Dollar per Square Foot is based upon the home’s interior living space and does not include garages, unfinished attics and basements, rooms built without permit, patios, decks or yards (though all those can add value to a home). These figures are usually derived from appraisals or tax records, but are sometimes unreliable (especially for older homes) or unreported altogether. The calculation can only be made on those home sales that reported square footage.