The square footage listed in the county assessors records for real estate are typically not questioned. These numbers are taken from the original documents and are generally accurate.
Most real estate appraisers measure the exterior of the home to calculate the gross living area.
Many real estate agents and builders will include all finished "walkable" areas when totaling the square feet of a house.
At the bottom of most listings is a disclaimer that says in part ..."Buyer to verify square footage."
Also in the listing it will say where the square footage came from.
If you're interested in buying a particular house and want to know the size expressed in square feet, my advice would be to make an appointment to visit the home and bring your tape measure.
Good luck, Terri and Larry Mulinix, Keller Williams...(405) 834-7842