Just FYI, most basemants are separate interior cells. There's typically a Floor Stairs Down static that drops below the floor level of the ground floor. You can snap a doorway there and link it to your besemant cell.
Go into some of the existing houses that have a basemant and you'll see how it was done.
Some do use a basemant in one (that is, no loading a new cell), but they use one of the two-story interiors matched to a one story exterior. Psychology does the rest. If you want three stories inside, then yes, this is the only way.