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

Under Sec. 15-80 of the county's Countywide Stormwater and Flood Plain Ordinance, the Director of Stormwater Management first fixes the base flood elevation (BFE) from the current FEMA-adopted flood plain study, or from a Director-published hydrologic model where no FEMA study exists. Sec. H then sets the flood protection elevation (FPE) at the BFE plus one foot of freeboard, the county's regulatory buffer above the mapped 1% annual chance flood. Sec. 2 applies that FPE to residential construction: new buildings and substantial improvements in a Special Flood Hazard Area must have their lowest floor elevated to at least the FPE, and any enclosed space below that floor used only for parking, access or storage must be engineered, or designed to a minimum of two flood vents with a combined net area of at least one square inch per square foot of enclosed area, with no vent opening sitting higher than one foot above grade.

Sec. 7 lets an accessory structure such as a detached garage sit below the FPE only if it stays under 576 square feet or $15,000 in value, sits outside the regulatory floodway, and is fitted with the same flood-vent openings. Sec. 15-28 backs this up countywide: all usable space in new or expanded buildings in the flood plain must be elevated, floodproofed, or otherwise protected at least one foot above the base flood elevation, with any floodproofing device required to operate without human intervention and carry a professional engineer's certification. A stormwater management certification from the Director is required before construction begins.