DuPage County, IL Environmental Rules: Stormwater Management (2026)

Chapter 15, the Countywide Stormwater and Flood Plain Ordinance, applies to all development in the county since February 15, 1992, administered by a Director on behalf of the county's Stormwater Management Planning Committee, or by a municipal Administrator in a 'waiver community' that runs its own compliant ordinance. Sec. ' Sec. 15-27 sets the underlying design standards: development may not unreasonably increase flood elevations or decrease flood conveyance capacity, may not increase flood velocity without a watershed benefit, and may not degrade surface or ground water quality; site runoff storage and compensatory storage facilities must be built before or concurrently with general construction and functional before any building work that adds impervious area.

Sec. 15-28 requires that in the regulatory flood plain, all usable space in new or expanded buildings be elevated, floodproofed or otherwise protected so the lowest entry sits at least one foot above the nearest base flood elevation, with floodproofing devices that operate without human intervention and are certified by a professional engineer. Projects with building permits, approved engineering, or exempting agreements predating February 15, 1992 can qualify for the county's official exempt-development list under Sec. 15-23(B).