Chicago Zoning History

[Chris Witt]

Does anything know how I can access historical zoning maps for Chicago?

I have a two flat in an area currently zoned R2 (two flats are prohibited in this area) however the code allows legal non-conforming uses to continue under certain situations. 2004 is the effective date of current zoning map but looks like my property was converted around to two flat around 2000 so I need to know if two flats were allowed at that time.

Thanks