Any developer who proposes a residential project with 10 or more units and receives either land or financial assistance from the city, a zoning change that increases density, or is in a downtown planned development or a TOD project, must provide some number of housing units designated as “affordable” and subsidize the cost of those units.
If it’s a zoning change, TOD, or planned development, the developer must provide 10 percent affordable units, and if they receive assistance, 25 percent must be affordable.
Developers must build at least 25 percent of them on site (in the same building or nearby for downtown projects). The developer can pay an “in-lieu” fee for the remaining 75 percent of the units.
Use Chicago Cityscape to determine the in-lieu fee for any address.

Look up any Address Snapshot and we’ll tell you the ARO status of low, high, or downtown, and remind you of the fee.
In the 29 community areas that are designated as “low to moderate income”, that fee is $50,000 per unit. In the 44 community areas and parts of three others labeled “high income”, the fee is $125,000 per unit. In the Loop community area and the remaining parts of those three community areas, the fee is $175,000 per unit.

Prior to the City Council’s change to the Affordable Requirements Ordinance in 2015, the fee was $100,000 per unit.