Madison, WI Environmental Rules: Flood Elevation Requirements (2026)

18(4)(a) requires new construction and substantial improvement of structures in zone AO to elevate the lowest floor, including any basement, to or above the flood depth shown on the FIRM plus two additional feet of freeboard, or five feet above the highest adjacent natural grade if no depth is specified; zone AH structures must reach the "flood protection elevation," the code's term (defined elsewhere in Chapter 21) for two feet of freeboard above the Regional Flood Elevation. 17(1)(a) requires new residential construction, including manufactured homes, to have the lowest floor elevated on fill to or above the flood protection elevation, with fill extending at least one foot above the regional flood elevation and at least 15 feet beyond the structure's limits.

17(1)(b) allows an alternative: a basement or crawlspace floor set two feet above the regional flood elevation if it's engineered watertight, with walls able to resist hydrostatic and hydrodynamic loads, no floor of any kind may sit below the regional flood elevation itself. 17(2). 20(2)(h) requires rebuilding with the lowest floor, including basement, elevated to two feet above the base flood elevation using fill, pilings, columns, posts, or approved perimeter walls, and mechanical, electrical, and HVAC equipment elevated to the same standard.