Sak: Everything, nothing and anything! In most areas you will note that the field is left blank. Realtors generally do not calculate square footage. There is also the question of value of some square footage. Cathedral ceilings are fine but how much do they add? Does an 10-foot wide hall add utility over an 8-foot wide hall? The principal source of square footage for Realtors is found in the public record but many municipalities do not report square footage of the buildings. Of course, we have NO idea how it is calculated. If the builder's square footage on his advertisement or in brochures is available, that may be used as well. Again, there is no standard and the number can be anything the builder cares to use as his base.
Appraisers may be required to use a formula for purposes of a mortgage. This information is not generally available to the Realtor at the time down the road when the property is sold again. At least one REO company asks for square footage of their agents but then asks for the source of the "estimate."
What Realtors generally do, although some leave the fields blank, especially in REO deals, is to measure the rooms that are stated on the MLS form. You will see that the figures do not include hallways or "mud rooms, etc.." On the other hand, sometimes you will see a "family room" listed with dimensions, only to find out it's in a basement with no windows. Some attics are heated and have a closet, especially in 1920's semi detached construction. Some are all set up but have no heat. You will find that the bedroom count varies from 3 to 4, depending on no more than whim.
Finally, raised ranchers with "English" windows, usually double sash but shorter than standard, have a whole floor of rooms, often with dimensions listed. Half the bedrooms are down there. Since the era of "raised ranches" seems to be over, Iâ€™m not sure if the "English" windows meet the newest codes for egress that were formulated so that a fireman, equipped with an Air Pack, can get into and out of the room. Most of these are grandfathered but Iâ€™ll bet you that today you might have a hard time getting permits from the building department to add such a room with undersized windows.
Bottom line: Total house square footage is avoided by Realtors, who generally only use the sources discussed above and who refrain from coming up with their own interpretation. There is no law or standards that have been generally promulgated.