Frequently Asked Questions
Illinois ADU answers.
Are ADUs legal in Chicago?
Yes. After banning coach houses outright in 1957, Chicago's City Council voted 46-0 on September 25, 2025 to legalize ADUs citywide (ordinance SO2024-0008918). The ordinance takes effect April 1, 2026, expanding the initial eligibility area from about 12% to 29% of the city — all multifamily and mixed-use zoning districts plus residential pilot areas — with room to grow further as individual alderpeople opt single-family blocks in.
How much does an ADU cost in Illinois?
In Chicago, total ADU project costs typically run $80,000 to $300,000+ depending on unit type. Basement and attic conversions tend to run $80,000–$150,000, while detached coach houses can reach $200,000–$350,000+ once utility connections and finishes are included.
Does Illinois have a statewide ADU law?
NationwideADU's September 16, 2026 review classifies Illinois as local control; no statewide ADU siting mandate is in effect. City, county, zoning, building, utility, and site rules control. A statewide preemption bill, SB 4071 (the BUILD Act), would require every municipality to authorize ADUs by ordinance beginning January 1, 2027, with no added lot-size, setback, aesthetic, or parking requirements — but it remained in the Senate Assignments Committee, without a floor vote, as of this review. Local zoning, building, utility, site, and private-covenant requirements may still apply; verify the current rules for the property before design or construction.
What is Illinois's BUILD Act (SB 4071)?
SB 4071, the BUILD Act, is a statewide zoning-preemption bill introduced February 24, 2026 that would require every Illinois municipality to authorize one ADU per single-family lot by ordinance beginning January 1, 2027, with no added lot-size, setback, aesthetic, parking, or familial-relationship requirements. It gained several Senate co-sponsors through March 2026 but remained in the Senate Assignments Committee without a floor vote as of this review — it has not become law.
How long does it take to get an ADU permit in Chicago?
The representative ADU permit-review planning timeline for Illinois is ~4 months. This is not a statewide deadline or guarantee. Timing can change with the local authority, project complexity, corrections, utilities, site conditions, and application completeness; confirm the current timeline locally.
Can I build both a coach house and a basement conversion in Chicago?
No. Under Chicago's ordinance, a property with 1–4 existing units can add one coach house OR one conversion unit — not both. Properties with 5+ units can't add coach houses but may add conversion units up to 33% of existing legal units.
What is the cheapest ADU to build in Illinois?
Basement and attic conversion units in Chicago are generally the least expensive option, often $80,000–$150,000, since the building shell already exists.
What is the ROI on an ADU in Illinois?
The adopted Illinois one-bedroom planning range is $775–$1,450 per month, derived from the HUD FY2026 revised FMR market basket under ROI-CM-1.4. Payback is calculated from the homeowner's budget, ADU type, size, rent range, and a 30% operating-expense reserve. Property value uses a 15% national planning input with a 10%–20% uncertainty range; it is not a valuation.
