Section 28-102.4. Existing buildings

The  lawful  use  or occupancy of any
  existing building  or  structure,  including  the  use  of  any  service
  equipment  therein,  may  be  continued  unless  a retroactive change is
  specifically required by the provisions of this code or other applicable
  laws or rules. The continuation of the unlawful use or  occupancy  of  a
  building  or structure after the effective date of this code contrary to
  the provisions of this code or other applicable law or rule shall  be  a
  violation of this code.