Buildings of the fourth grade are dwellings. A dwelling means and includes a building which is used or designed or intended to be used as the home or residence of not more than two separate and distinct families. “Dwelling” includes:
(a) All buildings used or to be used by one family.
(b) All buildings used or to be used by two families living side by side.
(c) All buildings used or to be used by two families living one above the other.
(d) That part of all buildings used by one family, having another grade of occupancy either above or below or adjacent to the residential part of the building.
(1974 Code § 155.06)