Can I legally have a 2 flat "by right" in RS-3 Chicago?

[Biib Salmaas]

First of all, this is my first post, long time lurker.  I hope this is the right subforum.  

I am under contract for a property that is a 2 flat, converted to SFH. I would like to be an owner occupant of this property. There are 2 levels to the house and would require minimal work to separate the upper floor from the first floor.

In RS-3, am I allowed, "by right" to have this be a 2 unit property? The home started as a 3 unit, at some point was deconverted to a SFH (whether this is the actual zoning I do not know, there are no records for this property because its been tax exempt for 30 years). The seller has yet to provide a zoning certificate. I have tried to read as much as I can about this, but havent come up with a concrete answer. The block in question also has 60% + Multi unit density. I have an appointment with the Alderman but its not for a week. If this is going to be a legal nightmare for me to have this be a legal 2 flat, I do not want to proceed. Any help would be appreciated! Thank you!