Mechanical Voids and the 2019 Rule
A mechanical void, a plain instance of a definition shaping a skyline, drawn from the entitlements law Michael Stern covers, is an ordinary thing made strange by its size: a floor of equipment rooms, electrical chases, pipe shafts and air handling plant, stretched to several times the height such rooms need. Gothamist’s coverage of the city’s move against it describes the object plainly as empty space between floors that lets the apartments above it sit higher, where they are worth more, and cites a proposed residential tower whose void ran about 160 feet.1 Friends of the Upper East Side’s summary of the amendment records the measurement that made the object a scandal: the Department of City Planning found the average mechanical floor in the high density towers it studied to be about 12 feet tall, against empty spaces exceeding 100 feet in new designs.3
The object existed because of a line in a definition. The Resolution’s definition of what counts sweeps in almost every enclosed space, basements and penthouses and largely enclosed balconies alike, then names what the sum omits: cellars, rooftop bulkheads, water tanks, cooling towers and the rooms that house the building’s equipment.8 The omission is old and reasonable. A 1958 American Planning Association survey of early ordinances shows the question is as old as the device: Chicago counted mechanical floors except those on the roof, Washington counted them only above six feet six inches of headroom, and Bismarck left out basement rooms that housed heating plant.6 No one writing those definitions imagined the mechanism they allowed: because a lot’s allowance of floor area caps area and not height, and because the densest residential districts answer to a sloping exposure plane instead of a fixed height cap, as CityLand’s report on the amendment explains, an exempt floor is a free lift for everything above it.4
The numbers in the 2019 text
- Height above which an enclosed mechanical void counts as floor area
- 25 feet
- Minimum vertical spacing between voids before they count together
- 75 feet
- Portion of a mixed building below which its commercial mechanical space is caught too
- 25 percent
- The residential districts the rule applies to, with their commercial equivalents
- R9 and R10
The Municipal Art Society’s testimony to the Council describes how the object had grown. Its 2017 report on out of scale towers had found mechanical and structural voids adding 100 feet or more to building heights, and the testimony cites a 161 foot void that would have been a fifth of one 775 foot tower, and a 510 foot building whose top 12 floors were to sit on a 150 foot pedestal of stilts that counted as no floor area at all.5 Gothamist’s account notes the distinction the Department of Buildings drew between the two: an enclosed mechanical floor and an open structural void exposed to the air are not governed by the same rules, and the open kind had precedent in older buildings on columns.1
What the 2019 text changed in the count
The amendment did not touch the exclusion; it put a height on it. The Department of City Planning’s summary of the text sets out three rules for residential buildings in the densest residential districts and the commercial districts equivalent to them: mechanical voids taller than 25 feet count as zoning floor area, voids located within 75 feet of one another count as floor area together so they cannot be stacked, and a mixed building whose non residential uses fill under a quarter of it has its commercial mechanical space caught by the same 25 foot limit.7 Mechanical space itself remains exempt below the line, and nothing in the text, as Friends of the Upper East Side observes, requires a space labeled mechanical to hold equipment at all; the limit is a height, not an inventory.3
The procedure that produced those numbers is itself instructive about how a definition is changed. CityLand records the Commission certifying the text in January 2019, thirteen community boards weighing in through February and March, many of them asking for a 12 to 15 foot limit instead of 25, Manhattan’s borough president asking that the spacing rise from 75 to 90 feet, a Commission hearing with 23 speakers for and 18 against, including engineers who argued that good practice needs flexible mechanical space, a Commission vote that loosened the threshold to 30 feet, and a Council vote in May that pulled it back to 25 and adopted the text 47 to one.4 That sequence is the timetable set out in ULURP, run on a text rather than a map.
The empty floor before and after
| Before the amendment | After the amendment | |
|---|---|---|
| Any enclosed mechanical floor | Exempt from floor area at any height | Exempt up to 25 feet; the excess counts |
| Several mechanical floors | Each exempt, however close together | Within 75 feet of each other, counted as one and capped |
| Open structural voids and stilts | Not floor area | Still not floor area; the amendment reaches enclosed space only |
| Where it applies | Everywhere | High density residential districts and their equivalents |
| Projects already permitted | Unaffected | Unaffected; nothing applied backward |
What the rule left untouched is as telling as what it caught. The amendment reaches only enclosed mechanical space in a few districts, as Friends of the Upper East Side notes, and it did not apply to buildings already permitted, so towers that had vested under the old count kept their voids.3 West Side Rag’s report on the Council vote quotes the local Council members on the gap: the text limits the height of voids and spaces them apart but places no limit on building height and no limit on how many voids a building may hold, and a state bill then pending would have counted every mechanical floor once mechanical space passed a small fraction of the building.2 The Municipal Art Society asked for more, that open structural voids, stilts and terraces be caught, that the rule run citywide and reach commercial towers, and that the Department of Buildings assess whether the equipment actually fills the floor.5
Measured after the rule, the object is still there, shorter and farther apart. An enclosed mechanical floor in a high density residential tower may now be a little over twice the height the equipment needs before it begins to cost floor area, while a second one must sit at least 75 feet away. The lesson of the void for anyone reading a zoning text is older than the amendment: every exclusion from a count is a place where a building can grow for free, and the city’s answer in 2019 was not to abolish the exclusion but to measure it.
