The provisions of Section 32-422 (Location of floors occupied by commercial uses) are modified to permit residential uses on the same story as a commercial use provided no access exists between such uses at any level containing dwelling units and provided any commercial uses are not located directly over any story occupied in whole or in part by dwelling units. However, such commercial uses may be located over such a story occupied by 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.