There are many buildings in New York that do not have a certificate of occupancy, and do not need one. But that said they do still have a legal use.
Certificates of Occupancy only came into existence in the 1930's so if a building was built before then and the use has never been changed, it may well NOT have a C of O.
The quick way to check on the legal use is to check what HPD have it listed as. The legal use will come out with a full title search. And most Banks do accept legal use.
Please feel free to contact me if you have further questions,
Kathryn Lilly, Broker
Realty on the Greene, LLC