DuPage County, IL Environmental Rules: Flood Zones (2026)

Sec. A sets the county's baseline rule for the regulatory flood plain: no development activity may result in an adverse hydraulic impact to upstream or downstream properties, and any fill placed in the flood plain must be offset with compensatory storage under Sec. D. Sec. 15-82 tightens that further inside the mapped regulatory floodway, where only a short list of uses may even be considered for a certification: flood control structures and public works relating to drainage, flooding, erosion, water quality or wildlife habitat; utility crossings and storm or sanitary sewer outfalls; recreational facilities such as trails and ballfields; bridges, culverts, roadways, sidewalks and railways; floodway regrading without fill to restore a positive slope; floodproofing of existing structures without enlarging their footprint; and in-ground swimming pools built without fill.

Any other floodway development is not an eligible use under this chapter. Sec. D requires that all approved floodway work preserve effective conveyance so there is no increase in the 100-year flood elevation, flow, or velocity, unless the increase sits inside a public flood easement and delivers a watershed benefit. The county holds delegated authority from IDNR-OWR to issue floodway construction permits, but a parallel application must still go to IDNR-OWR, and the county's delegated authority does not extend to federal or state agency projects, jurisdictional dams, or work in public bodies of water. Sec. A makes it unlawful to undertake any development in the county, or in a waiver community, without first securing a stormwater management certification.