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, 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 were 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 decided that the stormwater programs submitted by the Phase I permittees are equivalent and are seeking comments on our decision and the stormwater programs. The permit modification process includes two workshops and hearings with a formal public comment period on the draft permit modification. We plan to make a permit modification decision at the end of this process in Fall of 2021. The current permit expires on July 31, 2024.

Public comment period

We are pleased to announce that the draft modification of Appendix 10 of the Phase I Municipal Stormwater Permit is available for public comment. We invite comments on the draft permit documents beginning July 21, 2021 until 11:59 p.m. on Aug. 27, 2021. After the close of the comment period, we will publish a Response to Comments as an appendix to the fact sheet to address comments submitted during the public comment period.

For documentation of each change submitted and approved by Ecology, contact Emma Trewhitt,

How to provide comments

We will accept comments from July 21, 2021, until 11:59 p.m. on Aug. 27, 2021.

You may submit comments

Online: Submit your comments through our online comments form. (preferred method)

By mail:  
Emma Trewhitt
Washington State Department of Ecology
PO Box 47696
Olympia, WA  98504-7696

In person: Comments are accepted during any of our public hearings.

Workshop and hearings

We will host online workshops, immediately followed by a public hearing on the following dates:

Morning: Tuesday, Aug. 24, 2021 at 9 a.m. –  Webinar
Click here to register for the webinar

Evening: Tuesday, Aug. 24, 2021 at 5 p.m. –  Webinar
Click here to register for the webinar

During the workshop, we will explain the proposed changes to the permit and answer questions. The hearing will begin after the public workshop and will end when public testimony is complete. During the hearing, you may give oral testimony on the proposed changes. Written comments will receive the same consideration as oral testimony.

Current permit documents

Supporting documents

Overview of permit timelines

Permit timelines for new, continuing, and secondary permittees will be available in the coming weeks. These timelines will provide an overview of major deadlines for implementing permit requirements.

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

Join our municipal stormwater permit listserv to receive updates about this permit.