If you’re planning a development project in Toronto — whether it’s a new commercial building, a residential subdivision, or a mixed-use site — chances are the City or the Conservation Authority will ask for a Stormwater Management Report before approving your plans. But what exactly is this report, why is it required, and when do you need one?
In this guide, we break it all down— so you can walk into your next pre-application meeting fully prepared.
What Is a Stormwater Management Report?
A Stormwater Management Report (often called an SWM Report) is a technical engineering document that explains how rainwater and snowmelt will be collected, controlled, and discharged on your property after development.
When it rains on a natural, undeveloped site, water soaks into the ground, flows slowly through vegetation, and reaches nearby streams gradually. But when a site is developed — covered with buildings, roads, and parking lots — that natural process is disrupted. Impervious surfaces like asphalt and concrete prevent water from absorbing into the ground, causing faster and larger volumes of runoff. Left unmanaged, this leads to:
- Basement flooding in downstream neighbourhoods
- Erosion of local streams and creeks
- Pollution of rivers and Lake Ontario
- Overloading of Toronto’s aging sewer infrastructure
The Stormwater Management Report is how you demonstrate to the City and the Conservation Authority that your project will not make these problems worse — and ideally, will improve conditions compared to the pre-development state.
What Does the Report Include?
A complete Stormwater Management Report prepared for a project typically covers the following components:
| Component | What It Does |
| Pre-development analysis | Establishes how water currently drains on the site before any construction |
| Post-development modeling | Predicts runoff volumes, flow rates, and water quality after development |
| Drainage design drawings | Shows storm sewers, catch basins, grading, and lot drainage plans |
| Water quality treatment | Outlines measures like oil-grit separators or bioswales to filter pollutants |
| Erosion & sediment control | Plans to protect nearby streams during and after construction |
| Flood plain analysis | Required for sites near TRCA-regulated watercourses and floodplains |
When Do You Need One in Toronto?
The short answer: for almost any significant development or construction project. The City and the conservation authority require a Stormwater Management Report as part of the Site Plan Approval process in most cases. Here are the most common situations that trigger the requirement:
1. New Commercial or Industrial Development
Any new commercial, retail, office, or industrial building on a previously undeveloped or redeveloped site will require an SWM Report. This applies whether you’re building a gas station plaza, a self-storage facility, or a convention centre .
2. Residential Subdivisions and Multi-Unit Housing
With Ontario’s zoning bylaw updates now permitting duplexes, triplexes, and fourplexes on many residential lots, more infill projects are triggering the SWM requirement than ever before.
3. Mixed-Use Development and Official Plan Amendments (OPA)
Mixed-use projects going through Official Plan Amendment (OPA) or Zoning By-law Amendment (ZBA) applications almost always require an SWM Report as part of the application package. The report must be submitted alongside the Traffic Impact Study, Site Servicing Plan, and Planning Justification Report.
4. Sites Near TRCA Regulated Areas
If your site is near a river, creek, valley, or flood plain regulated by the TRCA, you will need both a City of Toronto SWM Report and a TRCA permit. The TRCA has jurisdiction over most of Toronto’s ravine system and any development within or near these areas faces additional stormwater requirements.
5. Major Renovations and Site Expansions
Adding a significant building addition, expanding a parking lot, or altering the grading of a site may also trigger the requirement for an updated SWM Report — even if the original site already has one on file. Always check with the City or your engineer before proceeding.
Common Mistakes Developers Make
Leaving it too late
The SWM Report should be started at the concept design stage, not after Site Plan is submitted. Early integration avoids costly redesigns.
Underestimating TRCA involvement
If your site touches a TRCA-regulated area even partially, you need to budget significant additional time for their review process.
Ignoring water quality requirements
The Toronto Green Standard Tier 1 requires enhanced water quality treatment for most new developments. This must be designed in from the start — retrofitting it later is expensive.
Not coordinating with the site servicing plan
Your SWM Report must be consistent with your Site Servicing Plan. Inconsistencies between the two documents are the most common cause of revision requests from City reviewers.
How n Engineering Prepares Your SWM Report
At n Engineering Inc., we have prepared Stormwater Management Reports for a wide range of project types across Toronto, Brampton, Mississauga, and the broader Ontario region — from gas station plazas and self-storage facilities to convention centres and mixed-use developments.
Our process:
- Site analysis — We review topography, soil conditions, existing drainage patterns, and any TRCA-regulated features on or near the site.
- Hydrologic and hydraulic modeling — We build pre- and post-development models to calculate runoff volumes and flow rates at key design storm events.
- Stormwater system design — We design the detention ponds, storm sewers, infiltration systems, and quality treatment measures required for your site.
- Coordination with Site Servicing — We ensure your SWM Report and Site Servicing Plan are fully consistent from day one, eliminating the most common source of revision requests.
- Agency Submission support — We prepare the final report package and liaise directly with City of Toronto and TRCA reviewers throughout the approval process.
Planning a development in Toronto or Ontario?
n Engineering Inc. provides expert Stormwater Management Reports, Site Servicing Plans, Traffic Impact Studies, and the full range of civil engineering services required for development approvals across Toronto and Ontario. Contact us today to discuss your project.
📞 416.256.9741 ✉️ info@nengineering.com 🌐 www.nengineering.com