Everything homeowners ask about building an ADU in Chicago — answered with 2026 data.
Yes, ADUs are allowed in By right in every RT (two-flat, townhouse and multi-unit) and RM (multi-unit) district citywide and in the B1, B2, B3, C1 and C2 business and commercial districts (use tables 17-2-0207 rows 7-8 and 17-3-0207 rows 5-6). In RS1, RS2 and RS3 single-family districts only on lots inside one of the ten ADU-Allowed RS Areas defined in Sec. 17-7-0574 (as replaced by SO2026-0022453, effective April 1, 2026), which the City says cover parts of 34 of the 50 wards; an RS lot outside those areas cannot get a permit (17-9-0131(1), 17-9-0201-F(1)). Shown as not allowed in C3, and the ordinance did not amend the use tables of the downtown D or manufacturing districts. In B and C districts a coach house must sit in the rear 30 ft of the lot, and a conversion unit below the second floor is by right only if 40% of the ground-floor frontage stays commercial (B2 excepted); otherwise it needs Zoning Board of Appeals approval under 17-9-0131(7) in Chicago, IL. The maximum size for a detached ADU is any size that fits the coach house bulk rules; Chicago sets no floor-area cap since the pilot's seven-hundred-square-foot dwelling-unit limit was repealed with the citywide ordinance (effective April 1; see recent changes). What sizes the building: 22 ft height limit, one dwelling unit per coach house, a 15 ft gap behind the main house, the rule that all accessory buildings together may cover no more than 60% of the required rear setback area, and a 3 ft open strip along one side. Coach houses are exempt from floor-area-ratio, lot-area-per-unit and rear-yard open-space minimums but may not eat into open space the main building needs. Attached ADUs can be up to any size the existing building can provide; a conversion unit has no floor-area cap in the zoning code. It must be inside a principal building that has lawfully existed for at least 20 years. Chicago has an ADU-friendliness score of 7/10. Check our full Chicago ADU rules page for complete zoning details.
ADU construction in Chicago typically costs $150,000-$350,000. The cost per square foot averages $200-$400. Garage conversions are more affordable at $75,000-$200,000, while prefab ADUs cost $125,000-$250,000. Permit fees add $2,525-$3,050 for a new coach house in 2026 permit records (DOB building permit fee minimum $2,450 for new residential construction in Table 14A-12-1204.3(3), plus a $75 zoning review fee on most permits; a few paid $1,500 zoning fees). $1,875-$3,150 for a conversion unit (minimum $1,800 for a permit that adds a dwelling unit, Table 14A-12-1204.3(4), plus zoning). The formula is construction factor x review factor x floor area (14A-4-412.2.2.1; $0.44 per sq ft x 0.75 for a wood-frame Group R coach house), so the minimums govern any ADU-sized project. A $300 deposit is paid at application (14A-12-1204.1). DOH lists no pre-certification fee; an affordable ADU costs $500 to register to your budget. See our detailed Chicago ADU cost guide.
Detached ADUs in Chicago can be up to any size that fits the coach house bulk rules; Chicago sets no floor-area cap since the pilot's seven-hundred-square-foot dwelling-unit limit was repealed with the citywide ordinance. Attached ADUs are limited to any size the existing building can provide; a conversion unit has no floor-area cap in the zoning code. It must be inside a principal building that has lawfully existed for at least 20 years. Junior ADUs (JADUs) have a cap of Not allowed as a separate category: Chicago has no junior ADU. A small unit carved out of the house is a conversion unit and counts toward the 33% conversion-unit allowance. Maximum height is 22 ft for a coach house, measured as building height under 17-17-0311, and no rooftop feature may rise above 22 ft either (17-9-0201-F(5)-(6)). Any other accessory building in a required rear setback is capped at 15 ft (17-9-0201-E). A conversion unit takes the height of the existing building. You are allowed up to one coach house per lot on a qualifying lot.
Rear setback: 2 ft from the rear lot line where the lot backs onto a public alley, waived if the coach house is at least 10 ft from the alley centerline as shown on a survey (17-17-0309). A coach house is one of the few structures allowed inside the required rear setback, but all accessory buildings together may occupy no more than 60% of that setback area (17-9-0201-D). A minimum 15 ft separation is required between the rear wall of the main house and the front wall of the coach house; on lots less than 125 ft deep the Zoning Administrator can cut it to 10 ft by administrative adjustment (17-9-0201-F(8), 17-13-1003-PP). Side setback: At least 3 ft of open, unobstructed space between the coach house and one side lot line for the full length of that wall, not required where that side line abuts an alley or street (17-9-0201-F(7)). A coach house may not encroach into a required side setback (17-17-0309); the 2021 amendment (O2021-1228) limited the RS side-setback table to the principal residential building, so ask DPD zoning review how the other side is treated on your lot. Front setback: N/A: a coach house is a rear accessory building and may only be built after the main house exists (17-9-0201-F(13)); it may not encroach into the front setback (17-17-0309). In B and C districts it must be within the rear 30 ft of the lot depth (17-9-0201-F(2)). Note that garage conversions may be exempt from setback requirements if the existing structure is already within the setback area.
None required for a conversion unit or a coach house (17-9-0131(5), 17-9-0201-F(4)). Parking the main building must keep may be reduced or waived by administrative adjustment when a coach house is built (17-9-0201-F(3), 17-13-1003-OO), so a garage can become the coach house's ground floor or be removed. Many cities and states have been relaxing ADU parking requirements in recent years. In California, no additional parking is required for ADUs near transit.
Depends on the zone. Not required on RT, RM, B or C lots or in ADU-Allowed RS Areas 1, 6 and 7. Required at pre-certification when the main building is a detached house in RS Areas 2, 3, 4, 5, 8, 9 and 10 (17-7-0573(2), 17-7-0574): DOH asks for a government photo ID plus a tax bill, utility bill, deed, mortgage statement or insurance declaration for the address. Nothing in the ordinance requires owner-occupancy after the permit issues. Note that owner-occupancy rules vary by state law — California and several other states have eliminated this requirement for standard ADUs.
The ADU permit review timeline in Chicago is typically not published by DOH or DOB. City permit records (Chicago Data Portal, permits issued January-September 2026 for a coach house, conversion unit or ADU) show Standard Plan Review permits issuing a median 120 days after the application date (middle half 78-183 days) and Self-Certification permits a median 58 days (35-98 days); DOB says self-certified applications typically issue within 10 business days after zoning approval. Add the DOH pre-certification review before either. Plan check: DPD zoning review comes first on every application; DOB then routes the drawings to its technical reviewers (building, structural, electrical, plumbing, mechanical, fire) through ProjectDox, with the balance of the permit fee due when the permit is ready. Plans must show the DOH pre-certification letter, the coach house height (22 ft), the 15 ft separation, the 60% rear-setback coverage calculation and the alley setback. Chicago has no ADU-specific plan checklist; the Standard Plan Review flowchart and the Self-Cert program pages are the published guidance. Pre-approved plans availability: Not available. Chicago has no pre-approved coach house or conversion-unit plan program as of September 2026; DOH's resources page lists guides (the NHS Better Basement Units Guide and a ULI video series) rather than plan sets, so budget for custom architectural drawings.
Average ADU rental income in Chicago: 1-bedroom units rent for $1,500-$2,200/month, 2-bedroom units for $1,800-$2,600/month. Short-term rentals: No. A conversion unit, or a coach house established after May 1, 2021, may not be rented, leased or offered for transient occupancy as defined in Sec. Expected ROI: 8-12% gross rental yield on a $175,000-$300,000 coach house renting at $1,600-$2,200/mo, before taxes, vacancy, insurance and financing; a $65,000-$150,000 basement conversion renting at $1,400-$1,900/mo returns 12-25% g.
The minimum lot size for an ADU in Chicago is none: conversion units and coach houses are exempt from the minimum-lot-area-per-unit rules of 17-2-0303-A, 17-3-0402-A and the overlay districts (17-9-0131(3), 17-9-0201-F(10)), and coach houses are also exempt from floor-area ratio (17-9-0201-F(12)). The practical limits are the 60% rear-setback coverage rule and the 15 ft separation, so a shallow lot may need the 10 ft administrative adjustment. This applies to detached ADUs — garage conversions and JADUs may have different or no lot size requirements.
Garage conversions are one of the most popular and affordable ADU options. In Chicago, a garage conversion typically costs $75,000-$200,000. The key advantages are lower cost (no foundation work), potential setback exemptions, and faster permitting. Read our complete garage conversion guide.
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