Illinois ADU laws are set almost entirely at the local level. The state has no statute guaranteeing your right to build an accessory dwelling unit, so whether you can add a basement apartment, an in-law suite, or a backyard coach house depends on your city’s zoning code. Chicago runs the most developed program in the state and allows both interior conversions and detached units across a growing portion of the city. Everywhere else, you have to read your own municipality’s ordinance.
Why Illinois Has No Statewide ADU Rule
Illinois gives municipalities broad authority to regulate land use, classify buildings, and set construction standards within their borders.1Illinois General Assembly. Illinois Compiled Statutes 65 ILCS 5/11-13-1 Each city or village decides on its own whether to allow ADUs, how big they can be, and where they can sit on a lot. No state law requires a municipality to permit them, and no state law sets minimum standards.
A pending bill, HB 1813, would change that. It would prohibit municipalities from banning ADUs outright, allow only reasonable regulations on size and location, and limit home-rule powers so that even home-rule cities could not block ADUs entirely. As of early 2026 the bill remains in committee. Until it passes, the patchwork stands.
One clarification worth making because it circulates online: Public Act 102-1051 is sometimes called an “Accessory Dwelling Unit Act.” It is not. That act amended laws on clinical laboratories, insurance, and pharmacy practice and has nothing to do with housing.2Illinois General Assembly. Public Act 102-1051
What Chicago Allows
The Chicago City Council first approved an ADU ordinance in 2020, creating five pilot areas on the South, West, and Southeast sides. The program has since expanded, and ADUs are now permitted in additional wards where the alderperson has opted in. Two types of units qualify:
- Conversion units built inside or attached to the existing building, such as a basement apartment, an attic unit, or a space above an attached garage.3City of Chicago. Additional Dwelling Units (ADU) Ordinance
- Coach houses, which are detached structures in the backyard separate from the primary residence.3City of Chicago. Additional Dwelling Units (ADU) Ordinance
Both types must include independent space for living, sleeping, cooking, and a bathroom. The ADU must be reachable without passing through the primary home.
Owner-Occupancy and Who Can Rent It
Owner-occupancy is not a blanket rule. In the South, West, and Southeast pilot areas, properties with one, two, or three dwelling units must be owner-occupied at the time the ADU permit is issued. When an alderperson opens RS-zoned blocks outside the pilot areas to ADUs, they decide whether to attach an owner-occupancy requirement for detached houses on those blocks.
Short-term rentals through platforms like Airbnb are flatly prohibited in any ADU permitted under the ordinance. Tenants have to occupy the space as a primary residence. The ordinance exists to add permanent housing, and the city enforces it on that basis.
Affordability Requirement
If a property adds two or more ADUs after May 1, 2021, the owner must keep half of those newly added units affordable. Rent on the affordable units cannot exceed 30 percent of income for a household earning 60 percent of the area median income, as published annually by the Department of Housing. The affordability obligation runs with the land for 30 years from the date of recording and binds any later owner.4Municipal Code of Chicago. Chicago Municipal Code 2-44-106 – Additional Dwelling Units
Size, Setbacks, Parking, and Fire Safety in Chicago
The specific dimensions depend on the property’s zoning classification (Chicago uses RS, RT, and RM designations for residential districts) and which type of ADU you build.
Size and Height
Coach houses are limited to 60 percent of the property’s required rear setback area. On a standard Chicago lot where the rear setback runs about 30 feet, that works out to roughly 450 square feet of living space. Larger lots support larger coach houses. The earlier pilot program had capped coach houses at 700 square feet, but that cap was removed when the ordinance became permanent.5NHS Chicago. Additional Dwelling Unit Grant Program Coach house height is capped at 22 feet to the top of the highest feature. Conversion units are not separately height-restricted because they sit inside the primary structure.
Setbacks
Side setbacks vary by zoning district. In RS1 zones, the two sides combined must equal 30 percent of lot width, with neither side less than 5 feet. RS2 follows the same formula but drops the minimum per side to 4 feet. In denser RS3 zones, the combined total is 20 percent with a 2-foot minimum per side. Rear setbacks for detached houses are 28 percent of lot depth or 50 feet, whichever is less.6American Legal Publishing. Chicago Zoning Ordinance 17-2-0300 – Bulk and Density Standards
Parking
Chicago does not require additional off-street parking for any ADU. That removes one of the biggest practical obstacles, particularly on narrow urban lots.
Fire Safety
Every residential unit, ADUs included, needs at least one smoke alarm on each level containing a habitable room. The alarm goes on the ceiling or within 4 to 12 inches of the ceiling on a wall, and within 15 feet of every room used for sleeping.7Municipal Code of Chicago. Chicago Building Code 13-64-130 – Smoke Alarms Location Carbon monoxide detectors are also required. Attached ADUs sharing a wall or floor-ceiling assembly with the primary home generally need fire-rated construction between the two living spaces. A one-hour rating for shared walls is a common standard in residential construction.
How Chicago’s Permitting Works
Chicago uses a two-step system. First you pre-certify with the Department of Housing. Then you apply for a building permit with the Department of Buildings.
Pre-Certification
Apply online through the city’s web portal. The application asks for property information and proof of owner-occupancy where it applies. When you submit, the system automatically notifies the alderperson’s office with your name, contact information, and project address. Department of Housing staff review the application and email you an approval or denial.8City of Chicago. How to Apply for an ADU
Building Permit
Once pre-certified, you submit your approval letter with a building permit application through the Department of Buildings.8City of Chicago. How to Apply for an ADU You’ll need detailed architectural plans covering fire exits, plumbing, ventilation, and how the ADU connects to existing structures. Inspectors visit at key milestones: foundation, framing, and final systems. Passing all inspections produces a Certificate of Occupancy, which clears a tenant to move in.
Costs, Taxes, Financing, and Insurance
Construction costs in Illinois run roughly $150 to $400 per square foot for new ADU work, meaning a 450-square-foot coach house might cost $67,500 to $180,000 before permits and design fees. Interior basement conversions tend to be the cheapest route and ground-up coach houses the most expensive. Professional architectural plans for a custom ADU typically run $3,500 to $12,000 or more. Chicago permit and plan review fees for a new structure can reach into the tens of thousands of dollars, so get a written estimate from the Department of Buildings before you lock in a budget.
On property taxes, building an ADU increases your property’s assessable area and in most cases its assessed value, but the Cook County Assessor’s office has clarified that the assessed value per square foot generally stays the same after adding an ADU. Construction on your property will not raise assessments for neighbors who haven’t built one. You may also qualify for a Home Improvement Exemption that offsets the tax bump from the added square footage for up to four years, and filing for it promptly after construction is one of the easiest ways to reduce the near-term hit.9Cook County Assessor. Assessor Kaegi Backs Proposed Chicago ADU Expansion
Few homeowners can pay cash. Two federal loan programs accommodate ADU work. Fannie Mae’s HomeStyle Renovation mortgage lets borrowers build a new ADU on a one-unit property they are buying or refinancing, provided the property stays a single-family home; two- to four-unit dwellings and properties with multiple ADUs are ineligible. The unit must have independent living, sleeping, cooking, and bathing space and be accessible without passing through the main home.10Fannie Mae. Accessory Dwelling Units Once the ADU is built and occupied, rental income from it can count toward qualifying income on a later purchase or refinance, up to 30 percent of total qualifying income, documented on a Single-Family Comparable Rent Schedule (Form 1007).11Fannie Mae. Rental Income – Fannie Mae Selling Guide FHA 203(k) rehabilitation loans can also finance an ADU, but only for attached or interior conversions. Detached coach houses do not qualify, the property must be your primary residence, all work must be done by a licensed contractor, and projected future rental income cannot help you qualify.
If you rent out the finished ADU, the IRS treats it like any other residential rental. You report income and deductible expenses on Schedule E of Form 1040, and the construction cost itself is recovered through depreciation rather than deducted upfront.12Internal Revenue Service. Publication 527 (2025) – Residential Rental Property Passive activity loss rules can also limit your ability to deduct a rental loss against other income. The interaction of depreciation, Section 179, and passive-loss rules is complicated enough that working with a tax professional familiar with rental property pays for itself.
Call your insurer before construction begins. A standard homeowners policy’s “other structures” coverage may not reach the full replacement cost of a detached coach house, and renting the ADU usually requires a landlord endorsement or separate landlord policy to cover tenant-related risks a homeowners policy excludes.
If You Live Outside Chicago
The ADU picture outside Chicago is fragmented. Some suburbs and smaller cities have updated their zoning codes to allow ADUs in certain residential districts. Many have not, and some effectively prohibit them through large minimum lot sizes, heavy setback rules, or bans on secondary kitchens in accessory structures. Home-rule municipalities have particularly broad latitude.1Illinois General Assembly. Illinois Compiled Statutes 65 ILCS 5/11-13-1
Start with your municipality’s zoning ordinance and land-use map. Search for “accessory dwelling unit,” “accessory apartment,” “in-law suite,” or “coach house.” If none of those terms appear, call the local planning or zoning department. Some communities allow these units through a special-use permit or planned development process even when the code is silent. If HB 1813 passes, that process would be simplified statewide. Until it does, every municipality makes its own call.