How do I turn a single family residential into a Duplex?

[John Weber]

I am currently going back and forth with offers on a SFR in Rosendale, Ny which is in between Kingston and New Paltz in Ulster County. There is currently an unfinished bedroom, finished living room and Water Hookups (old sink) in the room in between. I am thinking it might be possible to turn this house into a Duplex but I'm not sure where to look, or who to ask about rules and regulations for this type of renovation.

Any tips or guidance would be appreciated!