King County, WA Environmental Rules: Stormwater Management (2026)
Key Facts
- Review trigger
- 2,000 sq ft new/replaced impervious surface
- Land disturbance trigger
- 7,000 sq ft or more
- Pipe/ditch trigger
- 12 inches or larger
- Administering agency
- DLS Permitting Division
- Applies only
- Unincorporated King County
Summary
Unincorporated King County requires drainage review and stormwater control facilities once a project hits set impervious-surface or land-disturbance thresholds.
Drainage review is required when any proposed project is subject to a King County development permit or approval and:
1. Would result in two thousand square feet or more of new impervious surface, replaced impervious surface or new plus replaced impervious surface;
2. Would involve seven thousand square feet or more of land disturbing activity;
3. Would construct or modify a drainage pipe or ditch that is twelve inches or more in size or depth or receives stormwater runoff or surface water from a drainage pipe or ditch that is twelve inches or more in size or depth;
4. Contains or is adjacent to a flood hazard area as defined in K.C.C. chapter 21A.24;
5. Is located within a critical drainage area; or
6. Is a redevelopment project proposing one hundred thousand dollars or more of improvements to an existing high-use site.
B. The drainage review for any proposed project shall be scaled to the scope of the project's size, type of development and potential for stormwater impacts to surface water and groundwater to facilitate the preparation and review of project applications. If drainage review for a proposed project is required under subsection A. of this section, the department performing drainage review as specified in K.C.C. 9.04.070 shall determine which of the following drainage reviews apply as specified in the Surface Water Design Manual:
1. Simplified drainage review;
2. Targeted drainage review;
3. Directed drainage review
Full Breakdown
King County Code Chapter 9.04 triggers mandatory drainage review for any permitted project adding 2,000+ square feet of new or replaced impervious surface, disturbing 7,000+ square feet of land, or installing a drainage pipe or ditch 12 inches or larger. Reviewed projects must meet nine core requirements from the Surface Water Design Manual, including flow control and water-quality treatment facilities, construction erosion controls, and maintenance obligations. The department of local services, permitting division, administers review; Chapter 9.08 funds the countywide surface water management program. This applies only in unincorporated areas -- Seattle, Bellevue, Kent, Renton, Federal Way, Auburn, Redmond, Kirkland, Sammamish, Burien, Shoreline, Issaquah and other incorporated cities run their own stormwater codes.
Violations & Fines
The director enforces under K.C.C. Title 23's enforcement and penalty provisions; noncompliant drainage facilities can also be denied county maintenance acceptance under K.C.C. 9.04.120.
Frequently Asked Questions
Does this cover Seattle?
What triggers drainage review?
Sources & Official References
Other rules in King County
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