Hazel Crest

Cook County, Illinois - the second-most-populous county in the United States with 5.1 million residents - regulates accessory dwelling units in unincorporated territory through the Cook County Zoning Ordinance, administered by the Cook County Department of Building and Zoning. Cook County's unincorporated footprint is small (roughly 2% of county land area) because the great majority of Cook is incorporated by Chicago and 134 suburban municipalities. Chicago's own ADU pilot ordinance (Chicago Municipal Code Sec. 17-9-0102) governs ADUs inside city limits in five designated pilot areas, which is where the bulk of Cook County ADU activity occurs. Suburban Cook municipalities (Evanston, Oak Park, Cicero, Schaumburg, Arlington Heights, Skokie, etc.) each set their own ADU rules under home-rule authority. Cook County's county-level ordinance is meaningful chiefly for the unincorporated pockets along the western and southern county boundaries.

• Cook County Zoning Ordinance
• Chicago Municipal Code Sec. 17-9-0102 - Additional Dwelling Unit Pilot

State-floor overlay: Illinois has not enacted a statewide ADU preemption statute. Cook County and its 135 incorporated municipalities (Chicago plus 134 suburban) retain home-rule authority over ADU zoning. Illinois Constitution Art. VII grants home-rule powers to municipalities of population greater than 25,000.

Cook County administers flood-hazard, and (where mapped) coastal, wildland-fire, historic, and airport overlays that shape ADU project feasibility. The most consistent overlay across the county is FEMA NFIP floodplain regulation; other overlays apply to specific geographies inside the county.

• FEMA NFIP Special Flood Hazard Areas in Cook County — A new ADU in a mapped SFHA must be elevated to or above the Base Flood Elevation; cost impact on the project is often material.
• Historic districts and individually-listed historic resources

Cook County issues building permits for parcels in unincorporated territory through its development services / planning department, with separate review tracks for zoning conformance, building-code compliance, on-site sewage where applicable, floodplain compliance, and addressing. Inside incorporated municipalities, city departments handle their own permits; the county's authority is geographically limited to unincorporated territory. An ADU permit application is typically processed as a residential building permit with a zoning verification step against the county's ordinance for the parcel's zoning district.