King County, WA Environmental Rules: Flood Zones (2026)

King County participates in the National Flood Insurance Program (NFIP) and regulates floodplain development under KCC 21A.24 (Critical Areas) Part 6 — Flood Hazard Areas. Flood Insurance Rate Maps (FIRMs) issued by FEMA identify Special Flood Hazard Areas (SFHAs) — Zones A, AE, AH, AO, VE, and V representing areas with 1 percent annual chance of flooding (100-year floodplain). Major flood-prone rivers in King County include the Snoqualmie River (particularly Carnation, Duvall, and Fall City), Green River (Tukwila, Kent, Auburn valley floor), Cedar River (Renton), Sammamish River, White River, and Skykomish tributaries. Puget Sound coastal areas have V zones subject to wave action. Development in SFHAs requires a floodplain development permit under KCC 21A.24. New structures must be elevated with the lowest floor at or above the Base Flood Elevation (BFE) plus 1 foot freeboard for King County. Fully enclosed areas below BFE must have flood vents (minimum 1 square inch per square foot of enclosed area). Critical facilities (hospitals, schools, emergency services) face stricter 500-year flood protection. Substantial improvement or substantial damage (over 50 percent of structure value) triggers full current-code compliance for the entire structure. Fill in the floodway is generally prohibited — the floodway is the river channel and adjacent areas reserved for flood flow. The King County Flood Control District funds levee maintenance, home buyouts, and flood warning systems. Federal FEMA flood insurance is required for federally-backed mortgages in SFHAs. King County participates in the Community Rating System (CRS) providing flood insurance discounts.