A 100% STORM rating generally means that the modelled stormwater treatment strategy reaches the applicable best-practice water-quality benchmark for the development.
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
