Stormwater Code

See also: Grading Code, Grading PermitSide Sewer CodeSide Sewer Permit

What Is It?

The Stormwater Code and Manual are in the process of being updated and are scheduled to be effective July of 2021. See our Updating Stormwater Regulations page for more information.

The 2016 Stormwater Code and Manual became effective on January 1, 2016. 

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:

  • Drainage control submittal and review requirements
  • Where the stormwater from your site needs to go
  • On-Site Stormwater Management (previously referred to as Green stormwater infrastructure) and why it's important
  • Erosion control requirements for your project during construction and grading activities
  • Flow control and stormwater treatment requirements
  • How we enforce the Stormwater Code

Do I Need Drainage Review?

You need to get a drainage review for your construction or grading permit if:

  • You are disturbing more than 750 square feet of land
  • You are adding or replacing more than 750 square feet of hard surface, such as pavement
  • You are adding or replacing more than 750 square feet of a building (as measured by the roof outline)
  • A Grading Permit is required for your project 
  • Your project is within a Category I peat settlement-prone area per SMC 25.09.020 

Read the Code

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.  

Read the Manual

The City of Seattle Stormwater Manual consists of five volumes and a set of appendices.

Below is the Stormwater Manual, broken into sections.

Forms and Documents

Required Documents

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.

Reference Materials

Here's a list of documents to help you understand our stormwater code:

For Professional Engineers

For larger projects, you may need to submit additional documents.

  • Projects less than 10,000 square feet of new and replaced hard surface with flow control requirements may use the Pre-Sized Flow Control Calculator
  • Projects more than 10,000 square feet of new and replaced impervious surface requiring flow control must use the Seattle 158-year Precip/Evap series

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.

  • WWHM 2012 Time Series Selection Instructions
    • On the “Mapping Information Screen”, select the “Seattle” map
    • Next, select the “SPU 158 Year 5min” time series option
    • In the “View” pull down menu in the task bar, select “Options”, and go to the “Timestep” tab
    • Then select the appropriate time step for the type of facility that is being designed (see Table F.12, Appendix F, Section F-4 of the 2016 Seattle Stormwater Manual)
  • MGS Flood Time Series Selection Instructions
    • On the “Project Information” screen, in the “Select Climate Region” pull down box, select “32. Seattle 38 MAP”
    • Next, go to the “Simulate” screen
    • In the “Computational Timestep” pull down bar, select the appropriate time step for the type of facility that is being designed (see Table F.12, Appendix F, Section F-4 of the 2016 Seattle Stormwater Manual)

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:

Do You Need a  Construction or Industrial Stormwater General Permit?

A Department of Ecology Construction Stormwater General Permit is required for projects disturbing one or more acres of land, or for any sized project that has the potential to be a significant contributor of pollutants (e.g. contaminated sites).  

See the Department of Ecology Industrial Stormwater General Permit webpage to determine if an Industrial Stormwater General Permit is required for your site.