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

McHenry County's Unified Development Ordinance treats an ancillary dwelling unit (ADU) as its own use category under § 16.56.030O, not an ordinary accessory structure, and allows exactly one per lot or parcel (O.8). The unit may attach to the principal dwelling or stand alone (O.3), but it cannot exceed the size and scope of the principal residence (O.6) and is capped at two bedrooms (O.5). A detached ADU's footprint may not exceed 900 square feet (O.12), its height is limited to 35 feet as measured under § 16.60.010.F.1.a (O.10), and it must meet the same setback table that governs the principal structure, Table 16.36-1 (O.11).

The lot must remain owner-occupied: the owner may live in either the main house or the ADU, but no one besides the owner may occupy the owner's unit whenever the owner is off the property, and the county points to the homestead exemption under 35 ILCS 200/15-175 as evidence of that occupancy (O.7). Every ADU must satisfy building code and McHenry County Department of Health well and septic requirements (O.4), and one additional off-street parking space is required beyond what the principal dwelling already needs (O.9).

Mobile home trailers and recreational vehicles are flatly barred from serving as an ADU (O.13). ADUs that existed when this Ordinance was adopted are grandfathered as nonconformities under Chapter 16.80 or under whatever conditional use permit originally authorized them, and can be brought into full compliance by seeking a variation on any noncompliant standard (O.1). A property owner who disputes a Zoning Enforcement Officer's ruling on an ADU can pursue an administrative variation under § 16.20.020E.2 or a zoning variation before the County Board under § 16.20.020E.3.