Floor Area Ratio as a Mechanism

The ratio of floor area to lot area, the multiplier, within Michael Stern’s zoning field, from which every envelope in the city descends, is a fraction with two measurements in it, and each hides something. Planetizen’s definition of the term gives the numerator as a building’s gross floor area, the sum of every story, and the denominator as the buildable area of the lot, so a ratio of 2.0 means twice the lot’s area in floors.² The Housing Affordability Institute’s housing policy explainer makes the point with a 2,500 square foot lot at 2.0: five thousand square feet in floors, which can be a two story building covering the entire lot or a four story one covering half of it.⁵ That indifference to shape is the mechanism. This entry compares the two numbers, then what is counted against what is left out, then the ratio against the envelope rules it replaced and still works beside.

Two measurements, two kinds of hiding

The denominator looks fixed and is not. Planetizen notes that buildable land area excludes streets, rights of way and regulated ground, so two lots the assessor records as equal can carry different denominators.² StreetEasy’s explainer of air rights adds that the ratio itself shifts with the lot’s position: a wide street or a narrow one, a residential or commercial use, and whether the project offers the public something in return.³ The numerator hides more, because the word floor area is a defined term, not a tape measurement, and the definition decides which parts of a building are free. How those two lots become one denominator is the subject of Zoning Lot Versus Tax Lot.

What is counted and what is left out

The Zoning Resolution’s list of definitions takes the measurement from the exterior faces of exterior walls, or from the midline of a wall two buildings share, and then lists what the sum includes: basement space, elevator shafts and stairwells at every floor, penthouses, attics with eight feet of headroom, balconies and terraces whose perimeter is more than two thirds enclosed, and any space that is made inaccessible.⁹ The same definition then removes cellar space not used for dwelling, roof bulkheads, water tanks and cooling towers, uncovered steps, and space set aside for the building’s mechanical plant.⁹ Each exclusion is a place where height can grow without spending area, which is why the city revisited the mechanical exclusion in Mechanical Voids and the 2019 Rule. The American Planning Association’s 1958 report on the ratio shows the pattern was there from the start: the ordinances it surveyed in Chicago, Clarkstown and Washington all argued over whether basements, mechanical rooms and parking belonged in the count, and all agreed that a gross measure was fairer than a net one because the ratio relates a building to land.⁶

Figure

What the count includes

	Counted	Left out
Below grade	Basement space, unless specifically excluded	Cellar space, unless it is used for dwelling
Vertical shafts	Elevator shafts and stairwells at each floor	Elevator and stair bulkheads on the roof
At the roof	Penthouse floor space and attics with eight feet of headroom	Water tanks and cooling towers
Outdoor rooms	Balconies and terraces mostly closed in	Open terraces, uncovered steps
Service space	Loading berths beyond twice the required amount	Rooms for the building's own equipment

A rigid envelope against an expansible one

The report’s authors offered the comparison that still explains the device. Older bulk controls, they wrote, create a rigid envelope beyond which the structure cannot extend, and the wedding cake shape of the city’s business districts came from filling that envelope to its edges; the ratio instead creates an envelope with expansible dimensions, limiting gross floor area but not height except indirectly, so a designer can trade footprint for stories.⁶ The Skyscraper Museum’s zoning history portal traces the idea to the public housing technicians of the 1930s, who wanted low density and open space, and records that a maximum ratio became the city’s fundamental bulk control in the code enacted in 1961.⁷ Jason Barr’s account of how the ratio arrived adds that it appeared in a 1940 revision for residential districts before it became the comprehensive tool, and that the 1961 code combined use, height and open space rules into single districts instead of three separate maps.⁸ michaelsternjds.com carries the full history of that code, in the 1961 Zoning Resolution; the older geometry it displaced is the subject of The Sky Exposure Plane.

How the number becomes a building

The arithmetic runs in one direction and the design runs back the other way. Lot area times ratio gives a total; bonuses raise it; the definition decides what the total must pay for; and the designer distributes the result across the lot, knowing that limits on height, setbacks and yards will trim whatever the ratio alone would allow.² Planetizen warns against reading the ratio as density: it measures bulk, and only indirectly the number of people, though codes often hang parking, unit counts and service loads on it, and bonus programs that are called density bonuses are usually paid out in ratio.² The Housing Affordability Institute describes the same multiplier from the other side, as the upper limit on a structure’s total size, often further cut by height and impervious surface rules, and argues that low ratios suppress housing supply where demand is high.⁵ Where a bonus is paid for affordable apartments, the mechanism is described in Inclusionary Housing Bonuses.

Figure

How a multiplier becomes an envelope

1. 01
   Measure the lotTake the buildable area of the lot in square feet, after streets and other encroachments are removed.
2. 02
   Apply the ratioMultiply by the ratio the district assigns to get the gross floor area the lot may carry.
3. 03
   Add or subtractBonuses for plazas or below market apartments raise the total; the exclusions decide what the total must cover.
4. 04
   Choose a shapePile the area across the entire lot or on a fraction of it; the ratio alone allows either.
5. 05
   Meet the other rulesHeight, setback, yard and coverage limits then cut the envelope down to what can be drawn.

The ratio read against the existing city

Two comparisons close the entry. The first is with the buildings already standing. In the data The New York Times mapped for the code’s centennial, about 17,000 of Manhattan’s 43,000 buildings, roughly two in five, broke at least one current rule, through too much residential or commercial area, too many units, too few parking spaces or too much height; a 1910 tenement on Jones Street, rebuilt to today’s rules, would drop from 24 apartments to eight.¹ The second is with another city’s version of the same fraction. In Japan, Plaza Homes explains, every land use zone carries both a floor area multiplier, from 50 to 1,300 percent, and a building coverage ratio for the footprint, with the ratio cut further where the fronting road is narrow and relaxed for entrance porches, balconies, lofts and shared stair halls.⁴ The pairing of a volume number with a footprint number is the rigid and expansible envelopes written into one code, and it is taken up in Tokyo, Floor Area and Sunlight Rules.