The sum of the areas of the floor or floors of a building which are enclosed and usable for human occupancy or the conduct of business. These areas shall be measured between the inside face of exterior walls, or from the centerline of walls separating two uses. The areas shall include areas finished for human occupancy, having a sloping roof or ceiling where the roof or ceiling height is not less than six feet. The areas shall not include areas below the average level of the adjoining ground, garage space, or accessory building space.