McHenry County, IL Accessory Structures: Tiny Homes (2026)

Section 16.56.030(O) of the McHenry County Unified Development Ordinance is the county's only zoning path for a small second home on a residential lot, and it governs whether a tiny house can legally sit on the property at all. An ancillary dwelling unit 'may be attached to the principal dwelling unit or detached as a standalone structure,' but it 'shall not exceed the size and scope of the principal residence' and is 'limited to no more than 2 bedrooms.' A detached unit's footprint is capped at 900 square feet, its height cannot exceed 35 feet as measured under Section 16.60.010.F.1.a, and it must meet every setback that applies to the principal structure under Table 16.36-1. Only one ancillary unit is allowed per lot or parcel, and it must provide one parking space beyond what the principal home already requires.

Ownership matters as much as size. The lot must be owner-occupied: the owner can live in either the principal home or the ancillary unit, but 'no one else shall occupy the owner's unit when the owner is not residing on the property,' and the code notes the homestead exemption under 35 ILCS 200/15-175 can serve as evidence of that occupancy. Mobile home trailers and recreational vehicles are expressly 'prohibited from being used as an ancillary dwelling unit,' so a tiny house on wheels does not qualify; it must be a permitted structure meeting building code and McHenry County Department of Health well-and-septic requirements. Units already in place when the ordinance was adopted continue under the nonconformity chapter, Chapter 16.80, or their original conditional use permit, and can be brought into compliance by seeking a variation from the Zoning Board of Appeals for whichever standard they miss.