Square footage standards are set forth by the National Home Builders Association (NHBA) as approved by the National Association of Realtors (NAR) and accepted by the American Institute of Architects (AIA).

Square footage is calculated as follows:

First Floor - all space considered "habitable space" by the building code, including exterior and interior walls for all "conditioned" (heated and cooled) space including all closets and staircases:  in other words - the entire first floor excluding garage space - but including the width of the wall between the house and garage if they are attached. 

Second Floor - same definition as first floor with the following exceptions: the garage, obviously; any "unconditioned" attic area and/or space below roof eaves; any space which opens to below; the staircase is considered "open to below".

Attic Floor - same as the second floor except that the staircase is counted.
 
Basement Floor – finished area only is measured from the inside of the concrete foundation wall and includes all interior stud walls and staircase.

Exterior Areas – porches (covered or screened), decks and patios, are not counted.