ADU rules in Cook County

Unincorporated Cook County has no accessory dwelling unit use. Every residential district lists its accessory uses and then closes the door: 'All uses not expressly authorized in Sections 4.1.3 through 4.1.6 are prohibited' (and the parallel clause in each of R-2 through R-8). The two accessory uses that come closest are a private guesthouse, defined in Section 14.2 as a detached accessory building 'containing living quarters for temporary guests' whose 'quarters shall not be rented', and detached living quarters 'for persons employed on the premises and occupied only by such persons and their immediate family', which 'shall not be rented or otherwise used as a separate dwelling'. Section 8.3.5 independently bars a second house: 'A zoning lot shall not have more than one principal detached residential building.' A private guesthouse also needs a minimum lot area of 40,000 square feet and a minimum lot width of 150 feet, so even the guest-only version is out of reach on a typical suburban lot.

There is no internal or attached ADU as such, but in four districts a second unit is reachable by converting the house into a two-family dwelling. Section 14.2 defines a dwelling unit as rooms with 'individual bathrooms and complete kitchen facilities, permanently installed', a single-family dwelling as 'a building containing only one dwelling unit', and a two-family dwelling as one 'containing only two dwelling units'. R-1 through R-5 permit only 'Single-family detached dwellings' and group homes, so adding a second kitchen-and-bath unit inside the house creates a use the district does not allow. R-5A permits two-family dwellings and townhomes, and R-6, R-7 and R-8 permit two-family and multiple-family dwellings, so in those districts the conversion is a permitted use if the lot clears the density gate: 3,000 square feet per unit in R-5A, 5,000 square feet for a two-family dwelling in R-6, and 5,000 square feet per dwelling unit in R-7 and R-8. The result is legally a duplex, not an accessory unit, and it must also meet the district's minimum unit size (600 square feet per unit in R-6, 720 square feet in R-5A).

Because the ordinance authorizes no accessory dwelling unit, there is no ADU floor-area cap to quote. The limits that would bind an accessory building holding living space are Section 8.5: a detached accessory building 'shall not occupy more than 50 percent of a required yard area' and, when located in a required rear yard, 'shall not exceed the height of the principal building or 15 feet, whichever is less', with three-foot side and rear setbacks and at least ten feet of separation from the house. Every residential district also folds accessory buildings into the lot's floor area ratio ('Floor area ratio of accessory buildings shall be included in the total allowable floor area permitted on the zoning lot'), and those ratios for single-family dwellings run from 0.10 in R-1 and R-2 up to 0.25 in R-4, 0.40 in R-5 and 0.50 in R-5A. A private guesthouse or detached employee living quarters additionally needs a minimum lot area of 40,000 square feet in every residential district. Note that the district minimum-size rules are floors, not ceilings: structures 'occupied in whole or in part for residential purposes' must contain at least 1,000 square feet of livable floor area in R-1 through R-3, 900 in R-4, and 720 in R-5 and R-5A.

The Cook County Zoning Ordinance nowhere requires an owner to live on the property. Where a second unit is lawful, that is, a two-family dwelling in R-5A, R-6, R-7 or R-8, no occupancy condition attaches to the owner at all. What the ordinance does impose is tighter than owner occupancy in the accessory-building case: an accessory use is allowed only if 'it is operated and maintained under the same ownership and on the same lot as the permitted use', a private guesthouse holds 'living quarters for temporary guests' that 'shall not be rented', and detached living quarters are reserved for 'persons employed on the premises'. So the restriction runs to who may occupy the accessory space, not to whether the owner lives in the main house.

Section 11.2.6(A)(1) is flat: 'One-family dwellings and two-family dwellings. Two off-street parking spaces shall be provided for every dwelling unit.' There is no transit reduction and no ADU discount, so converting a house into a two-family dwelling in R-5A, R-6, R-7 or R-8 takes the lot from two required spaces to four. Article 11 gives no ratio for a private guesthouse or detached employee living quarters, since neither is a dwelling unit under the Article 11 definition ('one room or a suite of two or more rooms designed for or used by one family for living and sleeping purposes with only one kitchen or kitchenette'); Section 11.1.1(A) leaves the amount for uses 'not otherwise provided for in this ordinance' to the Department of Building and Zoning, the Zoning Board of Appeals and the Department of Highways, with the most stringent determination controlling. Residential parking must sit on the same lot as the use served. PA 104-0457, Art. 5 (People Over Parking Act), effective 6-1-2026, bars any unit of local government from imposing or enforcing minimum automobile parking requirements on a development project (a permit to construct or reconstruct a dwelling qualifies) within one-half mile of a public transportation hub (any rail transit station, or an intersection of 2 or more bus routes with combined 15-minute peak frequency) or one-eighth mile of a 15-minute-peak bus corridor, binds home-rule units (Section 5-90), and grandfathers site plans approved before the effective date (Section 5-15(b)). Multiple Metra and CTA rail stations sit within a half mile of unincorporated pockets, so the act plausibly voids the two-space minimum for new dwelling construction on some unincorporated lots, and Pace corridors could extend the zone where the 15-minute tests are met; parcel-level mapping and headway verification were not attempted.

There is no ADU permit, so what applies depends on the route. In R-5A, R-6, R-7 and R-8 a two-family dwelling is a permitted use, which means ministerial review: 'An application for a building permit shall be deemed an application for a zoning certificate', the Zoning Administrator reviews and approves it, and a certificate of occupancy issues after final inspection, with the department required to answer within 14 days. In R-1 through R-5 there is no ministerial path at all, because a second dwelling unit is simply not an authorized use; a homeowner would have to seek a unique use under Section 8.9.8, which is expressly for 'a use which is not expressly enumerated as a permitted, special, temporary or accessory use' and is decided through the special use procedure (Zoning Board of Appeals public hearing, published and mailed notice, County Board action), or seek a map amendment. A variance is not a substitute: Section 13.6.1 frames variances as relief from 'unreasonable difficulties or unnecessary hardships' in applying the ordinance, and the Zoning Administrator's administrative adjustment authority is limited to ten percent or less of bulk and location standards.

The two accessory living-space uses are written to block rental outright: a guesthouse's 'quarters shall not be rented' (sec. 14.2) and detached living quarters 'shall not be rented or otherwise used as a separate dwelling' (sec. 4.1.6(J)). Where a two-family dwelling is a permitted use (R-5A, R-6, R-7, R-8), both units may be rented long term, and the Cook County Residential Rental License Ordinance applies in unincorporated Cook County: 'It shall be unlawful for any Person to rent any Dwelling Unit unless the Owner has obtained a current Residential Rental License from the Department', although that article defines Dwelling Unit by reference to a Multi-Unit Building of 'four or more Dwelling Units'. Short-term rental of a dwelling unit is regulated in the zoning ordinance itself: a stay is 'at least 24 hours but shall not exceed 30 consecutive calendar days', a Supervised short-term rental (host present) is an accessory use needing a home-based business occupancy and may use no more than 25 percent of the floor area of any one story, and a Non-Supervised short-term rental (host absent) 'shall obtain a special use', is capped at six bedrooms, may not run more than one reservation at a time, and loses its special use if ten percent or more of the dwelling unit's ownership changes.