My realtor described it as everything under air meaning that if the space is heated or cooled, then it's counted. So the garage is out, but bathrooms and hallways and closets are in.
That's not exactly correct - part of my basement is heated/cooled but I'm pretty sure that doesn't count towards square footage (at least, the appraisal didn't count it).
Are you somewhere where people don't have basements?
Usually it's about finished vs. unfinished. I've never thought about whether my basement is heated or not. I mean it doesn't have radiators but that's where the furnace is and if the basement got too cold that would be problematic because of pipes. But it's very unfinished :-)
