Seems like an obvious answer, but I’m getting conflicting answers from friends.
Do I use the exterior dimensions length x width x number of floors, or do I add up all the interior measurements? If the latter, do I include halls, closets, attic and basement? If the attic has a square footage of 450 feet, but the roof slopes in so steeply that roughly half of that space is unusable, does that change anything?
In my experience you use the exterior dimensions to give a square footage “under roof”. You would not consider outdoor porches/patios or attic area unless it was “heated/cooled living space”. Same for the basement; if it’s habitable living space, include it; if not, leave it out.
There is no one way that square footage is done. You have to ask the right questions when an agent or seller quotes you the square footage. Usually, no unfinished areas are included in the calculation (such as garage, attic, unfinished basement). So what is considered a finished living area? Read this for the answer and more details.
I wouldn’t always count on that to be correct. In my case, I have a neighbor three houses down that has the exact same house I do - except my house has an office on the upper level that my neighbor doesn’t have (It’s just a vaulted ceiling in her house). Both of our houses are listed as having the exact same square footage in the county tax records even though I have an extra 10’x11.5’ room…