Sun Prairie, WI Environmental Rules: Flood Elevation Requirements (2026)

28, the city's floodplain-overlay chapter, sets a freeboard cushion on top of the base flood level rather than letting builders stop at the calculated flood line. 030 defines "flood protection elevation" as an elevation of two feet of freeboard above the Regional Flood Elevation, and that elevated line, not the raw flood elevation, is what governs construction in the floodfringe district (FF). a, any structure, including a manufactured home, that is newly constructed or substantially improved in the floodfringe must have its lowest floor elevated to or above the flood protection elevation on fill, and the fill placed around the structure must extend at least one foot above the regional flood elevation and reach out at least 15 feet beyond the structure's limits.

ii a basement or crawlspace floor may sit at the regional flood elevation instead, but only if it is designed watertight below the flood protection elevation, with walls substantially impermeable to water and structural components able to resist hydrostatic and hydrodynamic loads; no floor of any kind may go below the regional flood elevation itself. Accessory structures get a lighter standard: they need only be built on fill with the lowest floor at or above the regional flood elevation, without the extra two feet of freeboard.

Contiguous dryland access from the structure to land outside the floodplain is also required, with a narrow exception where the city has written assurance from police, fire and emergency services, or a DNR-approved evacuation plan, that wheeled-vehicle rescue is possible even when access roads sit below the regional flood elevation.