Square foot information is usually available at the Assessor's online office for the town where the property is located. Search online for "Millville (or whatever the town's name is) assessor's database." Once you are in the database, enter the property's address. This should bring you to the Assessor's records for that property. You may find that there is a sketch showing the building's exterior dimensions, and you can make the calculations. Or you may find that the Assessor has made the calculations for you. You are looking for the "living area." That area excludes porches, decks, and unheated loading docks. You are probably not looking for the "gross area" or "land area." The "living area" is typically the area you want.