Converting single family home?

[Kay Ferdous]

I live in Long Island, NY area and I was wondering how difficult is to buy duplex single family homes that needs rehab or is foreclosed and convert them to multi family. I.e. Add a second entry to second floor add kitchen, bath etc. By difficult I mean do you get pushback from banks/city/village. Is there a big delay in getting the proper paperwork from the time you buy to the time you actually can start work or maybe some towns simply don't allow it ? Would love to here your experiences . Thanks