Mount Prospect, IL Accessory Structures: ADU Rules (2026)

Section 14.312 of the Village Code, titled Elimination of Unpermitted Second Housekeeping Units, is the Village of Mount Prospect's rule on garage apartments, basement apartments and similar second units. It does not create a permit route for an accessory dwelling unit. It prohibits the unit unless an exception applies.

The prohibited acts are broad. No person can enter into a lease or rental agreement of any kind, or otherwise maintain or permit for living purposes, an unpermitted second housekeeping unit. No person can establish, install or construct one within the corporate boundaries of the Village. The section also reaches conduct after January 1, 2000, barring any lease or rental agreement or use for living purposes of such a unit.

The exceptions are specific: group community residences and family community residences as defined in section 14.2401; housing for religious institutions maintaining a house of worship in Mount Prospect when it is for ministry, outreach or program personnel; residences built or modified as two flats or two apartment buildings before May 2, 1944 that have not been converted to a single dwelling unit; any residence officially sanctioned by the Village as a two flat by variation, special use or other action of the president and board of trustees; otherwise permitted sleeping rooms or separate living quarters without second kitchen units; and units with free and unlimited physical access to the rest of the dwelling unit. If the pre-1944 or sanctioned status is disputed, the owner carries the burden of proof by clear and convincing evidence.

A lock on the only door, or hinges on the only passageway, between the second unit and the rest of the home raises an irrebuttable presumption that free and unlimited access is missing. Nothing in the section permits more than one family to occupy a dwelling unit. Real estate brokers and agents cannot advertise a residence as usable contrary to the section. A sworn complaint from a Mount Prospect resident, plus one other indication such as separate trash pickup or separate utility meters, is probable cause for a village official to seek an administrative warrant to inspect.