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

The floodplain regulations sit in County Code Title 8, Chapter 19, adopted to reduce flood losses inside the areas of special flood hazard shown on the county's Flood Insurance Study and FIRM maps, most recently dated September 26, 2008. Section 8-19-6 puts the Planning and Zoning Administrator in charge: no structure, manufactured home, or land-disturbing activity, including fill, may go forward inside a mapped flood hazard area in unincorporated Twin Falls County outside the cities' areas of impact until that office issues a development permit.

Applicants must submit scaled plans showing existing and proposed structures, drainage facilities, and elevations, plus the lowest floor's elevation relative to mean sea level and any floodproofing elevation, certified by a registered engineer or architect for nonresidential floodproofing. Once the lowest floor or floodproofing level is finished, the permit holder must give the administrator an as-built FEMA elevation certificate; if that certificate has deficiencies, the administrator can halt work with a stop-work order until they are corrected. The administrator also reviews every application against floodway encroachment standards and notifies neighboring communities before any watercourse is altered.

Anyone who disagrees with a floodway boundary call or a permit decision can appeal to the Planning and Zoning Commission under section 8-19-7, and from there to the Board of County Commissioners; a variance from the flood elevation standard is granted only in narrow cases, such as a small or irregular lot surrounded by existing development already built below base flood level.