Mirrormont Environmental Rules: Erosion Control Rules (2026)

King County regulates land clearing, grading, and erosion control to protect water quality, prevent landslides, and preserve salmon habitat. Under KCC 16.82 (Clearing and Grading), any land-disturbing activity clearing more than 7,000 square feet of native vegetation or moving more than 50 cubic yards of earth requires a clearing and grading permit. Smaller activities may still be subject to erosion control requirements. Required Best Management Practices (BMPs) specified in the King County Surface Water Design Manual Volume II include silt fences, fiber rolls (wattles), construction entrance rock pads to prevent tracking, straw or hydromulch for exposed soils, temporary and permanent seeding, and inlet protection. A Stormwater Pollution Prevention Plan (SWPPP) is required for construction sites over 1 acre under the Washington Department of Ecology Construction Stormwater General Permit. BMPs must be installed before clearing begins and maintained throughout construction. The county's wet season (October 1 through April 30) imposes additional restrictions — disturbed areas over certain sizes must be stabilized within 2 to 7 days depending on forecasts. Steep slope hazard areas (over 40 percent slope) and landslide hazard areas under KCC 21A.24 trigger geotechnical review and stricter controls. Critical areas including wetlands, streams, and their buffers have minimal allowed disturbance. Violations that cause sediment discharge to salmon-bearing streams are treated seriously — King County and Ecology coordinate enforcement including stop-work orders and restoration requirements.