Certificate of occupancy in fourplex

[Isabel Rao]

Hello BP community:

I am under contract for a four story, 3 unit building in east Garfield park. 
I am looking into building a fourth unit/garden unit on the first story. RM5 zoning. 

I was reading about permits and such, adding another unit converts my building into R2 occupancy classification therefore I will need a certificate of occupancy before I can rent it out. The fire department might also want me to retrofit fire sprinkler system since its an R2 occupancy. 
How true is this? The garden unit is 650sqft, so relatively small compared to the whole building size.