There is a slight nuance to building area based on the context. As Mac stated, normally it is measured to the outside surface of the exterior walls.
However, if we are dealing with energy efficiency, it is the inside surface of the walls of the conditioned space.
Tthis is siilar to the thought experient called The Ship of Theseus. If you are to replace a ship peice by peice at some point it goes from an old ship to a new ship, but when along the process does it happen? A deck with a roof over it is an exterior space, but a closed in screen room is part of the building, but when does one transition from one to the next.
Similarly, we were discussing exemptions from the building code for our province. People felt a building of 10' x 12' should be exempt, so I asked the next logical question, should a building 12' x 12' be exempt as well? People all answered no and a few saw where I was going with my line of questioning.