Section 171. Alterations

1.  A  building,  other  than a frame building,
  originally occupied as  a  one-family  or  two-family  dwelling  may  be
  altered  to  a  converted  dwelling  if its bulk and volume has not been
  increased since April eighteenth, nineteen hundred twenty-nine,  and  if
  it is converted in conformity with the other provisions of this article.
    2.  Except  as  permitted  in  subdivision  nine  of  section nine and
  subdivision  six  of  section  one  hundred  seventy-two,  it  shall  be
  unlawful:
    a.  To  increase  the  height  or  number  of stories of any converted
  dwelling or to increase the height or number of stories of any  building
  in converting it to a multiple dwelling.
    b. To reduce the room or window area of any converted dwelling or make
  any  other  alteration  therein  if such alteration decreases the light,
  ventilation, fire protection or sanitation thereof.
    c. To convert to a multiple dwelling any dwelling  which  exceeds  six
  stories in height unless it was occupied as a multiple dwelling on April
  fifteenth, nineteen hundred thirty;
    d.  To convert any dwelling to a class A multiple dwelling occupied by
  five families or more upon any story unless such dwelling  is  fireproof
  or is converted in conformity with the requirements of subdivision three
  or  was  occupied  as  a  class  A multiple dwelling on April fifteenth,
  nineteen hundred thirty.
    e. To combine any  converted  dwelling  with  another  building  which
  exceeds six stories in height.
    f. To enlarge or extend any converted dwelling so as to exceed by more
  than  twenty-five  per  centum  the  area which such dwelling had on any
  floor at the time of its conversion, except as provided in paragraph h.
    g. To convert to a multiple dwelling any  dwelling  not  so  converted
  before  April  fifteenth, nineteen hundred thirty, if after such date it
  has been increased  in  number  of  stories  or  combined  with  another
  building  more  than six stories in height or enlarged or extended so as
  to exceed by more than twenty-five per centum the area of such  dwelling
  or its area on any floor as shown by the plan approved by the department
  before its erection, except as provided in paragraph h.
    h.  Paragraphs  f  and g shall not apply to, or be deemed to prohibit,
  any rear  extension  of  the  first  story  of  any  converted  dwelling
  permitted under the provisions of section one hundred seventy-two.
    3.  Converted  dwellings six stories or less in height may be combined
  if such dwellings when combined conform  to  the  requirements  of  this
  subdivision.
    a. No frame building shall be a part of any such combination.
    b.  Every  entrance  hall,  stair  hall,  public hall, stair and stair
  enclosures shall be constructed of fireproof  materials  throughout  and
  every  door  opening therefrom and door assembly shall be fireproof with
  the door self-closing and without transoms. Every public hall  shall  be
  at  least three feet six inches in clear width and enclosed with masonry
  walls not less than eight inches in thickness.  Every  stair  and  stair
  hall  shall  be  constructed  and  arranged  throughout  as  provided in
  sections thirty-five, thirty-six, fifty and fifty-two, and  as  provided
  in  sections  one  hundred two, one hundred five and one hundred six for
  fireproof dwellings erected after  April  eighteenth,  nineteen  hundred
  twenty-nine,  and  shall  be at least three feet in clear width with all
  landings at floor levels at least three feet six inches in clear  width.
  In  every combination of three or more such dwellings, fire-stairs shall
  be located so that there shall be horizontal access in the  public  hall
  from  every  apartment  to two or more fire-stairs, one or more of which
  shall be within fifty feet horizontally in the line of travel of one  or
  more  required  means  of  egress  from  such  apartment.   Every stair,

  fire-stair and fire-tower shall extend to the roof through  a  fireproof
  bulkhead  constructed  in  compliance with the provisions of section one
  hundred four for fireproof multiple dwellings.
    c.  The  ceiling of the cellar, or of the lowest story, if there be no
  cellar, shall be fire-retarded.
    d. There shall be at least two means of egress from every apartment or
  suite of rooms containing three or more living rooms. Such  means  shall
  be remote from each other, with the first means opening to a public hall
  which  is  connected  with a stair not more than fifty feet distant from
  such means of egress. The second  means  of  egress  shall  be  directly
  accessible to a fire-tower or fire-stair, or to an outside fire-escape.
    e.  No  window  to  the  outer  air shall be required opening from any
  public hall, vestibule, stair, or stair hall  artificially  lighted  and
  ventilated  mechanically  as  provided  in section one hundred seven for
  fireproof multiple dwellings. If any  windows  are  provided,  they  and
  their  assemblies  shall be fireproof and glazed with wire glass, and if
  such windows are used in lieu of mechanical ventilation  and  artificial
  lighting,  they shall also be arranged as provided for stairs and public
  halls  in  non-fireproof  multiple   dwellings   erected   after   April
  eighteenth, nineteen hundred twenty-nine.
    f. No elevator shall be required, but should one be installed it shall
  conform to the provisions of section fifty-one relating to elevators and
  section one hundred five for fireproof dwellings.
    g.  None  of  the  combined dwellings shall be required to comply with
  sections one hundred seventy-eight, one hundred eighty-five, one hundred
  eighty-seven, one hundred eighty-eight and one hundred  ninety  or  with
  subdivisions one, two and three of section one hundred eighty-nine.
    h.  When  any  business  is  conducted  in any portion of dwellings so
  combined, the ceilings and walls of such portion shall be  fire-retarded
  and  every  door  and  door  assembly  shall  be fireproof with the door
  self-closing.
    4. Nothing in this article shall be deemed to prohibit  a  combination
  of class A and B occupancy in any converted dwelling. If more than fifty
  per  centum  of  the  rooms  therein are used for class A occupancy, the
  dwelling shall be deemed  a  class  A  dwelling,  otherwise  a  class  B
  dwelling.