McLean County, IL Environmental Rules: Flood Elevation Requirements (2026)

Section 350-41B defines Flood Protection Elevation (FPE) as the elevation of the base flood plus one foot of freeboard at any given location in the floodplain, with the base flood itself defined as the flood having a one-percent annual chance of being equaled or exceeded, commonly the hundred-year flood. Section 350-41G(1) requires all buildings in the floodplain to be protected from flood damage below that flood protection elevation whenever a new building is constructed, an existing building is altered or added to at a cost over $1,000 or more than 70 square feet, a substantial improvement increases floor area more than 20% or equals 50% of market value, a substantially damaged building is repaired, a manufactured home is installed, or a travel trailer sits on a site more than 180 days a year.

Section 350-41G(2)(a) sets out how residential and nonresidential buildings meet that standard when built on permanent fill: the lowest floor, including the basement, shall be at or above the flood protection elevation, the fill must be placed in layers no greater than six inches before compaction and extend at least 10 feet beyond the foundation, and it must be protected against erosion by vegetative cover, riprap or other structural measures. Critical facilities such as fire and police stations, schools and hospitals face an even higher bar under Section 350-41 requiring elevation to the five-hundred-year flood frequency elevation or three feet above the hundred-year level, whichever is greater.

Every applicant must first secure a development permit from the Director of Building and Zoning, who compares the site elevation to the base flood elevation before issuing it.