In C4 or C6 Districts, the underlying provisions of Section 32-422 (Location of floors occupied by commercial uses) shall be modified, for mixed buildings that are developed or enlarged, to permit dwelling units on the same story as a commercial use provided no access exists between such uses at any level containing dwelling units and provided no commercial uses are located directly over any dwelling units. However, such commercial uses may be located over dwelling units by authorization of the City Planning Commission upon a finding that sufficient separation of residential uses from commercial uses exists within the building.