DuPage County

DuPage 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.

Process overview: Typical workflow: (1) jurisdictional verification (parcel confirmed in unincorporated DuPage County, not inside city limits or extra-territorial jurisdiction); (2) zoning verification against the county ordinance; (3) building-code plan review against the adopted state building code; (4) site-plan, septic (where applicable), and floodplain review; (5) issuance, construction with inspections, and certificate of occupancy.

Impact fees: DuPage County permit fees are itemized at intake. Counties in Illinois commonly do not levy municipal-style impact fees on residential additions in unincorporated areas; verify current fee schedule at the development-services counter.

The DuPage County property assessor / equalization office maintains parcel-level assessment records for all real property in DuPage County. ADU additions are typically captured as improvements to the host parcel via shared permit data with the building department. Illinois property-assessment rules govern annual revaluation cycles, homestead or principal-residence caps where applicable, and the procedures for protesting an appraisal.

Assessment policy: Improvement value for an ADU is added to the parcel record on the next regular revaluation cycle. Homestead / principal-residence caps where applicable shield the existing structure from rapid valuation increases but do not exempt new improvement value.

DuPage 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 DuPage 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