Legal Non Conforming Two Flat

[John Travis]

I'm going to look at a legal non-conforming two flat in a suburb just outside of Chicago. I looked up the zoning map for the particular city and it is zoned in a single family residence area. What do I need to watch out for? Is the property grandfathered in or would it have to be converted to single family after purchase? I know this likely varies from town to town, but just wanted to know what to look out for be diving in. Thanks