Renton, WA Environmental Rules: Flood Elevation Requirements (2026)

d, the City's Specific Standards for flood hazard areas set the actual construction floor. In AE and A1-30 zones, or other A-zoned areas where a base flood elevation (BFE) has been determined or can reasonably be obtained, new construction and substantial improvement of a residential structure must have its lowest floor, defined to include a basement or attached garage, elevated one foot or more above the BFE. Mechanical equipment, ductwork, and utilities carry the same one-foot freeboard requirement. Nonresidential construction (commercial, industrial, or other) is held to the same one-foot floor elevation or to the elevation required by ASCE 24, whichever is greater, and its mechanical equipment and utilities follow the same rule.

Manufactured homes placed or substantially improved in Zones A1-A30, AH, and AE must sit on a permanent foundation with the lowest floor at least one foot above BFE and be anchored against flotation, collapse, and lateral movement. Where a BFE cannot reasonably be obtained (an Unnumbered A zone), the lowest floor, including any attached garage floor, must instead sit at least two feet above the highest adjacent grade; the code specifically warns that failing to reach that two-foot mark can raise flood insurance premiums. Any fully enclosed area built below the lowest floor may be used only for parking, building access, or storage, and must include flood vents: a minimum of two openings totaling at least one square inch of net area per square foot of enclosed space, with opening bottoms no higher than one foot above grade, unless a registered engineer or architect certifies an alternative design. ii.