Phase I Municipal Stormwater Permit
Open comment periods
MS4 Phase I permit modification
Comment period: Aug. 5 through Nov. 2, 2026
Online public hearing: Sept. 9, 2026, 2 p.m.
MS4 Phase I and II permit: Early input for 2029 reissuance
Early input period: July 24, 2026, through Feb. 19, 2027
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 has been in effect since Aug. 1, 2024, and expires July 31, 2029.
The Phase I Municipal Stormwater Permit regulates municipal separate storm sewer systems (MS4s) 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 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.
Proposed modification
We're proposing to add the Phase I municipalities' local program to the official list of manuals and ordinances (Appendix 10, Part 3) each municipality uses to comply with permit requirements. We have reviewed the programs and found that they are equivalent to the 2024 Stormwater Management Manual for Western Washington.
The draft modification is available for review and public comment. By request, Ecology is extending the comment period for the draft modification. The comment period will now close at 11:59 p.m. on Nov. 2, 2026, instead of Sept. 18, 2026. After the public comment period closes, we will publish a response to comments as an appendix to the fact sheet to address the public comments submitted.
Documents for the draft modification
- Draft modified Appendix 10 - Equivalent Programs for Runoff Controls for New Development, Redevelopment, and Construction Sites (see Part 3 of appendix)
- Appendix B: Fact Sheet for the draft 2026 phase I modification
- Presentation from the public meeting and hearing on the draft modification (September 2026)
- Washington State Register public notice of draft release (WSR 26-15-127, Aug. 5, 2026)
Public involvement
We will accept comments on the draft permit from 9 a.m. Aug. 5, 2026, through 11:59 p.m. Nov. 2, 2026. Comments should reference specific permit text when possible.
Ways to comment
- Submit comments online
- By U.S. mail
Abbey Stockwell
WA Department of Ecology
PO Box 47696
Olympia, WA 98504-7696 - At our public hearing
Public meeting and hearing
During the comment period we will host an online public meeting and hearing where we will explain the proposed changes to the permit and answer questions prior to the formal hearing. The hearing provides opportunity for you to give formal spoken testimony on the proposed draft permit. The public hearing begins immediately following the meeting and concludes when public testimony is complete. Spoken testimony will receive the same consideration as written comments.
Online Public Meeting and Hearing
Sept. 9, 2026 – 2 p.m.
Register in advance for this meeting
After registering, you will receive a confirmation email containing information about joining the meeting.
Next steps
We will review and consider all comments received. We will make appropriate revisions based on public comments, publish an official response to comments as an appendix to the fact sheet and make a final decision on permit issuance in December 2026.
Anticipated timeline
- August 8 – November 2: Formal public comment period on the draft permit modification
- September 9: Public meeting and hearing
- December 2026: Decision on reissuance
Annual reporting
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 permit versions and learn about past permit cycle processes, such as appeals, public comments, and draft reviews, see municipal stormwater permit history.