What issues can be caused by a bonus unit?

[Ben Staples]

I'm looking at a multifamily 4 unit with a "bonus" basement unit in Illinois.  Cash flow wise, it only really makes sense to do the deal with that bonus units cashflow contribution.  

• Are there issues that could come up with the closing?
• Are there issues that could come up down the road?  I.E. if I needed to renovate?
• Should I underwrite this as a strict 4 unit, and call the 5th the icing on the cake?