Legal 2 Flat with 3rd (Illegal) Attic Apartment

[Ryan Redenius]

I'm looking for a 3 flat in Chicago as my first investment property and recently came across a legal 2 flat with a 3rd illegal apartment in the attic.  Currently all 3 unites are occupied.

I know there are some risks with this as an investor but that it also goes on quite rampantly.  I'm not seriously considering this property right now but was curious if anyone had any experience or insight into renting out an illegal 3rd unit.  What sort of questions should I be asking?  

Thanks in advance!