Square footage is used as one of the key ways to describe a home. Most of the time it’s a value that someone has come up with but rarely is there something (like a floor plan) to show how the total sq footage was calculated.

The following comes from this article. http://www.dcd.com/oleary/oleary_janfeb_2005.html

Square footage needed for the sale of buildings.
In the real property field, real estate brokers often compare the value of properties and offer them for sale on the basis of their relative floor areas. Frequently, sellers tell their real estate sales people their understanding of the square footage and this, in turn, is quoted to prospective buyers, often without confirmation. The seller’s understanding of the building area may not always be realistic and in some cases may be intended to deceive an unwary buyer.

Sometimes, owners or their brokers attempt a field measuring of the building and, not always knowing the proper measuring method or appropriate geometry, they arrive at erroneous or questionable results. Also, there is no consistency in knowing what areas to include or exclude in the calculation.

Another article on this topic can be found here