New Policy: You must submit all drainage and sanitary legal documents for review when you apply for a building or site work permit.
The 2016 Stormwater Code and Manual became effective on January 1, 2016. It replaced the 2009 Stormwater Code and Manual.
Our stormwater regulations protect people, property, and the environment from damage caused by stormwater runoff. Our stormwater codes also satisfy the City’s obligation to comply with our Municipal Stormwater Discharge National Pollutant Discharge Elimination System (NPDES) Permit, issued by the Washington State Department of Ecology.
Our Stormwater Code addresses:
You need to get a drainage review for your construction or grading permit if:
The 2016 Stormwater code is Title 22, Subtitle VIII of the Seattle Municipal Code (SMC). You can find the stormwater code on the Seattle Municipal Code website.
The City of Seattle Stormwater Manual consists of five volumes and a set of appendices.
Below is the previous version of the Stormwater Manual, broken into sections.
Projects with more than 750 square feet of land disturbing activity must submit the following documents.
Note: If your project has 5,000 square feet, or greater, of new plus replaced hard surface area, your application must be prepared by a professional engineer. See the “For Professional Engineers” later on this webpage.
On-site Stormwater Management Documentation
In the 2016 Stormwater Code and Manual, “On-site Stormwater Management” (OSM) requirements replace the previous Green Stormwater Infrastructure (GSI) requirements. On-site Stormwater Management feasibility, Best Management Practice selection, and Best Management Practice sizing must be documented in the On-site Stormwater Management – List Approach Calculator.
Infiltration Investigation Requirements
If your project requires On-site Stormwater Management or must meet Flow Control or Water Quality Standards, you must first investigate the feasibility of infiltration facilities (such as a dry well) to meet these standards. The exception to this is if your project is located in area that is mapped as "Infiltration Investigation Not Required." The “Infiltration Investigation Not Required” map is available to you as a Base Map Layer on our GIS website. If you are required to do infiltration testing, you must submit the infiltration checklists with your plans. If infiltration is not required, you must indicate the reason on the Site and Drainage Control Summary.
Nutrient Critical Receiving Waters
At the time this Manual was developed, there were no nutrient-critical receiving water segments determined to be impaired due to phosphorus contributed by stormwater. In the future, the City may designate a waterbody as a nutrient-critical receiving water as defined by the SMC, Section 22.801.150.
Here's a list of documents to help you understand our stormwater code:
For larger projects, you may need to submit additional documents.
Note: The Seattle 158-year Precip/Evap series is included as an option in the 2012 Western Washington Hydrology Model (WWHM 2012) and MGS Flood continuous runoff models.
Note: We do not provide technical support for the approved continuous rainfall hydrologic models or in the application of the model to specific projects. Consultants who would like training in the use of the approved models may contact these companies directly:
A Department of Ecology Construction Stormwater General Permit is required for projects disturbing one or more acres of land.