Section 13. Application of chapter to existing dwellings

Any building
  erected and occupied on or before  April  eighteenth,  nineteen  hundred
  twenty-nine, or thereafter, as a tenement, which is not recorded as such
  in  the  department, shall be required to comply with all the provisions
  governing dwellings of like class  or  kind  erected  after  such  date.
  Except  as  otherwise expressly required in this section and in sections
  nine  and  twenty-five,  subdivision  six  of  section  thirty-one,  and
  sections thirty-three, sixty-six and sixty-seven and in articles six and
  seven,  nothing in this chapter shall be construed to require any change
  in the construction, use or occupancy of any multiple dwelling  lawfully
  occupied  as  such  on  April  eighteenth, nineteen hundred twenty-nine,
  under  the  provisions  of  all  local  laws,  ordinances,   rules   and
  regulations applicable thereto on such date; but should the occupancy of
  any such dwelling be changed to any other kind or class after such date,
  such dwelling shall be required to comply with the provisions of section
  nine.