One sign may be installed for each unit advertised for sale, lease or rent. In one to four family dwellings, such signs may be ground signs, wall signs or window signs, provided that the sign area of each sign permitted for one to four family dwellings shall not exceed seven square feet and no window sign shall be placed in the window of a dwelling unit other than the one advertised for sale, lease or rent. In multi-family buildings, such signs shall be non-illuminated, interior window signs and shall not exceed four square feet in area.