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

Mason County's Board of Commissioners created the county's storm and surface water utility under Chapter 14.46, relying on RCW 36.32.120 and RCW 36.89.030, and Chapter 14.48 lays out the substantive stormwater management rules that apply only in unincorporated Mason County under § 14.48.040; the City of Shelton, the Skokomish and Squaxin Island reservations, Olympic National Park and Olympic National Forest sit outside the utility's boundary under § 14.46.010. The county adopts the Department of Ecology's Stormwater Management Manual for Western Washington by reference in § 14.48.050, and the public works director administers the chapter under §§ 14.48.030 and 14.48.180.

New single-family homes and duplexes that add two thousand square feet or more of impervious surface must file a small parcel erosion and sediment control plan and a residential site improvement plan under § 14.48.150(1)(C); commercial and multifamily projects crossing five thousand square feet of new impervious surface, or disturbing an acre or more, need a large parcel plan and a permanent stormwater quality control plan stamped by a licensed civil engineer under § 14.48.150(1)(A). Illicit discharges to the drainage system, from paint and solvents to pet waste and sediment, are banned outright under § 14.48.130, and anyone aware of a spill must notify the county's spills hotline within twenty-four hours.

The director can issue a stop work order under § 14.48.210 and a civil penalty of fifty to one hundred dollars per site per day under § 14.48.220, appealable to the Board of County Commissioners within ten business days.