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.
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.
- Errata for 2019-2024 Permits
- Fact Sheet
- Appendix B: Response to Comments
- Appendix C: Fact Sheet for 2021 Permit Modification
- Appendix D: Response to Comments for 2021 Permit Modification
- State Register Notice
- Stormwater Management Action Plan (SMAP) Guidance
- Quality Assurance Project Plan (QAPP) Templates
- Stormwater Discharge Monitoring SOPs
- IDDE schema (.xsd file) - Use this file to help export data from your tracking database to the MS4 annual report
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.