Mount Prospect, IL Environmental Rules: Flood Elevation Requirements (2026)

The Village's freeboard is written into its definitions. Section 22.102 defines the flood protection elevation (FPE) as the elevation of the base flood or 100-year frequency flood plus two feet (2') of freeboard at any given location in the SFHA.

Where the base flood elevation comes from is set by section 22.105. The protection standard is based on the flood insurance study for the village. For the Des Plaines River, Feehanville Ditch, McDonald Creek, McDonald Creek Tributary B and Weller Creek, the elevation is read from the 100-year flood profiles in the Cook County flood insurance study prepared by FEMA and dated August 19, 2008, as amended. Where no elevation is available, the best data in the Illinois state water survey floodplain information repository governs, and for a nonriverine SFHA the base flood elevation is the historic flood of record plus one foot (1'), unless calculated by a detailed engineering study and approved by the Illinois state water survey. A party who disagrees with the best available data can finance a detailed engineering study and submit it to IDNR/OWR and FEMA.

Section 22.109C applies the elevation. New buildings, and nonconforming structures that are enlarged or substantially improved, must be protected below the FPE by one of the listed methods. On permanent land fill, the lowest floor including the basement must be a minimum of two feet (2') above the base flood elevation, and the fill is placed in layers no greater than one foot (1') deep before compaction and should extend at least ten feet (10') beyond the foundation. On an open foundation such as a crawl space, stilts, piles or walls, the lowest floor and all electrical, heating, ventilating, plumbing and air conditioning equipment and utility meters must be located a minimum of two feet (2') above the FPE. The permanent openings must be no more than one foot (1') above grade, with a minimum of two openings and a total net area of not less than one square inch for every one square foot of enclosed area. The director of community development keeps an elevation certificate for each building in the permit files under section 22.104G.