For mixed buildings, the underlying provisions of Section 32-421 (Limitation on floors occupied by non-residential uses) shall not apply. In lieu thereof, Use Groups 6, 7, 8, 9 or 14, other than offices listed in Use Group 6B, shall not be located above the level of the second story ceiling. Offices shall be permitted above the level of the second story, provided that where any floor space allocated to such offices is located on the same story as a dwelling unit, no access exists between such uses, and further provided that no floor space allocated to such offices is located directly over dwelling units.