A project commonly falls below the required STORM rating when too much runoff remains untreated or the proposed treatment measures are not properly sized, connected or supported by realistic assumptions.
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:
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.
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needs further work, but it can also occur because the assessment has been prepared using incorrect, incomplete or outdated information.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.
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:
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.
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:
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.
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:
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}
Tank capacity, connected roof area and reuse demand need to work together.
A poor match can occur where:
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.
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:
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:
The proposed treatment must remain compatible with site levels, vehicle access, drainage and maintenance requirements.
A raingarden needs sufficient treatment area for the catchment draining to it.
A low result may occur where:
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.
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:
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.
Permeable paving can improve a STORM result, but only where a genuine permeable pavement system is proposed.
Problems may arise where:
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.
An apparently strong result may fail during review if relevant impervious surfaces have not been included.
Frequently overlooked areas include:
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.
The sum of the modelled roof, paving, landscape and other surface areas should reconcile with the assessment boundary.
Common area errors include:
An unexplained difference between the site area and surface schedule can undermine confidence in the result and may lead to further questions from council.
A STORM report can become outdated when the architectural or landscape design changes.
Changes that may affect the result include:
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.
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:
The report, architectural drawings and hydraulic documentation should describe the same treatment arrangement.
A modelled solution may reach the numerical target while remaining unsuitable for the physical site.
Examples include:
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.
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:
A coordinated combination of measures can produce a more practical and resilient result than relying on one element to compensate for every untreated surface.
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:
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.
A 100% result does not automatically mean that the full submission will be accepted.
Council may still ask whether:
The rating demonstrates modelled stormwater treatment performance. It does not replace council’s review of the broader design and submission.
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:
This helps distinguish between a genuine design shortfall and an input or coordination problem.
Depending on the project, possible improvements may include:
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.
A low result does not necessarily mean:
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.
A successful or failed STORM result relates primarily to stormwater treatment performance.
It does not ordinarily determine:
A project may achieve its stormwater treatment target while still requiring separate drainage, detention or flood-related work.
Confirm that the assessment references the current architectural, landscape and hydraulic documents.
Check that every relevant roof, driveway, path, patio and landscaped area is included once and classified correctly.
Treating a large driveway or roof catchment may provide more benefit than making small adjustments to minor surfaces.
Tank capacity, roof connection and reuse demand should be reviewed together rather than independently.
Confirm that runoff can reach the proposed treatment measures based on roof drainage, site levels and the developing hydraulic design.
Do not rely on raingardens or tanks that conflict with access, services, landscaping or required open space.
For new suitable small developments, council may expect BlueFactor or another accepted output rather than a new certificate from the former STORM Calculator.
Check whether council also requires detention calculations, discharge information or civil engineering documentation.
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:
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.
It commonly fails because too much runoff remains untreated or the proposed measures are not properly sized, connected or supported by realistic operating assumptions.
Not necessarily. It usually means that the assessment inputs or treatment strategy require further review before the target can be demonstrated.
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.
The connected roof area or reuse demand may be too small, or untreated driveways and paving may be limiting the overall result.
Yes. A shared driveway can form a major untreated impervious catchment and may significantly affect the project result.
Yes. Its contribution depends on its treatment area and the size of the catchment draining to it.
Yes. Omitted impervious areas can make the site schedule and treatment result inaccurate.
Only where a genuine permeable pavement system is proposed and the modelled area matches the drawings and specification.
Yes. Council may review whether the inputs are accurate, the measures are practical and the report remains consistent with the submitted plans.
Possible responses include correcting the inputs, treating more runoff, increasing credible tank reuse, resizing treatment measures, reducing impervious area or combining several treatment approaches.