Single family home in manufacturing zone. Can I build an extension?

Good afternoon, I am planning to buy a small 1350sqf single family house on a 2500 sqf lot in an M1-1 zone in Greenpoint. I would like to build an extension of 500-600sqf for private family use. I have no heard that since the house was build 1899 we are allowed to renovate the house but not build an extension. We have a non-conforming use that we can’t expand. Others told me I can build an extension but only on the ground floor.

Any advise on this would be most welcome! We are ready to buy but want to wait until this is sorted out. Not an investor but buying for private use.

Many thanks!