Looking for Chicago code defining a basement unit

[Alex F.]

Hi, I'm looking for someone who could point to Chicago code defining a basement unit, or someone who may have experience or knowledge regarding this type of situation.

When I purchased my multiunit house, the seller provided a a Zoning Certificate indicating 3 dwelling units. This was the expected number of units. However, at the time, I had not noticed that the certificate specifies that the dwelling units are on the first, second, and third floor. In reality, they are at the garden level and the two floors above.

The garden unit is only about 2 feet below grade and is otherwise a regular unit... full size windows in every room, high ceiling, front and back door, separate utilities and ventilation. I am wondering if this can actually be classified as the first floor unit. 

Does anyone know what specific criteria classify a unit as a "basement" vs a "first floor" unit? Is there something that says that a first floor must be 100% above grade?

Thanks for your help.