Will County, IL Accessory Structures: ADU Rules (2026)
ADU rules in Will County, IL, also called accessory dwelling unit regulations or granny flat ordinances, cover setbacks, owner-occupancy, parking, and permit requirements.
Key Facts
- Max ADUs per lot
- 1
- Max size
- 650 sq ft or 49% of home
- Owner occupancy
- 6 months per year minimum
- Detached ADU setback
- 10 ft behind principal dwelling
- Max height
- 20 ft or house height
Summary
Unincorporated Will County allows one accessory dwelling unit per lot with detached houses, capped at 650 square feet or 49% of the main home's floor area.
(h) Size. The floor area of an ADU may not exceed 49% of the gross floor area of the principal dwelling unit on the subject lot (excluding any attached garage), or 650 square feet, whichever is less. (d) Owner occupancy. The owner of record must reside in either the principal or accessory dwelling unit for a minimum of six months of each calendar year.
Official source re-checked September 7, 2026: no newer edition of the code had been published (publisher’s edition: 2025 S-25: Supplement contains: Local legislation current through Ordinance No. 24-100, passed 4-18-2024 and Res. 23-387, passed 11-16-2023; and State legislation approved through 2024 Pamphlet #2).
Full Breakdown
Will County Code § 155-10.10(C) permits one accessory dwelling unit (ADU) per lot occupied by a detached or attached house, created by interior conversion, addition, or a new detached structure. The owner of record must live in either the principal or accessory unit at least six months a year. ADU floor area is capped at 49% of the principal dwelling's gross floor area (excluding attached garage) or 650 square feet, whichever is less. Detached ADUs built after January 18, 2018 must sit at least 10 feet behind the principal dwelling, match its exterior finish, trim, and roof pitch, and stay at or below 20 feet or the main house's height. This zoning code governs only unincorporated Will County; Joliet, Bolingbrook, Romeoville, Plainfield and Lockport apply their own municipal ADU rules.
Violations & Fines
Building or occupying an ADU without approval is a zoning code violation enforceable by the Land Use Department, with daily fines and a court order to remove or bring the unit into compliance.
Frequently Asked Questions
Can I add an ADU on any lot in Will County?
Does Will County require extra parking for an ADU?
Do Joliet or Bolingbrook follow this county rule?
Sources & Official References
Other rules in Will County
How Will County compares: Easiest Cities to Build an ADU·Compare Will County to another location·View the Illinois accessory structures overview
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