit locations or sizes;
• the legal point of discharge;
• on-site detention storage;
• peak discharge rates;
• overland flow paths;
• flood levels;
• finished surface levels;
• structural design of treatment assets; or
• the complete civil drainage design.

These matters may require separate hydraulic, civil or flood-related documentation.

Practical Considerations for Victorian Projects

Measure the Current Design

The calculation should use the latest architectural and landscape plans. Changes to footprints, driveways or paving can alter the score.

Include All Relevant Surfaces

Small untreated areas may appear insignificant individually but can collectively affect the overall result.

Use Achievable Treatment Measures

The assessment should not rely on a tank, raingarden or permeable area that cannot fit within the proposed development.

Coordinate Rainwater Reuse

Toilet, laundry or irrigation reuse included in the model should be documented in the relevant plans and specifications.

Check the Drainage Path

A catchment should only be connected to a treatment measure where the site levels and drainage arrangement make that connection credible.

Confirm the Required Output

The relevant council may request a STORM result, BlueFactor output, MUSIC model or another form of treatment assessment depending on the project.

How Certified Energy Can Help

Certified Energy can assess the proposed site areas and treatment measures and prepare a stormwater treatment result for suitable Victorian developments.

The assessment process may include:

• measuring roof, driveway, paving and landscape areas;
• identifying treated and untreated catchments;
• reviewing rainwater tank capacities and reuse demands;
• assessing raingardens and permeable surfaces;
• testing different treatment combinations;
• identifying why a result is below the target;
• recommending practical improvements;
• documenting the final modelling assumptions; and
• coordinating the result with the current project plans.

Where the project is too large or complex for a STORM-style assessment, the need for MUSIC modelling or separate civil and hydraulic input should be identified.

Explore the STORM Assessment Knowledge Hub or send through the current plans and council request for an initial project review.

Frequently Asked Questions

How is a STORM score calculated?

The assessment tool models runoff from the development’s impervious surfaces, applies the performance of connected treatment measures and compares the resulting pollutant reductions with the applicable best-practice objectives.

What information affects the score?

The result is affected by site areas, impervious catchments, untreated surfaces, treatment type, treatment size, connected catchments and rainwater reuse assumptions.

What score is usually required?

A result of 100% is generally used to demonstrate that the modelled strategy meets the benchmark represented by the assessment tool. Council requirements should still be confirmed for the individual project.

Does 100% mean all pollutants are removed?

No. It means the modelled reductions reach the relevant best-practice benchmark. It does not mean that the development removes 100% of every pollutant.

Can the score exceed 100%?

Some outputs may indicate performance above the benchmark. The treatment strategy must still be practical, coordinated with the plans and acceptable to council.

Does a larger tank always improve the score?

No. Tank performance also depends on the roof catchment and regular reuse demand. Additional storage may provide limited benefit when those other inputs remain unchanged.

Can untreated areas be excluded?

No. Relevant impervious areas should be included so the assessment accurately represents the proposed development.

Can a raingarden improve the result?

Yes. Its contribution depends on its treatment area, connected catchment, configuration and whether runoff can physically drain to it.

Does permeable paving improve the score?

It may improve the score where a genuine permeable pavement system is proposed and correctly represented in the assessment.

Does the score assess flooding?

No. Flooding, detention, drainage capacity and discharge are separate matters that may require specialist engineering assessment.

Suggested Internal Links

• STORM Assessments and Reports Knowledge Hub
• What Is a STORM Assessment?
• What Does a STORM Report Include?
• What Does a 100% STORM Rating Mean?
• Can Rainwater Tanks Improve a STORM Rating?
• Common Reasons a Project Fails a STORM Assessment
• How to Improve a STORM Rating
• Do Raingardens Improve a STORM Assessment?
• Permeable Paving and STORM Compliance
• What Information Is Needed for a STORM Assessment?

In Brief

A STORM report generally includes the project details, assessed site areas, impervious catchments, proposed stormwater treatment measures, modelling assumptions and the final treatment result.

In Brief

A Victorian council may request a STORM assessment where a development must demonstrate how runoff from roofs, driveways, paving and other impervious surfaces will be treated.

In brief

A STORM assessment evaluates stormwater runoff from the impervious areas of a development and measures the treatment performance of proposed measures such as rainwater tanks, raingardens and permeable surfaces.

It is commonly associated with Victorian planning applications where a council needs evidence that a project can achieve an appropriate stormwater-quality outcome.

The assessment is focused on runoff treatment and pollutant reduction. It is not a substitute for drainage design, on-site detention calculations, flood modelling or a civil engineering report.

Current terminology note: Melbourne Water has replaced its original STORM Calculator with BlueFactor. However, “STORM assessment” and “STORM report” remain familiar terms in existing council documentation, earlier planning permits and project discussions. The relevant council should confirm which assessment tool or reporting format it currently accepts.

How a STORM Assessment Works

When a site is developed, surfaces that once absorbed rainfall may be replaced by roofs, driveways, paths, paved areas and other impervious surfaces. Rainfall landing on these areas can become stormwater runoff and carry sediment, nutrients, litter and other pollutants into the drainage system and surrounding waterways.

A STORM assessment examines how much of this runoff is generated by the proposed development and how effectively the proposed treatment measures manage it before it leaves the site.

The assessment usually considers:

• the total development or site area;
• roof catchment areas;
• driveways, paving and other impervious surfaces;
• landscaped or permeable areas;
• the destination of runoff from each surface;
• proposed rainwater tanks and their connected catchments;
• the expected demand for stored rainwater;
• raingardens, infiltration systems or permeable surfaces;
• untreated impervious areas; and
• the combined stormwater treatment performance of the design.

The resulting score or output helps demonstrate whether the proposed treatment strategy reaches the applicable best-practice stormwater-quality objective.

For a broader overview of STORM reports, treatment measures and Victorian project requirements, visit the Certified Energy STORM Knowledge Hub.

What Does a STORM Assessment Actually Assess?

A STORM assessment is primarily concerned with the relationship between:

• the impervious areas generating runoff;
• the treatment measures connected to those areas; and
• the resulting stormwater treatment performance.

Each roof, driveway, path or paved surface should be accounted for. The assessor then identifies whether runoff from that surface is directed to a treatment measure or discharged without treatment.

For example, roof water may be directed to a rainwater tank that supplies toilets or landscape irrigation. Driveway runoff might be directed through a raingarden, while a small paved area may remain untreated.

The assessment combines these different catchments and treatment responses into an overall result for the development.

What Is the Purpose of a STORM Assessment?

The purpose of a STORM assessment is to provide a clear, measurable response to stormwater treatment requirements.

Rather than simply stating that a project includes sustainable drainage features, the assessment tests whether the selected measures are appropriately sized and connected to the runoff-generating areas of the site.

This gives councils and project teams a practical way to review whether:

• all relevant impervious surfaces have been included;
• the nominated treatment measures are connected to suitable catchments;
• rainwater storage is supported by an appropriate reuse demand;
• untreated runoff has been identified;
• the proposed measures are large enough for the areas they treat; and
• the overall design achieves the required treatment outcome.

Why It Matters

Urban development changes how rainfall moves through a site. An increase in impervious area can increase the volume and frequency of runoff entering drains and waterways.

Stormwater may also carry pollutants from roofs, driveways, paved surfaces and landscaped areas. Without suitable treatment, these pollutants can affect the health of creeks, rivers, bays and other receiving environments.

A STORM assessment matters because it turns the proposed treatment strategy into a measurable project outcome. It can help the project team identify whether the design needs:

• a larger rainwater tank;
• a greater connected roof catchment;
• additional rainwater reuse;
• a raingarden or biofiltration area;
• more permeable surface area;
• better allocation of runoff between treatment measures; or
• changes to the proposed site layout.

Considering these matters early can reduce the risk of late redesign after architectural drawings, landscape plans and hydraulic services have already been developed.

How It Relates to Victorian Planning Requirements

Victorian planning provisions include stormwater management objectives for various forms of subdivision, buildings and works. Depending on the development type and applicable planning controls, an application may need to show how the proposed stormwater management system addresses runoff quality, retention, detention and discharge.

A STORM-style assessment may be used as supporting evidence where a council requires a quantitative stormwater treatment result for a relatively small or straightforward development.

Projects that may encounter stormwater treatment requirements include:

• townhouse developments;
• multi-dwelling residential projects;
• apartment developments;
• small commercial developments;
• industrial buildings and works;
• public-use developments;
• subdivisions; and
• some additions or alterations that materially change the developed area.

The exact requirement is not identical for every project. It can depend on the planning scheme, development type, project size, permit pathway, local policy and council expectations.

For that reason, a STORM assessment should not be described as automatically mandatory for every Victorian building project. The planning permit requirements and relevant council guidance should be checked for the particular site.

The Transition from STORM to BlueFactor

Melbourne Water’s original STORM Calculator has now been replaced by BlueFactor.

BlueFactor performs a similar planning role for small-scale developments by assessing stormwater runoff, pollutant generation and proposed treatment measures. It is intended for relatively small and straightforward residential or commercial developments rather than complex catchments or extensive treatment trains.

This means a new project described informally as requiring a “STORM assessment” may now be assessed through BlueFactor, depending on the council and the applicable submission requirements.

The distinction is important:

• STORM refers to the original Melbourne Water calculator and the established assessment terminology.
• BlueFactor is the replacement tool for suitable small-scale Victorian developments.
• MUSIC is a more detailed modelling platform used for more complex sites, catchments and treatment trains.

Existing reports and permits may still refer specifically to a STORM score. New applications should follow current council requirements rather than assuming that an older calculator output will always be accepted.

How It Relates to WSUD

STORM and Water Sensitive Urban Design are closely connected, but they are not the same thing.

WSUD is the broader design approach. It considers how water is managed across a site through planning, architecture, landscaping, reuse, infiltration and stormwater treatment.

A STORM assessment is a quantitative assessment. It tests the stormwater treatment performance of selected measures and catchments.

A project may use WSUD measures such as:

• rainwater harvesting and reuse;
• raingardens;
• biofiltration systems;
• permeable paving;
• infiltration areas;
• landscaped treatment zones; and
• other approved stormwater treatment measures.

The assessment then checks how those measures contribute to the required runoff treatment outcome.

Read the dedicated comparison in STORM vs WSUD: What Is the Difference?.

How It Relates to MUSIC

STORM and MUSIC can both be used to evaluate stormwater treatment, but they are suited to different levels of project complexity.

A STORM-style assessment is generally associated with smaller developments and relatively simple arrangements of catchments and treatment measures.

MUSIC can represent more complex catchments, drainage connections and treatment trains. It uses time-step simulation to model rainfall, runoff and the performance of connected stormwater treatment measures.

A larger or more complicated project may therefore require MUSIC rather than a basic STORM or BlueFactor assessment.

The relevant council or drainage authority should confirm which method is appropriate. See STORM vs MUSIC: Which Assessment Does Your Project Need? for a focused comparison.

What a STORM Assessment Does Not Replace

A STORM assessment should not be treated as a complete stormwater engineering package.

It does not ordinarily replace:

• a detailed drainage design;
• pit and pipe sizing;
• on-site detention calculations;
• legal point of discharge documentation;
• overland flow analysis;
• flood modelling;
• civil engineering drawings;
• hydraulic design; or
• a complete Stormwater Management Plan where one is required.

The STORM assessment owns a narrower technical territory: quantifying stormwater treatment performance.

A project may need both a treatment assessment and separate drainage or civil engineering documentation. One does not automatically satisfy the purpose of the other.

Practical Considerations for Victorian Projects

Account for Every Impervious Surface

Roof areas are only part of the assessment. Driveways, paths, patios, balconies and other sealed areas may also generate runoff and need to be included.

Confirm Which Surfaces Drain to Each Treatment

A treatment measure cannot simply be shown somewhere on the plan. The assessment needs a credible relationship between the runoff-generating catchment and the treatment receiving that runoff.

Coordinate Rainwater Tank Reuse

A tank generally performs more effectively when stored water is regularly reused. Tank volume, connected roof area and nominated end uses should therefore be coordinated rather than selected independently.

Allow Space for Treatment Measures

Raingardens, tanks and permeable systems require physical space and suitable connections. They should be considered before the site plan and landscape design become fixed.

Check Council-Specific Requirements

Councils may differ in the information, drawings, annotations and assessment outputs they expect. Some may also have specific preferences for treatment measures or maintenance documentation.

Keep the Report and Drawings Consistent

The tank capacity, connected catchment, reuse connections and treatment areas shown in the assessment should match the architectural, landscape and hydraulic documents.

What Information Is Usually Needed?

A preliminary STORM assessment can often begin using architectural plans, provided the site areas and proposed treatment strategy are sufficiently clear.

Common inputs include:

• project address;
• site plan;
• roof plan;
• total site area;
• roof catchment areas;
• driveway and paved areas;
• landscaped and permeable areas;
• proposed rainwater tank capacity;
• areas connected to the tank;
• proposed uses for captured rainwater;
• raingarden or biofiltration dimensions; and
• any council request or planning permit condition.

Read What Information Is Needed for a STORM Assessment? for a more detailed document checklist.

How Certified Energy Can Help

Certified Energy can review the available project drawings and identify the information needed to prepare a stormwater treatment assessment for a Victorian planning submission.

The assessment process may include:

• reviewing the proposed site and impervious areas;
• identifying the catchments connected to treatment measures;
• assessing rainwater tank capacity and reuse assumptions;
• testing the proposed treatment performance;
• identifying practical opportunities to improve the result;
• coordinating assessment inputs with the project drawings; and
• preparing clear documentation for the project team or council submission.

The correct tool and reporting scope will depend on the project, its complexity and the requirements of the relevant council.

Explore the STORM Assessment Knowledge Hub or send through the available plans for an initial project review.

STORM Assessment FAQs

What is a STORM assessment?

A STORM assessment is a quantitative assessment of how runoff from a proposed development will be treated. It considers impervious areas and treatment measures to determine whether the design achieves the required stormwater-quality outcome.

Is the STORM Calculator still used in Victoria?

Melbourne Water has replaced the original STORM Calculator with BlueFactor. The STORM name remains widely recognised, but new projects may need to use BlueFactor or another assessment method accepted by the relevant council.

What does a STORM assessment measure?

It measures the stormwater treatment performance associated with runoff from roofs, driveways, paths and other impervious surfaces, taking account of the treatment measures connected to those areas.

Does a STORM assessment assess flooding?

No. Its primary purpose is stormwater treatment and water-quality assessment. Flood behaviour, drainage capacity, detention and overland flow may require separate civil or hydraulic assessment.

Is a STORM assessment the same as WSUD?

No. WSUD is an integrated approach to water-sensitive planning and design. A STORM assessment quantifies the treatment performance of particular stormwater measures used within the design.

Is STORM the same as MUSIC?

No. MUSIC provides more detailed modelling of catchments, drainage connections and treatment trains. A STORM-style assessment is generally suited to smaller, simpler developments.

Can a rainwater tank improve a STORM result?

Yes. A tank can improve the result when it captures runoff from an appropriate roof area and is connected to regular reuse demands such as toilet flushing or landscape irrigation.

Does every Victorian project need a STORM assessment?

No. The requirement depends on the development, planning controls, permit pathway and council expectations. Some projects need a different assessment method or may not require a quantitative treatment assessment.

What plans are needed for a STORM assessment?

A site plan and roof plan are commonly needed, together with dimensions or area information for roofs, driveways, paving, landscaping and proposed treatment measures.

When should the assessment be completed?

It is best considered before the design is finalised. Early assessment leaves more opportunity to coordinate tank locations, catchment connections, landscaping and treatment areas without major redesign.

Suggested Internal Links

• STORM Assessments and Reports Knowledge Hub
• When Is a STORM Assessment Required in Victoria?
• What Does a STORM Report Include?
• How Is a STORM Score Calculated?
• What Does a 100% STORM Rating Mean?
• STORM vs WSUD: What Is the Difference?
• STORM vs MUSIC: Which Assessment Does Your Project Need?
• Can Rainwater Tanks Improve a STORM Rating?
• Common Reasons a Project Fails a STORM Assessment
• What Information Is Needed for a STORM Assessment?