Mirrormont Fence Regulations: Retaining Walls Rules (2026)

Under KCC Title 16 and the adopted 2021 International Residential Code, a building permit is required for any retaining wall exceeding 4 feet in height measured from the bottom of the footing to the top of the wall, or for any wall supporting a surcharge such as a driveway, structure, or swimming pool. In King County, retaining walls are especially scrutinized because much of the unincorporated area lies in landslide hazard areas, steep slope hazard areas (over 40 percent grade), or erosion hazard areas mapped under KCC Title 21A.24 (Critical Areas). Any retaining wall proposed in a landslide or steep-slope hazard area typically requires a geotechnical report prepared by a licensed engineer regardless of wall height. Walls within 200 feet of salmon-bearing streams are reviewed under the Critical Areas Ordinance and the Shoreline Master Program, and must avoid disturbing riparian buffers. Engineered drainage (perforated pipe, free-draining backfill) is required to prevent hydrostatic pressure and slope failure. Stacked segmental block walls that appear as tiered terraces may be aggregated when the horizontal offset is less than twice the height of the lower wall.