It oftentimes depend on the bank and where you live. If the basement if finished out, most banks will include the square footage in their calculations. If the basement is not finished, they treat the entire area like they would your garage. Beware of places like Texas because most bankers don’t have a clue of how to include it. I have heard cases in which the basement finished was only half calculated, and a few times it was not calculated at all as part of its square footage.
The rule in places like Texas and Oklahoma is, expect them to have no rule that they follow. I think this is great because you can shop for the bank that does what you want. You want it calculated so tell them you’re looking for a bank that calculates the basement into the square footage. On the other hand, if you want to keep the fact that your basement is living space hidden from the tax man, then look for a basement that does not calculate it as living space.