One of the confusions about spiral staircases in Brooklyn is whether they can be the only means of egress or does there have to be another set of straight stairs as well. Some architects would say you need both.

But the truth according to my sources is that you can get away with ONLY the spiral stairs in most situations. This is great because in space craved Brooklyn it is crazy to have two stairs in a floor.

According to the code I have seen there are three exceptions to the need for a straight stair egrees, which means you only need the spiral:

1. only within dwelling units
2.from a space not more than 250 square feet in area and serving not more than five occupants
3. from galleries, catwalks and gridirons

The green show house is a dwelling unit so 1 would apply. The space is 600sq and not a gallery so 2 and three don’t apply.

Here is what one of my consultants and green minded architects, Nicholas Liberis, has to say:

Its kosher, the section is from the International Building Code/New York Building Code, and the 250sf area and occupant load pertains to a non residential mezzanine application. The area and number of occupants are elaborated on because they address the non residential application of the code.

From the official IBC published commentary for the International Residential Code, the explicitly Residential version of the IBC. See the highlighted below, commentary italicized.

R311.5.8.1 Spiral stairways.
Spiral stairways are permitted, provided the minimum width shall be 26 inches (660 mm) with each tread having a 71/ 2-inches (190mm) minimum tread depth at 12 inches from the narrower edge. All treads shall be identical, and the rise shall be no more than 9-1/ 2 inches (241 mm). A minimum headroom of 6 feet 6 inches (1982 mm) shall be provided.

•:. A spiral stairway is one of several types of special
stairs that the code permits. Although a spiral stair may
be difficult to use to move furniture on from one level to
another, the code places no limitations on its use
within the egress system if it meets the size requirements
of this section. A spiral stairway that meets
these requirements may provide the only means of
egress from a level regardless of the occupant load or
size of area served.

A spiral stairway is one in which the treads radiate
from a central pole. Such a stair must provide a clear
width of at least 26 inches (660 mm). Each tread must
be identical and have a minimum dimension of 71/ 2
inches (191 mm) at a point 12 inches (305 mm) from its
narrow end. The stair must have at least 6 feet 6
inches (1981 mm) of headroom measured vertically
from the leading edge of the tread. The rise between
treads can be as much as, but not more than, 91/ 2
inches (241 mm). Commentary Figure R311.5.8.1
shows the required dimensions of a spiral stairway.

You are totally able to use this stair as a means of egress.