The Multiple Listing Service makes the rules, not everyone follows them. Square footage should be Heated, Above Ground, Living Space, so traditional basements would not be included, some raised ranches the lower level would be considered.
Sometimes owners want to list finished basement square footage, I don't recommend this at all, I would rather potential buyers pleasantly surprised by extra space than annoyed by basement space.
The MLS is looking at clarifying the rules on this, but it is fairly difficult to get consensus.
All the best!