@Alex Martinez - +1! I wanted to ask the same clarifying questions you posed, and my understanding was also the same in regards to "It seems legal nonconforming means a unit was established in accordance with zoning code effective at the time it was built but is not in accordance with the current zoning code (i.e. legal unit that was grandfathered in)", but I am not very confident in that assessment.
@John Warren @Jonathan Klemm - I want to reframe Alex's question a little differently as well: is the main difference between a non-conforming illegal vs. legal unit that one was created originally with the proper permits pulled and one was not?
Scenario A: a building was zoned in a such a way originally that allowed the conversion of an extra garden unit. At that time in the past, the owner applied for the permits to finish a basement and use it for extra income (with separate utilities hookups, etc.). Years later, the zoning was changed but since it was done legally to begin with, it's 'grandfathered in' and marked now as 'non-conforming' yet legal?
Scenario B: same situation as above, but this time the owner did not pull permits. Same building, same construction, same owner, same code changes, only difference was the owner was too lazy to get the permits. Now I want to buy this building. I must somehow find out if permits were originally pulled because if not, it's now non-conforming and illegal unit, no?
@Jonathan Klemm - you said everyone has illegal units and I have certainly seen that written elsewhere many places too. So it's not really that risky for me to go into a property with an unfinished basement (within Chicago limits) and do the conversion without permits? Also, how can you tell if a property's zoning may even just allow legally finishing the basement or not? Better to do it the right way if you can, I'd think. When I look at the zoning map at areas I know are littered with 2 and 3 flats, almost everything is RS-3