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, 2019, was modified Oct. 20, 2021, and expires on July 31, 2024.

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.

Permit modification

Phase I municipalities are required to adopt a stormwater program that provides equal or greater protection of receiving waters and pollutant control as compared to the 2019 Stormwater Management Manual of Western Washington.

We held a public comment period to gather input on the decision that the stormwater programs submitted by the Phase I permittees are equivalent. The permit modification process included two workshops and hearings with a formal public comment period on the draft permit modification held from July 21, 2021 - Aug. 27, 2021. All comments received are available on our online comment form

The modification incorporates Ecology's determination of equivalency for the stormwater programs listed in Appendix 10 of the permit. 

Current permit documents

Overview of permit timelines

Duty to Reapply for Permit in 2024 must be completed by Feb. 1, 2024.

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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