Renton, WA Environmental Rules: Flood Zones (2026)

RMC 4-3-050 folds Renton's floodplain rules into its critical areas chapter, covering the special flood hazard areas and channel migration zones mapped on FEMA's Flood Insurance Rate Maps for King County. No development, alteration, or regulated activity in a mapped flood hazard area can proceed without a development permit, critical area permit, or Administrator-issued letter of exemption; the Floodplain Administrator, a role assigned to the City's Administrator, grants or denies these permits and enforces the section. Floodways are treated as the most hazardous zone: fill, new construction, substantial improvements, and reconstruction of residential structures are flatly prohibited there unless the narrow encroachment exceptions in subsection G4e apply, and any structure that is allowed must show the project will not raise the base flood elevation.

Outside the floodway but still within AE or A1-30 zones, new and substantially improved residential structures must have their lowest floor, including basement or attached garage, elevated one foot or more above the base flood elevation, with mechanical equipment and ductwork elevated the same amount. Where a base flood elevation cannot be obtained (Unnumbered A zones), the lowest floor and attached garage floor must sit at least two feet above the highest adjacent grade. Enclosed areas below the lowest floor are restricted to parking, building access, or storage, and must include flood vents (minimum two openings, one square inch of net area per square foot of enclosed area, bottoms no higher than one foot above grade) so floodwaters can enter and exit automatically.

Critical facilities built inside a flood hazard area must have their lowest floor elevated three feet or more above the 100-year base flood elevation. The City relies on the FIS for King County, Washington and Incorporated Areas dated August 19, 2020, and its accompanying FIRMs, kept on file at Renton City Hall, 1055 South Grady Way.