Expanding non conforming legal units in M1-2 Zoning

[Ching Chung Wong]

Hello all.  This is my very first post and would love to get some help from the BP community.  I'm looking at a legal 3-unit-multi-family deal with a 1-story-single-unit building in the front and a multi-floor-2-unit coach building in the back.   The building is over 100 years old.

For the 2 unit coach house, there's room in the attic that would allow me to duplex up and there's also an unfinished "basement" or "first floor" that would allow me to duplex down.   The overall lot size is 3,000 and the current zoning is M1-2.  I checked cook county assessor and treasurer's websites and it's odd enough that the PIN was classified as multi unit (code 211)  and then got changed to single family less than 999 sq (202) around 2019.

My questions are:

1. Would the current property classification under 202 affect my ability to duplex up or down in the future?  Or would it affect anything down the road?

2. What would the reason be for the current owner to request for reclassification of the property from 211 to 202 while nothing was physically changed on the two buildings? 

3. Am i reading 17-15-0303-B correctly that if I'm in a M1-2 zone with grandfathered in residential building, I won't be able to expand the existing footage by more than 20%? 

4. Is the "basement" or "First floor" considered below the grade because both buildings are built below the grade because you'll have to walk a few steps down from the street level to get to the buildings.   Just wanted to see if the additional floor footage from duplexing down will be counted towards FAR.  

5. Does the certification of zoning indicate the current recorded floor footage? I'm asking because cook county assessor only indicates that it's a single family house with about 900 sq ft.  

Thank you all for your help and sorry for all the questions.