Chicago Rehab Question - RS-3 Floor area ratio

[Matt DiVerde]

Hi all, I have a question specific to Chicago zoning rules.  I am looking at properties in several neighborhoods, usually zoned RS-3, often with lot sizes of 2400 sq ft.  According to the zoning rules, an RS-3 property limits the Floor area ratio to 0.9.  Per my understanding, that means a 2400 sq ft lot would allow at most 2160 sq ft of finished, above grade indoor space.  The thing is, most properties have well beyond that amount of finished space.  How are people getting around this rule and should I be worried buying a property that has more than 2160 sq ft?  Further more, if I buy a property with less than 2160 sq ft, would I not be able to get a permit to add square footage beyond the 2160 mark?  What am I missing here?

Thanks in advance!