make a 2 family into a legal 3

How hard/time consuming/ expensive/ possible is this?

Currently in process of buying a home with an interesting basement (there are many properties like this in the Bronx).  The basement is 100% above ground on 3 sides (the windows are almost floor to ceiling, and there is a full door that leads to the backyard).  On the street side of the house, however, the basement is more than 3/4 underground.  Basically the house is built in a giant ditch that allows the basement to be above ground completely while at the same time being lower than street level.  I hope that makes sense.

Would this be something that could be legally converted into a 3 family?  

Another question - if this is impossible to convert legally to a 3 family, could it be legally used as a duplex with the first floor?

Thanks