Section 27-286. Classification

When  two  or more classes of construction
  occur within the same building, the entire building shall be subject  to
  the  most  restrictive occupancy and size limitations for the classes of
  construction involved. However, if the occupancies within the  different
  classes  of  construction  are completely separated by construction that
  meets the fire-resistance rating requirements for fire divisions  listed
  in  table  5-2 then each occupancy so separated may, for the purposes of
  this code, be considered as a separate building ("building section").