Legal Question in Real Estate Law in Illinois

Unfortunately this is too little information to be of help. If the rental is on the same zoning lot, or separate lot, is an issue. It may only be a building code issue, or a zoning issue - and you could be doing something that could affect how the two buildings are taxed for property tax purposes. So I think the first step would be to bring surveys of the house lot and rental lot (if they are separate lots) or whatever they are, to an attorney to review with you.