Snohomish County, WA Environmental Rules: Flood Elevation Requirements (2026)

120 sets the county's freeboard standard for every AE, A1-30, and other A-zoned special flood hazard area where FEMA has determined a base flood elevation. New construction and substantial improvement of a residential structure must have the lowest floor, including any basement, elevated a minimum of one foot above the base flood elevation, and mechanical, electrical, heating, and plumbing equipment must be waterproofed or elevated the same one foot. Nonresidential buildings follow the same one-foot rule or must be dry floodproofed watertight to that height, whichever is greater under ASCE 24 if that standard demands more.

110(1)(b). Enclosed areas below the elevated lowest floor, such as garages, are barred unless they carry flood vents: a minimum of two openings totaling at least one square inch of net area per square foot of enclosed space, with vent bottoms no higher than one foot above grade, sized to let floodwater automatically enter and exit. 360, face a stricter three-foot freeboard above base flood elevation, plus floodproofing to keep toxic substances from washing into flood waters. 100(2) still requires the lowest floor to sit at least two feet above the highest adjacent grade.

130: a completed FEMA elevation certificate, prepared by a registered land surveyor for elevation data and by a registered engineer or architect for floodproofing data, must be filed with Snohomish County Planning and Development Services before the building can be occupied. 070 oversees compliance.