Typically comes from the tax records, based on the legal amount of heated sqft for finished sqft. The Dutchess County MLS list details such as above gound, below ground, finished, unfinished and total sqft. Some other MLS's and many websites that advertise homes for sale can be confusing by not letting public know the total sqft is not the finished sqft.
Finished sqft is typically the measure of the heated area that that also has a certificate of occupancy/compliance for the finished space. Staiways & closets would be included. Finished basements without C/O would not be included in Finished sqft.