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:
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.
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.
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.
Stormwater Code: You can find the stormwater code on the City Clerk's website. We've also listed it here for 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:
See our Stormwater Code & Rules Update page for proposed changes.