Understanding ADU housing laws and regulations in Illinois for 2026

Your municipality decides. Illinois ADU laws exist only at the local level because the General Assembly has never enacted a statute saying whether, where, or how large an accessory dwelling unit can be. The Illinois Municipal League said so directly in a March 3, 2026 position statement: ADU regulation falls under municipal authority, including height, bulk, setback and parking.

An accessory dwelling unit, or ADU, is a second self-contained home on the same lot as a primary residence. It may be a coach house behind the main building, a basement apartment with its own entrance, a finished attic unit or an attached addition.

Illinois home rule municipalities write their own zoning ordinances, and zoning is where ADU permission lives. The same coach house that Oak Park approves by right in a single-family zone would be illegal in Naperville, which does not permit ADUs, and illegal in Springfield, whose code bars using any portion of a detached garage or accessory building as a dwelling.

For an investor holding property in several Illinois towns, that means checking each address against each town's zoning code separately. The two bills that would change this, HB5626 and SB4071, sat in committee as of October 2026.

Local control over zoning does not mean you build to whatever code your contractor prefers. Statewide building, energy and plumbing requirements can apply to an ADU even in towns that have never adopted a local building code. Accessibility rules apply only when a project reaches the statutory housing thresholds.

Public Act 103-0510 is the law most often miscited as an Illinois ADU statute, and it is nothing of the kind. The act is SB2368, sponsored by Sen. David Koehler, titled "AN ACT concerning business" and filed under building and residential codes; the Governor approved it August 4, 2023 and it took effect January 1, 2024. From January 1, 2025, it requires any municipality or county that enforces a building code to regulate residential buildings at least as stringently as the baseline residential code and to report its model code edition and amendments to the Capital Development Board (20 ILCS 3105/10.18).

In jurisdictions with no building code, the act sets the default for new residential construction contracts: the Energy Efficient Building Act code, the Illinois Plumbing Code, the National Electric Code, and the current International Residential Code. Its own definition of "building code" excludes zoning ordinances, which is why it says nothing about whether an ADU is allowed. IML puts the same point this way: "Having a municipal or county zoning ordinance is separate from having a municipal or county building code" (IML).

Three other statewide code systems may affect your project:

• Energy: The 2024 IECC as amended by Illinois took effect November 30, 2025 and applies to any new building with a permit application; additions and alterations must conform for the new work only (CDB, July 2026). Local governments cannot set residential energy standards more or less strictly than the state code, except municipalities of 1,000,000 or more (Chicago) and towns that adopted the Stretch Code (71 Ill. Adm. Code 600).
• Plumbing: The Illinois Plumbing Code, 77 Ill. Adm. Code 890, has no ADU-specific section and shows its most recent adopted amendment effective April 24, 2014. It applies to new plumbing and alterations, treats a building changed from one use to another as a new building (Appendix A §1100), and requires each habitable building to connect to a water supply and a sewer or private disposal system (Appendix A §1700). Section 42 of the Plumbing License Law reserves the minimum plumbing code to the State, including against home rule units (P.A. 97-0365).
• Accessibility: The Illinois Accessibility Code, 71 Ill. Adm. Code 400, effective October 23, 2018, applies to public facilities and multi-story housing, defined as 4+ stories with 10+ units; non-multi-story housing with 4+ units in one structure follows HUD's Fair Housing Accessibility Guidelines (410 ILCS 25). A single detached ADU sits below both thresholds.

The Chicago Metropolitan Agency for Planning counts nearly a dozen northeastern Illinois communities that permit ADUs or are considering amendments (CMAP).

Chicago's ordinance governs eligibility, unit types, owner-occupancy, rental restrictions and address screening. The subsections below explain conversion units and coach houses, the remaining area-specific limits, and how to check a property.

Chicago allows ADUs by right in every multi-unit residential district and in certain business and commercial districts, plus single-family (RS) districts in 34 wards, effective April 1, 2026. The City counts over 320,000 eligible parcels, up from about 116,000 under the pilot. ADUs are still not legal citywide: in RS districts that have not opted in, no conversion-unit permit can be issued, because §17-9-0131 bars them outside an ADU-Allowed Area. Community association declarations can also still block an ADU that zoning would otherwise allow (KSN Law).

City Council approved the original ordinance, SO2020-2850, on December 16, 2020, effective May 1, 2021, for five pilot zones overlapping 21 of the city's 50 wards. The zones and their community areas were:

The City's announcement lists East Garfield Park under both the Northwest and West zones. The South, West and Southeast zones carried two extra restrictions the North and Northwest did not: a cap of two ADU permits per block per year, and owner-occupancy for buildings with three units or fewer.

The pilot built almost entirely on the North Side. Of 215 building permits issued by mid-June 2024, 205 were in the North and Northwest zones, 8 in the South, 2 in the West and none in the Southeast (Block Club). By March 18, 2026, 816 ADUs had been pre-certified, roughly 85% of them on the North or Northwest Side (Block Club).

Expansion came in three ordinances. SO2024-0008918 cleared the Zoning Committee 13-7 on July 15, 2025 and passed Council 46-0 on September 25, 2025, legalizing ADUs by right in all non-single-family residential, business and commercial districts and letting single-family areas opt in. SO2025-0020513, passed December 10, 2025, added RS districts in 28 wards with limits set ward by ward. SO2026-0022453 added sections of the 5th, 31st, 34th and 36th Wards.

Conversion units and coach houses

A conversion unit is a new unit built inside a principal residential building that has lawfully existed for at least 20 years, typically in the basement or attic (DOH). How many you can add depends on the building's existing unit count (City requirements):

Conversion units are exempt from minimum lot area per unit and open space provisions and need no accessory parking (§17-9-0131). Where you add two or more, 50% of the new units must stay affordable at 60% of area median income for 30 years, with fractions rounded down (ordinance text).

A coach house is a detached unit at the rear of the lot. Under §17-9-0201-F, it may not exceed 22 feet in building height, must keep at least 3 feet of open space from ground to sky between itself and one side lot line, and must sit at least 15 feet behind the principal building's rear wall (10 feet by administrative adjustment on lots under 125 feet deep). One dwelling unit per coach house. In B and C districts it must sit in the rear 30 feet of the lot. Coach houses are exempt from minimum lot area, open space and floor area ratio rules, need no parking, and may not be built before the principal building.

New coach houses must be built by contractors enrolled in U.S. Department of Labor-approved Registered Apprenticeship Programs; conversion units are exempt from that rule (Block Club).

Owner-occupancy, block caps and rental limits

Owner-occupancy applies only where an ADU-Allowed RS Area imposes it and the principal building is a detached house, and it covers both conversion units and coach houses (City requirements). The operative language in §17-7-0573(2): "Where the principal building on the zoning lot adding a conversion unit or coach house is a detached house, the detached house must be owner-occupied at the time of pre-certification."

The West and South ADU-Allowed Areas go further, as does the Southeast Area. There, a building permit may not be issued for a conversion unit or coach house on a lot with three or fewer established units unless the principal building is owner-occupied at the time of permit application (ordinance text). Those areas also keep their per-block permit limits, while the North and Northwest areas have none (Abundant Housing Illinois).

In single-family districts the block caps run RS-1 one permit per year, RS-2 two, RS-3 three. An administrative adjustment to get around a cap costs $500. Conversion units and coach houses built after May 1, 2021 cannot be used as short-term rentals; the renting section below covers the fines.

How to check whether your address qualifies

Enter the address in the Department of Housing's online ADU application portal, which launched April 1, 2026 and took more than 30 applications in its first 12 hours (City). Pre-certification confirms the lot is in an ADU-Allowed Area and meets the ordinance; it does not guarantee a building permit, which the Department of Buildings issues separately (City).

Keep Chicago Moving, a third-party tracker rather than an official city release, reports that 446 of the applications filed between March 31 and September 14, 2026 (90.1%) were pre-certified at a median of 9.3 days (Keep Chicago Moving).

Outside Chicago, Evanston, Oak Park and Moline permit ADUs outright, while Urbana reviews them case by case. Naperville prohibits them, Springfield bars detached accessory structures from use as dwellings, and Rockford's use table has no ADU entry. The comparison, drawn from each town's code or official guide, looks like this:

Evanston adopted its ordinance September 29, 2020, allows one internal, attached or detached ADU per zoning lot, and caps height at 28 feet or two stories (20 feet for flat or mansard roofs). No additional parking is required, but existing required parking must stay or be replaced (Ordinance 86-O-20; 2026 ADU Guidebook).

Oak Park approves ADUs with no public meeting or commission review outside historic districts (Wednesday Journal). Height is 20 feet or the height of the house, whichever is less, and displaced garage parking must be replaced on-site (Village ADU guide).

Moline adopted its rules May 7, 2024 on a 5-2 vote (Council minutes). Detached units need no separate driveway or curb cut. You must record a covenant with the Rock Island County Recorder before the building permit issues (Moline ordinance). The section numbers come from the ordinance attachment and were not checked against the current codification.

Urbana's planning staff told the News-Gazette in May 2023: "We simply treat them as a second principal structure (i.e. a second house) and require an application for a conditional use permit, which is then reviewed by our Zoning Board of Appeals" (News-Gazette). Several have been approved over nine years. ADU-specific size and setback rules were not located.

Rockford's zoning ordinance, amended through July 1, 2024, lists no ADU, accessory apartment or coach house use. §20-004-G states unlisted uses are expressly prohibited. A July 2025 Platform Rockford piece said ADUs "remain largely absent from local policy conversations" (Platform Rockford).

Naperville's 2022 Land Use Master Plan recommended reviewing codes to accommodate ADUs, but no ordinance followed; a November 19, 2025 council item confirms the City does not permit them. Springfield's §155.068(b)(2) states "in no case shall any portion of a detached garage, accessory building, or structure be used as a dwelling unit or for any business or office use." The retrieved text covers detached structures; interior units are not separately addressed.

Illinois towns use six or seven labels for what are physically two things: a unit inside the main building, or a separate structure on the same lot. The label your town uses determines which code section governs you.

• Coach house: Chicago's term for a detached unit at the rear of the lot, often above a garage, governed by §17-9-0201-F. Evanston, Oak Park and Moline call the same structure a "detached" ADU, and Oak Park also uses "coach house."
• Conversion unit: Chicago's term for a basement or attic unit inside a principal building at least 20 years old. Evanston calls this an "internal" ADU and limits it to one level; Oak Park lists "basement or attic conversion"; Moline calls it an "interior" ADU and requires a separate entrance.
• Attached ADU: An addition sharing a wall with the house. Evanston, Oak Park and Moline each list it as a distinct type; Northbrook's code uses the label "AADU" (Northbrook).
• Garage conversion: The researched Illinois codes do not treat this as a separate category; a detached garage conversion falls under detached-ADU rules. Oak Park and Moline both require you to replace the displaced parking on-site. Builder Assembly Service reports most existing garage foundations cannot support a second floor (Assembly Service).
• In-law suite: A colloquial name, not a code term. If it has its own kitchen and bath and a separate entrance, it is a conversion, internal or interior unit under the relevant ordinance; Evanston's guidebook allows basement units with their own kitchen and bath.
• Granny pod or granny flat: A small detached unit, sometimes prefabricated. Capitol News Illinois used "granny flats" for the whole category (CNI). Legally it is a coach house in Chicago (22-foot height, 15-foot separation, apprenticeship contractors), a detached ADU in Evanston, Oak Park and Moline, and a prohibited dwelling in Naperville and Springfield.

Wilmette's 2019 handout uses yet another term, "accessory living unit," for all of the above (Wilmette).

The largest explicit ADU allowance in the towns covered here is an internal or attached unit in Evanston, which may exceed 1,000 square feet as long as it stays on one level and smaller than the primary dwelling. Every detached cap found in the research is 1,000 square feet or less: Evanston 1,000, Oak Park under 1,000, Moline 1,000.

Chicago's coach-house size is unsettled. The 2020 pilot ordinance capped coach-house units at 700 square feet (original ordinance). The current codified §17-9-0201-F contains no maximum floor area, and paragraphs 14 and 15 of the section read "Reserved." What does bind you is §17-9-0201-D, which limits all accessory buildings to 60% of the required rear setback; Abundant Housing Illinois says that typically caps a coach house at about 450 square feet on a standard lot (AHI). Confirm the current text with the Department of Buildings before you commit to a floor plan. No conversion-unit minimum or maximum size was found, and a pending ordinance, O2026-0026329, would exempt RS ADU areas from minimum lot area per dwelling unit rules (Councilmatic).

Height limits are tighter than size limits in most towns: 22 feet in Chicago, 20 feet in Oak Park, 20 or 28 feet in Evanston depending on roof type.

In the municipalities covered here, plan on obtaining a building permit for any ADU because the shed exemptions require no plumbing. Illinois has no statewide shed exemption; the model 2021 IRC exempts one-story accessory structures up to 200 square feet (ICC), but each jurisdiction adopts its own number. The thresholds retrieved for storage sheds:

Setback and zoning rules apply to a shed even when no permit does.

Chicago's path runs through three city departments plus Cook County. The Illinois Policy analysis, from an advocacy think tank, puts the 2024 average from application to permit at 121.6 days across 249 permits. The City's ADU process page gives this sequence:

1. Confirm zoning eligibility. In Chicago, apply for Department of Housing pre-certification through the portal; in Evanston, confirm your lot meets §6-4-6-10 and skip the variance; in Urbana, file a conditional use permit application for the Zoning Board of Appeals; in Moline, record the required covenant with the Rock Island County Recorder before applying for a building permit.
2. Hire a licensed architect or structural engineer. Chicago requires a plan-based permit with professional drawings for both conversion units and coach houses.
3. Submit for zoning review and the Department of Buildings permit. Chicago offers Self-Certification review for eligible projects (DOB); the permit fee deposit is non-refundable.
4. Arrange water and sewer. The Department of Water Management may require a new water service and meter whenever the unit count changes, sized under Plumbing Code §18-29-604. A coach house needs an easement recorded against the primary structure to protect its water service, and if it abuts a right-of-way with a water main it must connect directly to it (DWM). Developments with fewer than three units are exempt from Metropolitan Water Reclamation District permit fees unless a public sewer extension is involved (MWRD).
5. Pass inspections. DOB inspects the construction; DWM inspects the tap, with two inspections included in the tap fee and $500 per re-inspection.
6. Obtain a Certificate of Occupancy from the Department of Buildings.
7. File the Cook County Assessor's new-construction form within 30 days of occupancy or completion (Assessor). If the unit is an affordable conversion unit, register it with the City. In Evanston, register the unit and pass a rental inspection before any tenant moves in (Evanston).

A Chicago basement conversion runs $65,000 to $170,000 depending on whose numbers you trust and how deep you dig; a new coach house runs $150,000 to $350,000 before the overruns that turned one Wicker Park project from a $350,000 budget into nearly $700,000 in actual costs (Sun-Times). Fees are small by comparison.

The sections below cover permit and utility fees, project design and construction costs, and the potential effect on a Cook County property tax bill.

Permit, water and sewer fees in Chicago

The Department of Buildings' 2026 fee tables took effect January 6, 2026 (DOB). Water Management's 2026 amounts were not retrieved, so the water figures below are the 2025 schedule:

Two builder sources put total permits and fees at $2,525–$3,050 (ADU Zoning) and $14,000–$26,000 (General Construction Chicago); they do not reconcile, and the official tables above are the better guide.

Design, utilities and construction

Assembly Service estimates architecture and engineering for a coach house at $8,000–$40,000, and architect James Hall prices site-specific drawings from $14,000 (James Hall). Plans and engineering for a basement unit are cheaper, $2,000–$5,000 (Assembly Service).

The same Assembly Service page estimates $3,000–$8,000 to separate utilities so a basement unit has its own service and meters. Separate-meter connections to a coach house cost $15,000–$40,000 (ADU Zoning). Coach houses also need their own foundations (James Hall).

The construction ranges below come mostly from builder and guide pages without published methodology; the Chicago Association of REALTORS® figure and the permit-declared values are the firmer anchors:

Permit-declared values, which understate finished cost, were $100,000 for a North Center coach house in February 2025 and $140,000 for a Logan Square coach house with two-car garage in September 2025. Prefab coach-house shells run $125,000–$200,000 before foundation and utilities (ADU Zoning). Figures for Champaign, Peoria and Springfield in the research come from vendor pages and are not reliable enough to print.

Chicago's ADU Repair Grant Program offers a planning grant up to $10,000 and a construction grant up to $75,000 structured as a five-year forgivable loan (City).

What a Chicago ADU adds to your Cook County tax bill

Cook County assesses residential property at 10% of market value, multiplies by a state equalization factor, subtracts exemptions, and applies the local composite rate (Treasurer). The Assessor's office says an ADU "will increase the total assessable area of a property and, sometimes, its assessed value," with value per square foot usually unchanged, and that it does not raise neighbors' assessments (Assessor). A detached house that gains an interior unit may be reclassified as class 2-11, an apartment building with 2 to 6 units (Chicago Cityscape; class codes).

No source gives a typical ADU value premium, so the two added-value figures below are assumptions. The factor is the final 2025 Cook County multiplier of 3.0300 (IDOR); the 6.85% rate is a third-party median across Chicago tax codes for tax year 2025 (Cook County Tax Appeal), not an official parcel rate. The official 2024 City of Chicago composite rate was 6.618606% (Clerk).

The Home Improvement Exemption excludes up to $75,000 of added value for up to four years and applies automatically when you pull a permit (Assessor); the Assessor says ADU builders may in many cases qualify. At 2025 figures it is worth about $1,556.66 a year. Your first year is prorated from the occupancy or habitability date, and the factor and rate for the year your ADU is first assessed will differ from these.

Two exemption questions the research does not answer: no retrieved page says whether a new coach house gets its own PIN or stays on the existing one, and none says whether renting the ADU changes the $10,000 Homeowner Exemption, which requires owner occupancy as a principal residence (Assessor). One is clear: if you hold a Senior Freeze, the first-year EAV of the new unit is added to your frozen base (35 ILCS 200/15-172).