Turning a duplex into a triplex in Old Brooklyn?

[Phil Morgan]

I am closing on a duplex in Old Brooklyn on the 17th and it has a huge attic I am thinking about turning into an efficiency unit.  Question is do I need to do anything with the city since the property is already a multifamily?  Is there paperwork involved in doing this or to I just build it out and and find a tenant?  Anyone with experience doing something similar in this area??  Followup question, I will be looking for a couple people to quote this work if you can recommend a good contractor.

Thanks