Waukegan

Lake County, Illinois - 714,000 residents in northern suburban Chicago and along the Wisconsin border - regulates accessory dwelling units in unincorporated territory through the Lake County Unified Development Ordinance (UDO), administered by the Lake County Planning, Building and Development Department. Lake is significantly incorporated (Waukegan, North Chicago, Highland Park, Lake Forest, Mundelein, Libertyville, Vernon Hills, Buffalo Grove, Gurnee, Round Lake-area, Zion, Antioch, Grayslake, Lincolnshire); unincorporated Lake includes substantial rural and lake-front pockets. Each incorporated municipality sets its own ADU rules.

• Lake County Unified Development Ordinance

State-floor overlay: Illinois has not enacted statewide ADU preemption. Lake County and its incorporated municipalities retain home-rule authority.

Lake 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 Lake 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

Lake 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.