§ 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.