Madison County, IL Accessory Structures: ADU Rules (2026)
ADU rules in Madison County, IL, also called accessory dwelling unit regulations or granny flat ordinances, cover setbacks, owner-occupancy, parking, and permit requirements.
Key Facts
- Zoning status
- Special use, Agricultural District only, § 93.023(D)(44)
- Minimum lot
- 2 acres
- Size cap
- 1,000 sq ft, per § 93.007(B) definition
- Sewer
- Public sewer or approved private system required
- Kitchen
- Full kitchen allowed with inspection/approval
- Penalty
- $10-$500 per day, § 93.999
Summary
Unincorporated Madison County allows accessory dwelling units only as a special use in the Agricultural District, on lots of two acres or more, capped at 1,000 square feet, under Zoning Code § 93.023(D)(44). The countywide definition in § 93.007(B) sets that same size cap.
ACCESSORY DWELLING UNIT. A residential dwelling unit, not including mobile homes, within an accessory structure located on the same lot as a single-family dwelling. ACCESSORY DWELLING UNITS shall not take up more than 1,000 square feet of an accessory structure. ... (44) Accessory dwelling units on lots at least two acres in size, where the dwelling unit located within an accessory structure is a maximum of 1,000 square feet. Only permitted on public sewers or an approved private sewage system, and the sewer system of the accessory structure dwelling unit may be combined or connected with the principal dwelling structure's sewer system subject to inspection and approval by all necessary entities including, but not limited to, the Building and Zoning Department. Accessory dwelling units may be equipped with a full kitchen subject to compliance with all applicable codes, regulations, and laws and subject to inspection and approval by all necessary entities including, but not limited to, the Building and Zoning Department.
Full Breakdown
023(D)(44) lists them as a special use rather than a use allowed outright: the lot must be at least two acres, the unit within the accessory structure is capped at 1,000 square feet, and the property must be on public sewers or an approved private sewage system. The section lets the accessory unit's sewer connection tie into the principal dwelling's system, but only after inspection and approval by the Building and Zoning Department and any other necessary entity. 023(E)(9), which may have a kitchenette or bathroom but not a full kitchen, bedroom or living space.
178 before the County Board votes on it, the same procedure used for kennels and other Agricultural District special uses. The rule applies only to unincorporated county land; Edwardsville, Alton, Collinsville, Granite City and the county's other cities and villages set their own ADU standards, or none, under their own zoning codes.
Violations & Fines
Building or occupying an accessory dwelling unit without the special use permit, on a lot under two acres, or over the 1,000-square-foot cap violates § 93.023(D)(44) and is punishable under § 93.999: a fine of $10 to $500, with each day of continued violation a separate offense, plus possible Circuit Court action to force removal or correction.
Frequently Asked Questions
Can I build a granny flat on my Madison County property?
Can an ADU have its own kitchen?
What if my lot is smaller than 2 acres?
Sources & Official References
Other rules in Madison County
How Madison County compares: Easiest Cities to Build an ADU·Compare Madison County to another location·View the Illinois accessory structures overview
See something wrong?
Help us keep this page accurate. If you notice an error or outdated information, let us know.