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

The elevation rules sit in BMC 14.04.760(A)(3), which applies in all areas of special flood hazard. For residential construction in AE or other A-zoned areas with a determined base flood elevation (BFE), the lowest floor, including basement, must be elevated one foot or more above the BFE, and mechanical equipment and utilities must be waterproof or elevated at least one foot above it. In an unnumbered A zone with no obtainable BFE, the structure must be reasonably safe from flooding and the lowest floor must be at least two feet above the highest adjacent grade.

Commercial, industrial and other nonresidential buildings must meet the same one-foot standard or the elevation required by ASCE 24, whichever is greater, with equipment waterproofed or elevated to match. The alternative is dry floodproofing: the structure must be watertight below one foot or more above the base flood level, or to the ASCE 24 elevation if higher, and a registered professional engineer or architect must certify the design. Manufactured homes in Zones A, AE and X must sit on a permanent foundation with the lowest floor one foot or more above the BFE.

Fully enclosed areas below the lowest floor that are subject to flooding are prohibited unless designed to equalize floodwater. The code's minimum criteria are two openings with a total net area of not less than one square inch for every square foot of enclosed area, with the bottom of every opening no higher than one foot above grade. Those enclosed areas can be used solely for parking of vehicles, building access, or storage. The code also builds in a record: BMC 14.04.740(A) requires the city to keep the as-built lowest floor elevation of all new or substantially improved structures. Substantial improvement means work costing 50 percent or more of the structure's market value before construction starts.