King County, WA Environmental Rules: Stormwater Management (2026)

King County operates under a federal NPDES Municipal Separate Storm Sewer System (MS4) permit (Phase I) issued by the Washington Department of Ecology under the Clean Water Act. This permit requires the county to prevent pollutants from entering Puget Sound, Lake Washington, salmon streams, and other waters through its storm drain system. Implementation is through KCC 9.04 (Surface Water Runoff Policy) and the King County Surface Water Design Manual (most recently updated 2021). Any project that creates or replaces 2,000 square feet or more of impervious surface, or clears 7,000 square feet or more of native vegetation, triggers stormwater review and design requirements. Small Parcel Best Management Practices apply to smaller projects. Flow control (detention) is required to prevent downstream flooding and erosion — new development must match predeveloped (forested) flow durations. Water quality treatment is required for pollution-generating surfaces — parking lots, high-use driveways, and commercial surfaces must treat runoff through biofiltration, wet ponds, or approved proprietary devices. Illicit discharges to the storm system are prohibited under KCC 9.12 — discharges of anything other than clean stormwater including soap, oil, sediment, paint, pet waste, and chemicals violate federal and state law. Construction sites must implement erosion and sediment controls. Agricultural operations have Best Management Practices under the King Conservation District. Critical aquifer recharge areas (KCC 21A.24) impose additional treatment requirements.