Winnebago County, IL Accessory Structures: ADU Rules (2026)

Illinois delegates ADU authority to municipalities and counties through the Counties Code (55 ILCS 5) and the Illinois Municipal Code (65 ILCS 5/11-13-1 zoning powers). There is no statewide ADU preemption law as of 2026, though Cook County, Chicago, Evanston, and other Illinois jurisdictions have passed local ADU ordinances. Winnebago County's Unified Development Ordinance (Chapter 90 of the Code of Ordinances) regulates accessory living quarters in unincorporated areas only - the county Planning and Zoning Division has no authority within incorporated municipalities. Per the UDO's accessory use provisions, an accessory living quarter (sometimes called accessory dwelling) must have a minimum floor area of 300 sq ft and a maximum of 900 sq ft. The unit must be subordinate to the principal single-family residence on the lot, and additional setback, height, and parking requirements apply based on the underlying zoning district (AG, R-1, R-2, R-3, etc.). A zoning permit is required from the Winnebago County Planning and Zoning Division before construction. Properties inside Rockford follow the City of Rockford Zoning Ordinance (Chapter 28 of the Rockford Code), which has its own accessory structure rules and does not currently authorize separate-entry ADUs in most single-family zones. Loves Park, Machesney Park, Rockton, Roscoe, Cherry Valley, and South Beloit each have separate zoning codes governing accessory structures within their boundaries.