Existing Illegal Basement unit remodel and short term rental

[Matt Smith]

I am at the beginning of remodeling a multi-family house and looking for assistance/opinions on renting out garden unit at AirBnb once rehab is done.

The house is build before 1920s and has two condos (1st and 2nd floor). Previous owners converted basement space into livable space (kitchen, shower, bathroom), but it does not comply with legal ADU unit standards (one separate entry, 6'10ft ceiling). House is marked as RS-3 zoning. I am looking to do some updates and touch ups in basement unit as well.

While the house zoning requirements comply with ADU requirements, the basement is not up to ADU standards (ceiling is too low and one entry). Are there any exceptions in place for the houses that are that old and has existing converted unit in the basement? 

Digging down is not really an option for me because of additional cost. Not looking for long term rental in the basement so going through all the headaches of converting into legal ADU are not worth it. I am looking if there are some exclusions that apply in my case. My brother would be staying 3-5 months of the year, while other time I would like to rent it out on Airbnb. I have seen and been in Airbnb in various conditions/sizes, would that be a problem?

Experience and opinions are appreciated. 

House is in Chicago Area