ADU rules in Chicago

Coach houses (detached backyard units, one dwelling unit each) are legal citywide as accessory buildings in RT and RM multi-unit districts and in B and C business/commercial districts (where they must occupy the rear 30 feet of the lot), under ordinance SO2024-0008918 (passed 2025-09-25, effective 2026-04-01), which replaced the 2021 five-pilot-area program; from 1957 to 2021 new ADUs were effectively banned citywide. On single-family RS1, RS2 and RS3 lots, a coach house permit can only be issued inside an ADU-Allowed RS Area designated in Section 17-7-0574; ten mapped areas exist in the current codification and City Council keeps adding areas by ordinance (amendments of 2025-12-10 and 2026-02-18). Form standards: 22-foot height cap, 15-foot separation from the principal building (reducible to 10 feet on lots under 125 feet deep by administrative adjustment), 3 feet of side clearance, and construction only after the principal building exists. DOH pre-certification comes first, and coach house builders must use contractors enrolled in US DOL Registered Apprenticeship Programs (2-44-106(g)(4)). The DOH requirements page reads the code as allowing one coach house per property.

Conversion units (attic, basement or other interior conversions and additions within the principal residential building) are allowed in all R districts and in B, C1 and C2 districts, but only in principal residential buildings that have lawfully existed for 20 or more years; newer buildings are ineligible. A single-unit house may add one conversion unit; larger buildings may add 33% of the dwelling units that have existed 20-plus years (fractions of 0.5 and up round up), so 5 to 7 existing units yield two conversion units and 8 to 10 yield three. On RS1 to RS3 lots the same ADU-Allowed RS Area gate as coach houses applies (17-9-0131(1)). In B and C districts other than B2, a conversion unit below the second floor is by right only if at least 40% of the ground floor fronting the public way stays commercial; otherwise it needs special use approval; units above the ground floor are by right.

The permanent ordinance sets no square-footage ceiling for either ADU type (the pilot-era coach house floor area cap was not carried forward). Coach houses are limited by form instead: 22 feet maximum building height with rooftop features also capped at 22 feet, one dwelling unit per coach house, and all accessory buildings combined may cover at most 60% of the required rear setback. Coach houses are exempt from floor area ratio and minimum lot area per unit standards, and may stand in the required rear setback (2 feet off a rear line on an alley unless at least 10 feet from the alley centerline). Conversion units carry no size limit; their number is capped by the 33% density formula of 17-2-0303-C and 17-3-0309, and the DOH requirements page states a single coach house is allowed per property, alone or with conversion units.

No citywide owner-occupancy rule: in RT, RM, B and C districts an ADU can be added to a rental or investor-owned building. In seven of the ten ADU-Allowed RS Areas (areas 2, 3, 4, 5, 8, 9 and 10 in the 17-7-0574 table), limitation (2) applies: if the principal building is a detached house, it must be owner-occupied at the time of DOH pre-certification, proven with a government photo ID plus a document such as a property tax bill, utility bill, recorded deed, mortgage statement or homeowner's insurance declaration. The text frames this as an at-application test, not a recorded ongoing condition. Areas 1, 6 and 7 currently impose no owner-occupancy limitation. Illinois has no state ADU statute overriding any of this.

Coach houses may be established without any accessory parking to serve the coach house (17-9-0201-F(4)), and conversion units may be established without corresponding accessory parking (17-9-0131(5)). Building a coach house can also unlock relief for the main building: parking required to serve the principal residential building on the same lot may be reduced or waived by administrative adjustment under 17-13-1003-OO. The DOH requirements page confirms no additional parking is required for either ADU type.

The base path is administrative everywhere: Department of Housing pre-certification under 2-44-106(g), which includes mailed notice to the local alderman (notice only, not a veto), then a standard Department of Buildings permit; no hearing. Two carve-outs go further. In RS areas flagged with limitation (3) in the 17-7-0574 table (currently areas 2, 8 and 9), an administrative adjustment under 17-13-1003-QQ is required; it is still a Zoning Administrator decision (a $500 fee, mailed notice to abutting owners, and at least a 10-day wait, but no public hearing). And a below-second-floor conversion unit in a B or C district (except B2 or C3) that does not keep 40% ground-floor commercial frontage needs discretionary special use approval before the Zoning Board of Appeals (17-9-0131(7)).

ADUs are meant to be rentable long term: the RS-area purpose clause cites unsubsidized affordable rentals and owner income, and outside some RS areas no owner-occupancy applies. Short-term rental is banned: conversion units may never be offered for transient occupancy (as defined in 4-6-290, i.e. Airbnb-style stays under 31 days), and neither may coach house units lawfully established after May 1, 2021 (older legacy coach houses are exempt), except to members of the owner's or tenant's household; fines run $1,500 to $3,000 per day and four departments may enforce, so ADUs cannot be registered as shared housing units. Where two or more ADUs are added to a building after May 1, 2021, in any combination over time, 50% of them (rounded down) must be registered with DOH as affordable at 60% AMI rents under a 30-year recorded covenant with annual compliance affidavits (2-44-106(d), (i)); a homeowner adding a single ADU triggers no affordability requirement.