A Stormwater Management Report (SWM) shows how a development will control the quantity, quality, and rate of rainwater runoff leaving the site so it does not flood or pollute downstream systems. It is required for nearly all site plan, subdivision, and rezoning applications, and it is reviewed by both the municipality and, where a regulated feature is nearby, the conservation authority. It is prepared by a civil or water resources engineer and increasingly must incorporate Low Impact Development (LID) measures.
Stormwater is the study that quietly reshapes a site plan. The controls it requires, ponds, tanks, permeable surfaces, bioswales, take up land and dictate grading. A developer who fixes the building footprint before the SWM is scoped often finds the required storage does not fit, forcing a redesign that a earlier stormwater review would have avoided.
A Stormwater Management Report is an engineering study that demonstrates how a site will manage rainwater so that post-development runoff does not exceed pre-development rates and downstream water quality is protected. It models storm events, sizes the storage and treatment needed, and specifies the controls that deliver it.
Ontario practice increasingly requires Low Impact Development, meaning source controls such as permeable pavement, bioretention, and infiltration rather than a single end-of-pipe pond. Conservation authorities apply water balance and erosion targets that push design toward these measures, especially near sensitive features.
A Stormwater Management Report is required for developments that increase impervious area or alter drainage, including:
Not sure whether your site triggers a Stormwater Management Report? Enter your Ontario address and PreBuildIQ lists every study your specific property needs in 60 seconds.
Check my site →A Stormwater Management Report is prepared by a civil or water resources engineer licensed as a Professional Engineer (P.Eng.) in Ontario. It is usually authored by the same firm preparing the Functional Servicing Report and delivered as a combined report, since servicing and drainage are designed together.
A complete Stormwater Management Report typically contains:
| Factor | Typical range | What drives it |
|---|---|---|
| Consultant fee | $6,000 to $30,000+ | Site imperviousness, LID requirements, and conservation authority targets |
| Preparation time | 4 to 8 weeks | CA water-balance targets can require additional modelling |
| Municipal review | Municipal and CA review, weeks to months | Circulation to commenting agencies and revision cycles |
The land taken by stormwater storage and LID features is effectively lost developable area. On a tight urban site this can be the difference between one storey of parking and two, so factor storage into the yield calculation early.
Municipalities follow the provincial Stormwater Management Planning and Design Manual but layer their own criteria on top, and conservation authorities impose water balance, erosion, and quality targets that often exceed the municipal minimum. A site near a TRCA, CVC, or Conservation Halton regulated feature is designed to the CA standard, which is why the same building can need very different stormwater controls depending on which watershed it sits in.
A Stormwater Management Report (SWM) is an engineering study showing how a development controls the quantity, quality, and rate of rainwater runoff so it does not flood or pollute downstream systems. It models storm events, sizes the required storage and treatment, and specifies controls such as ponds, tanks, and Low Impact Development measures. It is prepared by a civil or water resources engineer.
A Stormwater Management Report is required for almost any development that increases impervious area or alters drainage: site plan applications, subdivisions, condominiums, and rezonings. Sites within or draining to a conservation authority regulated area will always trigger one, often to a stricter standard than the municipal minimum.
A Stormwater Management Report typically costs between $6,000 and $30,000 or more, driven by how impervious the site is, whether Low Impact Development measures are required, and the conservation authority’s water balance and erosion targets. It is usually combined with the Functional Servicing Report.
Low Impact Development (LID) refers to source controls that manage rain where it falls, such as permeable pavement, bioretention cells, and infiltration trenches, rather than a single end-of-pipe pond. Ontario conservation authorities increasingly require LID to meet water balance and quality targets, especially near sensitive natural features.
A Stormwater Management Report is reviewed by the municipality’s engineering or development-engineering staff and, where the site is within or drains to a regulated area, by the conservation authority. The two reviewers can apply different criteria, so the report often has to satisfy the stricter standard.
PreBuildIQ identifies every required study, including the Stormwater Management Report, for any Ontario address in 60 seconds, with the trigger and terms of reference for your specific municipality.
Run a site report →