Chicago basement/ garden unit legality question

[Dooreuhn Cee]

I am looking at 2-unit apartment buildings and many of them have a "3rd" unit as a garden or basement.  Because they are out of compliance with zoning, it seems that there could be some exposure to city violations, etc.

One solution is to incorporate the garden into the 1st floor unit as an apartment with a basement.  This has some expense in adding a staircase to the living room.  But the basement is part of a legal unit and thus voids the illegal unit issue.  The rent on a 3rd unit garden could be +$500 to the cash flow, while the rent for on the 1st floor for an extra 2-bedrooms and extra bathroom in the finished basement could be +$300 to the cash flow.  So for -$200 the legal issue disappears along with the extra tenant.

Here is where I need help.  I had an idea to rent out the 1st floor unit and give them the basement as well as related living, but without adding an internal staircase.  In other words, the 1st floor tenant will get keys to a 1st floor apartment and also have keys to the finished basement.  This is great for privacy too.  

Just wondering if that idea would bring me in compliance with zoning as a 2-flat.