For a single story house; you simply measure the sides of the foundation, and do the math. You excude the garage, it is not a "living area".
For a two or three story house, it get more complicated.
Notice that the sqfootage includes walls, stairs closets and spaces for ducts.
For a lot that is rectangular, it is simply length X width.
For a trapazoid,YOU will have to do the math:
Good luck and may God bless