Illinois housing - zoning & building rules
The honest state of ADU and addition rules in Illinois: there is no statewide mandate yet, so what your city or village allows is what you get. Here is the actual law, how local rules work, what communities already permit, and how to verify your own address.
Statewide mandate
None yet
ADU rules are set by each municipality.
Chicago
Permanent since 2026
ADU ordinance effective April 1, 2026, replacing the 2021 pilot.
Pending bill
Jan 1, 2027
SB4071 would require municipalities to allow ADUs if enacted.
Sources: Illinois General Assembly, City of Chicago, Chicago Cityscape, and CMAP. Rules change; this is a starting point, not legal advice.
Independent - not a government agency and not your attorney.
Illinois has no statute today that requires a city, village, or county to allow accessory dwelling units. That is not an oversight; it is the current reality. Whether you can add a granny flat, coach house, or in-law suite is decided by your local zoning ordinance, and many Illinois communities still do not permit them at all.
Two things are worth knowing because they are widely repeated and widely wrong. First, a bill before the legislature - SB4071- would require every municipality to authorize ADUs beginning January 1, 2027. As of this writing it is still in committee and has not become law. Second, the law most often miscited as the Illinois ADU law is Public Act 103-0510, which is abuilding-code alignment measure for new residential construction - not an ADU mandate.
Illinois's largest city went from banning ADUs (since 1957) to running a pilot (2021) to a permanent, citywide ordinance. It is the clearest illustration of how Illinois ADU rules work.
Chicago City Council voted 46-0 to approve the permanent ordinance (SO2024-0008918) on September 25, 2025. It replaced the pilot that had allowed ADUs in five areas since May 2021.
A conversion unit is added inside an existing building (attic or basement), and a coach house is a separate, detached building in the yard. Both need a pre-certification from the Department of Housing and then permits from the Department of Buildings.
ADUs are allowed by right in multifamily, mixed-use, and certain business and commercial districts, and in single-family (RS) districts within the former pilot areas or where an alderperson has opted in. In early 2026 that covered about 60% of the city's residential and mixed-use zoned land.
A conversion unit needs a principal building at least 20 years old. In some opt-in RS areas an owner must live in the principal house. Some blocks cap how many ADU permits are issued per year, and short-term rentals are restricted.
The summaries below come from each community's code; they are not the ordinance itself - verify with the linked source and your building department before you plan around any figure.
Ordinance 19-12-43, Jan 13 2020
An accessory residential unit (coach house, granny flat, or in-law apartment) needs a special-use permit, and is allowed mainly in the Downtown Form-Based Code district or a traditional neighborhood development.
Source: Village Code §10-5-3(G)
Section 11A, as amended by Ordinance 2019-20
One accessory apartment per parcel on agricultural or estate lots of 5 acres or more, at most 30% of the house's net floor area and two bedrooms. The owner must live in the house or the apartment, and the lot can't be subdivided.
Source: Zoning Code, Section 11A
Council Bill 2022-117, effective Jun 22 2022
ADUs are allowed by right in every residential zoning district, as detached, attached, or internal units, with one per lot and size caps of 600 to 800 sq ft depending on lot size. Detached units are capped at 24 feet tall.
Ordinance passed May 7 2024
Allowed in the R-2 one-family district, as attached, coach house, detached, or interior units, with one per lot capped at 1,000 sq ft. Exterior types need a lot of at least 6,500 sq ft; the owner must occupy the principal dwelling or the ADU; short-term rentals are barred.
Zoning code most recently amended Jul 15 2026
Attached ADUs only, by right in the R-1 through R-5 single-family districts; detached ADUs are not allowed. One per lot, at most 600 sq ft or 25% of the house floor area, and the owner must live there six months a year.
Ordinance 2025-11-1117O, Nov 11 2025
One ADU per lot. A stand-alone or accessory-structure ADU may sit only in the side, corner-side, or rear yard, at least 6 feet from the property line, and must match the principal dwelling's siding, trim, window design, and roof.
Source: Master Zoning Code §9-101(E)(1)
Coach houses 2017; other ADU types amended Jan 10 2022
One ADU per lot in the residential districts, as a coach house, a detached ground-floor or garage unit, an attached addition, or an attic or basement unit. The owner must live in the house or the ADU, and detached structures may cover at most 20% of the lot.
Source: Zoning Ordinance, Section 9.3.B
Unified Development Ordinance, 2010 edition as amended
An ADU is an accessory use to a single-family detached house, allowed only when the City Council approves it as part of a PUD or by special-use permit. Limits: one per lot, at most 33% of the floor area or 800 sq ft, behind the house, owner-occupied.
Source: UDO Chapter 7B, §7B.3.2(C)
Also allow ADUs, with rules summarized elsewhere: Evanston, Homewood, Lake Bluff, Park Forest, South Elgin, Wilmette. See the Chicago Cityscape city-by-city table and CMAP's Chicagoland table.
Across the communities that allow ADUs, the same few levers keep coming up. Knowing them tells you what to look for in your own municipal code.
Many ordinances require the owner to live in the principal dwelling or the ADU. Examples: Moline, Oak Forest, Oak Park, Northbrook (six months a year), and Woodstock.
Square-footage limits are common: Champaign 600–800 sq ft, Moline 1,000 sq ft, Oak Park under 1,000 sq ft, Woodstock 800 sq ft or 33% of the house, whichever is less.
Most ordinances allow a single ADU per lot, and several add minimum lot sizes — Moline requires 6,500 sq ft for exterior forms, Bull Valley requires 5 acres.
Several communities require the ADU to match the principal house's siding, trim, window style, and roof. Oak Forest and Woodstock spell this out.
Some require added off-street parking (Woodstock wants two spaces per ADU bedroom); others, like Champaign and Oak Park, add none beyond what the principal use already needs.
Some allow ADUs by right (Champaign, Northbrook), some by special-use permit (Antioch, Woodstock). By-right is easier; special-use adds a public hearing and council decision.
Moline and Northbrook both prohibit using a permitted ADU for short-term rentals like Airbnb. Expect this condition in many ordinances.
Because rules are local and change, this is the only way to know for sure what your property allows.
Start with the site's Illinois Permit Finder to find your jurisdiction's contact and process, then check the Illinois building codes explained and the contractor license check.
No. As of this writing Illinois has no statute that requires municipalities to allow accessory dwelling units, so whether you can build one depends entirely on your local zoning ordinance. A pending bill (SB4071) would require every municipality to authorize ADUs beginning January 1, 2027, but it has not become law yet.
Almost certainly what you read about was Public Act 103-0510, which aligns the residential building codes that local governments enforce with a state baseline for new construction. That is a building-code law, not an ADU mandate, even though it is often miscited as one.
Chicago's permanent ADU ordinance took effect April 1, 2026. It covers two forms (an attached conversion unit and a detached coach house) and, as of early 2026, applies to roughly 60% of the city's residential and mixed-use zoned land. Eligibility depends on your zoning district and whether the area has opted in, and a conversion unit needs a principal building at least 20 years old.
An addition is governed by zoning (setbacks, lot coverage, height) and the building codes your municipality enforces for the new work, not by ADU rules. The same advice applies: check your district's allowances and get the proper permits before building.
Treat silence as prohibition. ADUs were banned in many Illinois communities decades ago, and where an ordinance does not permit them, they are generally not allowed. That is why the first step is always to read your own municipal code or call your building department.