There are few things you need to check into.
1. Probably best option is to just renovate the curent house with option to extend to the max sf and that you can check on DOB web site
2. You can build a new house that will be 30x30 like you said yet the total sf of the house can be maximized by going with maybe more floors
Iam invest and work with investors who do this every day I can give you more info over the phone when I'll have more info on the specific property and how to find the info you need.
At the end anyway you need an architect to build or change the house so one will give you the most accurate info and some options for exceptions.
You can call me at 347-510-1616 Sam.