Hi Darin......As Eric said, the first thing to consider is if the building is zoned for the correct number of units that would include the garden unit. Oddly enough, I inspected a garden unit condo in a vintage building that had only one exit. I was certain that it was 'illegal' but just to make sure I called the Building Dept. The city inspector who answered my call told me that if the unit was 800 sq. ft. or less it didn't have to have a second exit. I was pretty surprised to hear that since it violates common sense and the building codes of pretty much everywhere else in the country. I used to investigate the cause and origin of fires for the CFD and I have seen the tragic results of inadequate exits in basements and attics. The 800 sq. ft. figure did NOT include bathrooms and closets so the true sq. footage could be 1,000 or so and still only one exit may be required. The City of Chicago and the State of Illinois have a long way to go in protecting the home owning and home buying public.

Also a low ceiling height and not having entrances/exits at both ends of the unit may make it illegal. Some owners that know they have an illegal apt may market it as an "in-law" or related family living space. Technically, no rent can be collected.

Buildings are zoned by the city of Chicago to have a certain number of units. A building that has 3 floors and a basement unit that has been zoned for 4 units would be a legal garden apartment. If the building has been zoned for only 3 units and there are 4 units in the building, most likely the garden would be considered illegal.

Similarly if a single family home has a separate apartment in the basement and the home is not zoned as a 2-unit building it has an illegal garden apartment.

There may be other requirement including a garden apartment requiring 2 exits, perhaps someone else can chime in on that.

If you have any doubts about whether an apartment is illegal you can request Zoning Certification from the city of Chicago to see whether a residential building is zoned for 1, 2, 3 or 4 units.