I have noticed the City of Chicago has changed their policy on legal non-conforming. Previously you could easily get these units legalized and keep them legalized. I have now noticed several cases where we have X number zoning doc from few years ago and when go to pull a new doc the city refuses to re-issue with X number and goes 1 less. I want to keep addresses private for client confidentiality but had this happen in bronzeville on a 4 unit city claimed 3 even with our 8 year old 4 unit doc, Avondale old doc from 2 years ago said 3 but city argued it was a mistake and it's 2 now, Logan Square 3 unit from 7 years ago but city refused to give us a 3 unit doc saying it's a 2 (this one I think I could of legalized though if put the time in). When this happens clients have option of closing or killing deal as your attorney puts a X zoning doc contingency, typically you will renegotiate down the price if still comfortable in the deal. 

I believe this change in policy is due to ADU soon being allowed throughout entire city, so they want to get permit fees, etc. vs giving out a quick X unit doc that produces no city revenue. You can view some of these legal 2 units but that clearly were 3 units as a long term play. If look in permits and past permits show 3 units or FOIA water bills show 3 you have a strong shot at either ADU these or as city policies revert back/with an attorney legalizing to create equity.