In Special Mixed Use Districts, in any building or portion of a building occupied by residential uses, commercial or manufacturing uses may be located only on a story below the lowest story occupied by dwelling units, except that this limitation shall not preclude the:

1. extension of a permitted business sign, accessory to such non-residential use, to a maximum height of two feet above the level of a finished floor of the second story, but in no event higher than six inches below the lowest window sill on the second story;
2. location of commercial or manufacturing uses on the same story, or on a story higher than that occupied by dwelling units, in buildings in existence on or prior to December 10, 1997, that are partially converted to residential use pursuant to paragraph (a) of Section 123-67 (Residential Conversion), or were previously so converted pursuant to Article I, Chapter 5 (Residential Conversion Within Existing Buildings); or
3. location of commercial or manufacturing uses on the same story, or on a story higher than that occupied by dwelling units, provided that:  
   1. no access exists between such uses at any level containing dwelling units; and
   2. the environmental requirements set forth in Section 32-423 shall be met where commercial or manufacturing uses are located above any story containing dwelling units.