Obtaining permits PRE purchase? NYC

[Veronica Taylor]

Excuse the super novice question here but here goes. 

If one purchases a 2 family with an illegal unit, what’s the protocol on making it legal? Property is in a 3 family zone. Say it won’t be an issue with the lender so long as a permit to convert to a 3 family is obtained. 

My question is, how long does obtaining permission take on average, if granted.
And how can I as the buyer not yet owner obtain one? Is there some sort of “conditional permit” dependent upon closing or something to that effect?