"livable space" is included in determining the square footage of a home. Garages, unfinished attics and basements for example are not included when calculating square footage.
"Livable space" is a bit tricky though. Hallways and closets are included when determining a homes square footage, and no one really lives in a hall or closet. However it'd be difficult to live in a home that didn't have closets and halls...
Additions to a home often cause a lot of difficulty in calculating square footage. The laws in your area may effect how additions are counted. In the Phoenix area, if a home addition is permitted, then that area can be included in a homes square footage. If the addition is not permitted, then it isn't included.
Square footage numbers are usually taken from previous sales data, tax records (which are often incorrect) or builders plans. An appraiser may be your best bet for determining the true square footage of a home as they are typically skilled in measuring square footage.
