You will need a zoning attorney, and be able to prove a long standing use of the unit as an apartment continuously back over 20 years or longer.
It will be inspected and will need to meet current code, if it is re-done, there will be a search for permits being pulled for the work, all very doubtful you can get this and meet the standards.
Move onto another building.
Start with the zoning certificate. I too had this happen where there was a question in regards to the zoning of a 2 flat and the certificate was issued as a 3. I would highly recommend you speak with the attorney handling your closing. The certificate is your starting point! If that doesn't solve your problem go to the alderman's office and work with them.
Another issue that may arise, if the above doesn't work & if it is brought to light, is when & was the work done legally meaning with permits. If not, you may still be able to convert however, now you may have to get permits for deconstruction and reconstruction....too many details for this forum.
Exercise due diligence and good luck!
I'm guessing this is an older two-flat that also has a finished basement. Over the years, it was quite common for property owners to add units to a two-flat to meet local demand. City inspectors overlooked this practice for a long time (for political reasons) until a series of fires in converted properties resulted in a number of deaths.
Regarding your question, the chances of legalizing the third unit depend on a number of factors, including:
--the year the conversion was completed
--the number, configuration, and location of exits to the third unit
--the lot size of the property
--the nature of other properties in the immediate vicinity of this property
--the policy of the local Alderman, whose support you'll need
A few years ago, the City of Chicago did a complete revamp of the Zoning and Building Codes which are administered, respectively, by the Zoning Ordinance Administration Division of the Department of Housing and Economic Development and the Department of Buildings. The Zoning Ordinance Administration Division establishes permitted uses, while the Department of Buildings enforces the code.
During the rewriting of the Code, the city decided to give certain relief to property owners who have buildings with units constructed long ago to meet post-WWII demand for housing. An ordinance offered to grandfather and legalize any dwelling unit that the property owner could prove was in continuous use since a Water Department survey of all city properties in 1957, with the caveat that the unit must have met all other building code requirements in effect at that time for things like ceiling height, required number of exits, etc.
But how to prove this? In practice, there were relatively few units legalized using this procedure, since the ownership of property changed over time and the current owner could not certify continuous use since 1957. You can read an interesting discussion of resources to certify use for historical purposes provided by the City on this page:
It explains how to get documents that show the original nature of the property and any legal changes made to it.
You can also make a guess at the potential for legalization by following the guidelines in Sections 17-13-1302-B-2 and 17-2-0303-B of the Chicago Zoning and Land Use Ordinance:
"When a building permit application for the repair, remodeling and/or alteration of residential buildings or buildings of mixed residential occupancy that have been in existence for 50 or more years and that contain no more than 6 dwelling units is filed with the Department of Housing and Economic Development, and zoning certification is sought for the present number of dwelling units existing at the time of submittal of such application, such zoning certification must be limited to certify not more than one unit over the number of units originally authorized. The permit application must be accompanied by such documents and be in such form as prescribed by the Zoning Administrator to substantiate the lawful existence of dwelling units in the building. Such documents may include, but not be limited to, affidavits, leases, utility records, or any other instruments deemed necessary by the Zoning Administrator to make a determination of authorized nonconformity." (17-13-13-2-B-2, partial quote)
"17-2-0303-B Exemption. In the RS3 district the minimum lot area per dwelling unit may be reduced to 1,500 square feet when 60% or more of the zoning lots fronting on the same side of the street between the two nearest intersecting streets have been lawfully improved with buildings containing more than one dwelling unit. This exemption will only allow for the establishment of a two unit building."
The first of these quotes would seem to indicate that there is some flexibility in the number of units that the City will authorize above the nominal Zoning Ordinance limit. Take a look at the entire ordinance on this page:
To determine the current zoning of any property in Chicago, take a look at this page:
This is an interactive Zoning map that has functions which allow the user to determine lot line measurements and other useful data which determine permitted use.
Bk, a good Chicago zoning attorney could give you an opinion (usually without charge) by looking at the information on the Zoning map and from other public records about whether to pursue a legalization. My colleague is correct when he says that you need to weigh the costs before going forward. The zoning attorney can assist you with this evaluation.
Don Pasek, CIPS, TRC, ADPR
Omniterra Real Properties
Nick Radakovich Home sales Realty
Sorry I am with my cell ..can't write