To my pleasant surprise, I just got a very quick (and favorable) emailed response from the NY State code department people, in case there are other New Yorkers reading this. The language could be easier to understand, but I’m pretty sure that the area “within the inside perimeter of the exterior walls” must not include the thickness of the exterior wall itself.
Here’s the response as it was written to me:
Thank you for contacting New York State’s Department of State Division of Building Standards and Codes (DBSC).
Your question (paraphrased):
How is gross floor area defined?
Our response:
As per the 2020 Building Code of NYS, Gross floor Area is defined as:
[BE] FLOOR AREA, GROSS. The floor area within the inside perimeter of the exterior walls of the building under consideration, exclusive of vent shafts and courts, without deduction for corridors, stairways, ramps, closets, the thickness of interior walls, columns, or other features. The floor area of a building, or portion thereof, not provided with surrounding exterior walls shall be the usable area under the horizontal projection of the roof or floor above. The gross floor area shall not include shafts with no openings or interior courts.
Thank you,
Keith G. Burger,II Assoc. AIA
NYS Code Enforcement Official, Division of Building Standards and Codes