Polk County, IA Environmental Rules: Flood Elevation Requirements (2026)

Article 7, Section 1(J) of the Polk County Zoning Ordinance sets the county's freeboard standard against the 'Base Flood Elevation,' defined in Section 1(Q) as the elevation floodwaters would reach at a site during a base flood, the flood with a 1 percent chance of occurring in any given year. Under Section 1(J)(1)(c), residential structures built or substantially improved after the ordinance's adoption must have the lowest floor, including the lowest level, elevated a minimum of one foot above the 1% Annual Chance flood level, built on compacted fill that stays at or above that one-foot elevation and extends at least 18 feet beyond the structure.

Nonresidential structures get the same one-foot minimum on the first floor, or must be floodproofed to that level instead. The same one-foot-above-BFE floor applies to new mobile homes on permanent foundations and to wastewater and water supply treatment facilities. Structural flood control works are held to a stricter standard: levees, flood walls and similar structures that are meant to protect against the 1% Annual Chance flood must carry a minimum of three feet of design freeboard above base flood elevation and provide adequate interior drainage, and any such structural flood control work needs Department of Natural Resources approval when required.

All of this construction must be certified by a professional engineer registered in Iowa as meeting these conditions before a Floodplain Development Permit is finalized.