Twin Falls County, ID Environmental Rules: Flood Elevation Requirements (2026)

Twin Falls County's freeboard standard lives in section 8-19-8(G), the specific standards subsection of the flood hazard reduction chapter that applies once base flood elevation data exists for a site. New residential construction and substantial improvements must have the lowest floor, including any basement, at least one foot above the base flood elevation, and any building utility system serving that structure must sit at the same one-foot margin above BFE. Nonresidential buildings get an alternative: elevate one foot above BFE the same way, or floodproof to that height with a design an Idaho-registered engineer or architect certifies to the floodplain administrator.

The county's own defined flood protection elevation in section 8-19-4 shows how that freeboard stacks: base flood elevation, plus any rise caused by floodway encroachment, plus the one foot of freeboard, and manufactured homes placed in a special flood hazard area must reach that full flood protection elevation, not merely the BFE. Fully enclosed areas below the elevated lowest floor, such as crawl spaces, must have flood vents sized at one square inch of opening per square foot of enclosed area, with the lowest opening no higher than one foot above grade, so hydrostatic pressure equalizes automatically.

Critical facilities like schools, hospitals, and emergency response buildings face a stricter three-foot margin above BFE, or must reach the 500-year flood height if that is higher, reflecting the county's judgment that even a slight flood risk to those uses is unacceptable.