McHenry County, IL Accessory Structures: Garage Conversions (2026)
Garage conversion rules in McHenry County, IL, sometimes called garage-to-ADU or accessory living unit conversions, govern permits, ceiling height, egress, and parking replacement.
Key Facts
- Accessory as dwelling
- Prohibited unless a permitted ADU
- Governing section
- UDO § 16.56.050A.1.a
- ADU pathway
- Must meet § 16.56.030O standards
- Health approval
- Well/septic sign-off required
- Applies to
- Unincorporated county only
Summary
In unincorporated McHenry County you cannot convert a detached garage or other accessory structure into living quarters unless it qualifies as a permitted ancillary dwelling unit under the UDO. Otherwise, an accessory structure may not be used as a dwelling.
No accessory structure may be used as a dwelling unit, unless permitted as an ancillary dwelling unit in accordance with § 16.56.030O.
Full Breakdown
UDO § 16.56.050A.1.a is explicit: no accessory structure may be used as a dwelling unit unless permitted as an ancillary dwelling unit under § 16.56.030O. That means converting a detached garage into a habitable unit is only allowed if it meets the ADU standards, including well/septic approval by the McHenry County Department of Health, the two-bedroom cap, owner occupancy, and the one-ADU-per-lot limit. Converting attached garage space into additional living area of the principal home is a building-permit matter subject to setbacks and building code. Incorporated cities such as Crystal Lake and Woodstock apply their own municipal codes.
Violations & Fines
An illegal conversion is a zoning violation enforced by the Zoning Enforcement Officer; unpermitted dwelling use must cease or be legalized as an ADU through the required approvals.
Frequently Asked Questions
Can I rent out my converted garage?
Do I need a permit to convert my attached garage?
Sources & Official References
Other rules in McHenry County
Compare McHenry County to another location·View the Illinois accessory structures overview
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