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Stormwater Code

What Is It?

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
  • Green stormwater infrastructure (GSI) 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)

Forms and Submittal Documents:

Projects with more than 750 square feet of land disturbing activity require a Construction Stormwater Control and Soil Amendment Standard Plan.

Most new construction projects and larger additions or alterations also require a Drainage Control Plan for Small Projects and the Green Stormwater Requirement Calculator. For example, you would need these documents if your project adds or replaces hard surface that covers 1,500 square feet for single-family residential projects or 2,000 square feet for other types of projects.

In certain areas of Seattle, there are no public drainage systems. If you are building a “small project” (less than 5,000 sf of new plus replaced hard surface) in one of these areas, you must infiltrate all of your stormwater into the ground. In order to determine the size of the infiltration facility (dry well), please see:

For Professional Engineers:

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

  • Projects that are at least 5,000 square feet of new and replaced impervious surface require a Technical Information Report and drainage and construction stormwater control plans stamped by a professional engineer.
  • Projects less than 10,000 square feet of new and replaced hard surface with flow control requirements require 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 6.21 in the Seattle Stormwater Manual, Volume 3, Section 6.5)
  • 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 6.21 in the Seattle Stormwater Manual, Volume 3, Section 6.5)

When you submit continuous modeling for review, a summary of the output using SPU Hydrostats must be provided.

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.

Read the Code

Stormwater Code: You can find the stormwater code on the Seattle Municipal Code website. We've also listed it here your convenience:

Stormwater Manual: The Stormwater Manual consists of the four Director's Rules listed below:

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

Proposed Changes

The proposed 2016 Stormwater Code and Manual update is scheduled to be effective January 2016. See our Stormwater Code & Rules Update page for proposed changes.

Recent Changes

The 2009 Stormwater Code and Manual was recently revised and Seattle Public Utilities created Director's Rule DWW-430.1, Flow Control Requirements for Projects in Identified Public Combined Sewer Basins. The code revision and Director’s Rule became effective on May 28, 2015.

What Do You Want To Do?

Still Need Help?

 Ask Us

  • Call us at (206) 684-5362
  • Email the Drainage and Sewer Review Desk
  • Visit the Applicant Service Center
    700 5th Avenue, 20th floor
    M, W, F: 8:00 a.m. - 4:00 p.m.
    T, Th: 10:30 a.m. - 4:00 p.m.
    (arrive before 2:00 p.m. for walk-in service)