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

Some Restrictions

Compiled from the official code textEditor Martyn O'NeillLast verified September 2026

ADU rules in McHenry County, IL, also called accessory dwelling unit regulations or granny flat ordinances, cover setbacks, owner-occupancy, parking, and permit requirements.

Key Facts

ADUs per lot: One ancillary dwelling unit maximum
Bedroom cap: No more than 2 bedrooms
Footprint cap: 900 sq ft for detached units
Height limit: 35 feet for detached units
Occupancy rule: Lot must remain owner-occupied
Penalty: $100-$500 per week, separate offense

ADU Eligibility Quick-Check

An Accessory Dwelling Unit (ADU) is a smaller, independent living unit on the same lot as a single-family home: converted garages, detached cottages, basement or attic conversions, and attached additions all qualify. Most states have streamlined ADU permitting since 2020, but local rules still control size, height, and setback.

Is your lot zoned single-family or multi-family residential?
Does your lot have at least 1,200 sq ft of buildable space?
Is your existing home owner-occupied (in most states this is not required, but some require it for ADU permits)?
Will the ADU stay under your state's size cap (commonly 850-1,200 sq ft for detached)?
Can you provide one off-street parking space (waived in many states within 0.5 mi of transit)?

If you answered 'yes' to most of these, you likely qualify in Illinois under recent ADU streamlining laws. The McHenry County, IL rules below cover local size, height, and setback requirements.

Summary

Unincorporated McHenry County allows one ancillary dwelling unit per lot under UDO § 16.56.030O, attached to the house or built as a freestanding structure. The lot must stay owner-occupied, the unit is capped at two bedrooms and a 900-square-foot footprint, and mobile homes or RVs cannot serve as the unit. Detached units cap out at 35 feet and follow the same setbacks as the principal home.

These county ordinances apply to unincorporated areas of McHenry County. Incorporated cities within the county may have their own rules that take precedence over county-level regulations.

O. Dwelling Unit, Ancillary. ... 5. Ancillary dwelling units shall be limited to no more than 2 bedrooms. 6. Ancillary dwelling units shall not exceed the size and scope of the principal residence. 7. Lots or parcels containing an ancillary dwelling unit shall be owner occupied. The owner may reside in either the principal residence or the ancillary dwelling unit. No one else shall occupy the owner's unit when the owner is not residing on the property. The homestead exemption per 35 ILCS 200/15-175 could be used as evidence for this requirement. 8. Only one ancillary dwelling unit is allowed per lot or parcel. 9. One (1) parking space shall be provided for an ancillary dwelling unit in addition to that required for the principal dwelling. 10. Detached ancillary dwelling units shall be no more than thirty-five (35) feet in height, measured in accordance with § 16.60.010.F.1.a. 12. The maximum square footage of the footprint of a detached ancillary dwelling unit shall not exceed 900 square-feet. 13. Mobile home trailers and recreational vehicles are prohibited from being used as an ancillary dwelling unit.

View official code

Official source re-checked September 7, 2026: no newer edition of the code had been published (publisher’s edition: 2025 S-5: Supplement contains: Local legislation current through Ord. A25-0037, passed 8-19-2025 | State legislation current through ALS 2025 Pamphlet No. 1).

Full Breakdown

8). 5). 11). 7). 9). 13). 1). 3.

Violations & Fines

Building or occupying an ADU that breaks any of these standards is a zoning violation under § 16.84.010, a petty offense carrying a fine of $100 to $500, with each week the violation continues counted as a separate offense. The County may also sue in circuit court under § 16.84.020 to force compliance, halt occupancy, or abate the violation, and any owner, tenant, contractor, or agent who participates can be charged and fined separately for the same violation.

Frequently Asked Questions

Can I rent out my McHenry County ADU to someone other than family?

Only if the owner still lives in either the principal residence or the ADU. Section 16.56.030O.7 bars anyone but the owner from occupying the owner's unit whenever the owner is away, and the county cites the homestead exemption under 35 ILCS 200/15-175 as proof the owner actually lives there. A tenant-only property with no resident owner does not qualify.

How big can a detached ADU be in unincorporated McHenry County?

Section 16.56.030O.12 caps a detached ADU's footprint at 900 square feet, and O.6 further requires it to stay smaller in size and scope than the principal residence. Height is limited to 35 feet under O.10, and it must meet the same building setbacks that apply to the main house under Table 16.36-1.

Can I put a mobile home or RV in my backyard as an ADU?

No. Section 16.56.030O.13 expressly prohibits mobile home trailers and recreational vehicles from being used as an ancillary dwelling unit anywhere in unincorporated McHenry County, regardless of lot size or zoning district.

What happens if my existing ADU doesn't meet the current standards?

Section 16.56.030O.1 grandfathers ADUs that existed when the Ordinance was adopted, letting them continue under Chapter 16.80's nonconformity rules or the conditional use permit that originally approved them. An owner can bring a nonconforming ADU into compliance by applying for a variation on whichever standard it fails to meet.

Sources & Official References

[1]McHenry County Unified Development Ordinance § 16.56.030O

Other rules in McHenry County

Shed & Carport RulesSome Restrictions Fire Regulations: Fire Pits & Backyard FiresLight Restrictions Tiny HomesSome Restrictions Garage ConversionsSome Restrictions Parking Rules: Loading ZonesSome Restrictions Animal Ordinances: BeekeepingSome Restrictions Animal Ordinances: Veterinary Clinic ZoningSignificant Restrictions Fire Regulations: Outdoor BurningSignificant Restrictions Landscaping Rules: Rainwater HarvestingFew Restrictions

All McHenry County rules

How McHenry County compares: Easiest Cities to Build an ADU·Compare McHenry County to another location·View the Illinois accessory structures overview

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