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needs further work, but it can also occur because the assessment has been prepared using incorrect, incomplete or outdated information.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?