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All habitable spaces shall be provided with natural ventilation in accordance with this section. Openings providing required natural ventilation to habitable spaces shall be windows and/or glazed doors to the outdoors.
Exception: Group R-3 occupancies. Openable skylights opening directly to the outer air, transparent or translucent panels, or other natural light-transmitting media may be substituted for window openings in Group R-3 occupancies provided that they meet the minimum dimensional requirements of Section 1203.5.1.2.1.
1203.5.1.2.1 Minimum Opening
The minimum openable area to the outdoors shall be 5 percent of the floor area of the habitable space being ventilated. Every opening providing required natural ventilation shall be at least 12 square feet (1.1 m2) of glazed area, providing a minimum of 6 square feet (0.56 m2) of openable area.
Exceptions:
  1. Where fresh air is furnished in any habitable room or space by mechanical means supplying a minimum of 40 cubic feet per minute (0.02 m3/s), the free openable area of the openings may be reduced to 21/2 percent of the floor area but each such opening shall provide not less than 51/2 square feet (0.51 m2) of openable area.
  2. The minimum free openable area of a mullioned casement window shall be 51/2 square feet (0.51 m2), provided that the minimum ratio of floor area to openable area is met.
1203.5.1.2.2 Adjoining Spaces
An alcove or room opening off another room or space shall be considered as a separate room in determining its requirements for ventilation.

Exceptions:
  1. Alcoves within Group R-3 dwelling units. In Group R-3 occupancies, where an opening between the alcove and the room or space is at least 80 percent of the area of the common wall and the floor area of the alcove does not exceed twice the area of the opening, the alcove and the room opening into the alcove may be considered as a single space.
  2. Balconies, partially enclosed. Exterior openings required for ventilation shall be permitted to open upon a partially enclosed balcony or space above a setback when:
    1. Such balcony or space faces upon a public street, space, alley, park, highway, or right of way; or upon a yard, court, plaza, or space above a setback where such yard, court, plaza, or space above a setback complies with Section 1206;
    2. The maximum depth of any habitable room is 30 feet (9144 mm) measured from the outer face of the exterior wall forming the partial or full enclosure of the balcony or space;
    3. The enclosure of the balcony or space is not more than one story in height; and
    4. The front of a partially enclosed balcony or space above a setback shall be open to the outer air with an open area equal to at least 75 percent of the floor surface area of such balcony or space.
  3. Balconies, fully enclosed. Exterior openings required for ventilation shall be permitted to open upon a fully enclosed balcony or space above a setback, including sunroom and patio covers, when:
    1. Such balcony or space faces upon a public street, space, alley, park, highway, or right of way; or upon a yard, court, plaza, or space above a setback where such yard, court, plaza, or space above a setback complies with Section 1206;
    2. The maximum depth of any habitable room is 30 feet (9144 mm) measured from the outer face of the exterior wall forming the partial or full enclosure of the balcony or space;
    3. The enclosure of the balcony or space is not more than one story in height;
    4. The building is of Type I or II construction;
    5. The outer enclosing walls of the balcony or space are glazed with an area at least 50 percent of the area of the interior enclosing walls of such balcony or space; the glazing may be clear plate glass or slow burning plastic;
    6. At least 50 percent of the glazed area required by Exception 3.5 of this section is openable; and
    7. No window from any bathroom, toilet room, kitchen or kitchenette shall open on such balcony or space.
  4. Mezzanines and split-level rooms. Where a habitable mezzanine or portion of a split-level room or space is not directly served by an opening providing natural ventilation in accordance with Section 1203.5.1.2, such mezzanine or space shall open for its full width with no obstruction, except for railings, to an adjoining habitable room which is provided with natural ventilation. The floor area of such mezzanine or portion of the split-level room or space shall be added to the floor area of the adjoining room to compute the required ventilation in accordance with Section 1203.5.1.2.1. The combined space shall also comply with Section 1203.5.1.2.4. When required natural ventilation is not provided, mechanical ventilation shall be provided to the mezzanine and split-level rooms in accordance with the New York City Mechanical Code capable of providing 2 cubic feet per minute (0.00094 m3/s) of fresh air per 1 square foot (0.09 m2) of floor area of mezzanine.
1203.5.1.2.3 Basements and Cellars
Where openings provide natural ventilation to habitable spaces located in basements or cellars, such opening shall also comply with the applicable provisions of Sections 27-2081 through 27-2087 of the New York City Housing Maintenance Code and Sections 26(8) and 34 of the New York State Multiple Dwelling Law.
1203.5.1.2.4 Maximum Depth of Room
No part of any room shall be more than 30 feet (9144 mm) from a window opening onto a street or yard unless such room also opens onto a court complying with Section 1206.
Exception: In dwelling units containing more than three habitable rooms in Group R-1 or R-2 occupancies in buildings of Type I or II construction, rooms may be greater than 30 feet (9144 mm) in depth provided that all other requirements of Section 1203.5.1.2 are met and that the required windows are so located as to properly light all portions of the room in accordance with Section 1205.

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