Extension on Landmarked House

i’m aware that one needs approval from both dob and lpc to do work on a landmarked building….if you’re looking at that building to buy, how can you know if such an extension would be permitted before making an offer on the house? (basically it would be a 15×15 bedroom off the garden with a parlor floor deck above…general ballpark on cost would also be greatly appreciated). thank you.

By jdlcnm |