The number of underground floors (or basements) is the total number of floor levels whose floorplate is below the base of the building. This does not include partial-height floors (less than standing room), elevator pits, cisterns, wells, or tanks.

The position of the floorplate relative to the building's base distinguishes an underground floor. A deep elevator pit or tank is not a floor merely because it lies below that base.

The count concerns levels rather than their area. Several rooms on one basement level do not create additional underground floors.