Phase I Municipal Stormwater Permit
We develop and administer Clean Water Act National Pollutant Discharge Elimination System (NPDES) municipal stormwater permits in Washington. The Phase I Municipal Stormwater Permit requires local governments to manage and control stormwater runoff so that it does not pollute downstream waters. This permit regulates stormwater discharges in the most highly populated areas in the state. The current permit is effective Aug. 1, 2024 and expires on July 31, 2029.
The Phase I Municipal Stormwater Permit regulates systems owned or operated by:
- Clark, King, Pierce, and Snohomish counties.
- Seattle and Tacoma.
- Incorporated cities with a population of over 100,000* people.
- Unincorporated counties with populations of more than 250,000* people.
*Populations are based on the 1990 U.S. census.
The permit also applies to MS4s owned by public entities located in a Phase I city or county including the ports of Seattle and Tacoma.
Overview of permit timelines
Annual reports for the calendar year are due by March 31 the following year.
Municipal stormwater permit guidance
For guidance about applying, changing, or implementing and complying with your municipal stormwater permit, see municipal stormwater guidance. Stormwater manuals provide stormwater permit implementation and management guidance. For more general stormwater permit guidance, see the stormwater guidance page.
Frequently asked questions
Have a question? Check out our frequently asked questions page.
Permit history
To access previous permits and learn about past permit cycle processes, such as appeals, public comments, and draft reviews, see municipal stormwater history.
Keep in touch with us
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