Lot Coverage Rules Decide How Much Ground a Tower Takes
By Michael Stern, JDS Development Group
Every site carries a limit on how much of its own ground a building may stand on, and that limit is settled in plan long before anyone draws a section. Two of our towers answer it in opposite directions, and what separates them is a decision about plan rather than about size.
This is written for the developer, the architect and the zoning consultant standing over a constrained lot, deciding a footprint. The footprint is the decision that closes the most doors behind it.
111 West 57th Street is the tallest and most slender residential building in the Western Hemisphere, so it stands on very little ground and takes its area upward. The American Copper Buildings occupy a former Con Edison site as twin towers with open ground held between them. One question about the ground, two answers.
What does lot coverage actually measure?
Coverage is the portion of a zoning lot that a building would cover when the lot is viewed directly from above. It is a measurement taken in plan, which is why it governs the shape a building makes on the ground rather than the quantity of building above it.
The phrase that does the work in the zoning text is any part of a building. Coverage is one projection, taken from overhead, of everything the building puts between the sky and the lot. So it is the outline that spends the ground, and the widest element in that outline is what the lot is charged for.
This has a consequence people miss on a tight site. A balcony, a bay or anything else that oversails ground the building does not touch is still in that projection, so a plan can gain usable area and lose coverage at the same time. The ground does not have to be built on to be spent.
There is a reason the rule is written as a projection rather than as a judgment about bulk. A projection can be drawn, checked and disagreed about on paper, which is what a rule applied across a whole city has to survive. It trades precision about how a building feels for certainty about what it covers.
Open space is the same rule read from the other side. The text describes it as the part of a zoning lot, courts and yards included, that is open and unobstructed from its lowest level to the sky and usable by the people living there. One provision says what may be covered and the other says what must stay open to the sky.
Left there, the second rule would be unbuildable, because a lot with any plumbing under it is not unobstructed to anything. So the text carries lists of what may sit in open space and in the yards and courts without counting against them. Those lists are where a workable plan is actually found.
All of it is measured against the zoning lot, which is a legal construct rather than a thing on the ground. The first honest question on a hard site is therefore what the lot is, which a licensed surveyor and the title record answer between them.
The order matters more than it looks. The lot is established, then the share of it that may be covered, then the open space that has to remain, and only then a plan that satisfies both at once. A plan drawn before those three are settled is a drawing of a building that may not be permitted to exist.
Lot coverage and floor area ratio are different controls
A floor area ratio answers how much building a lot may hold. A coverage limit answers how much of the lot that building may stand on. The first is a quantity and the second is a geometry, and a site can be generous on one and tight on the other.
In the residence districts they are not even set independently of one another. The permitted open space and the permitted floor area are determined together, through a height factor read off the zoning lot itself, so a team asking for more area is also accepting a different obligation about the ground.
Coverage is also only one of the layers stacked over a single lot, and the stack is where the surprises live. What makes the coverage rule arrive first is that it is the only one of them measured looking straight down, which means it can be tested on a site plan before a building has a height at all.
Yards and courts behave differently again. They work at the edges and at particular heights, carving the envelope where it meets a lot line or an interior space, while coverage works on the whole outline at once. A team that treats all of them as one bulk rule will get the plan wrong in a way that is expensive to discover late.
Why do tall buildings sit on small footprints?
Often because the ground is the scarcer permission. When the share of a lot that may be covered is small and the area the lot may hold is not, the building goes up rather than out, and a narrow tower is the shape that answer takes.
That is the reading I would defend from 111 West 57th Street. A footprint that small comes out of what the ground rules leave rather than out of a preference for slenderness, and the proportions of the tower follow from the footprint instead of the other way round.
The American Copper Buildings take the other road. Two towers on one site cover more of it between them, and the ground held open gives each tower a plate it can use rather than one it has to fight.
Open ground on a tight site is the permission the rest of the building is standing on.
Both answers are chosen in plan, early, and neither is easy to revisit. By the time the core and the foundation are designed, the building has committed to how much ground it stands on, and changing that is a new building rather than a revision.
The practical move on a site where coverage binds is to draw the permitted outline before drawing a building. Put the maximum covered area on the lot in the position the yards and the open space will tolerate, and then find out what plan fits inside it. Teams that start from the plan they want and test it against the rule afterwards discover the same outline from the wrong end.
The limits of reading a site from its coverage rule
A structural engineer could read this the other way round and would have a case. On a very slender tower the core, the elevators and the foundation establish a minimum plate of their own, and that minimum can be reached before a coverage limit ever becomes the binding constraint.
I still think the zoning control arrives first on most constrained sites, because it is settled while the core is still an assumption. I hold that loosely. On the narrowest sites the two constraints converge closely enough that arguing about which came first stops being useful.
The compromise I have accepted more than once is in the open space itself. Ground that satisfies the rule is often hemmed in by the building standing around it and gets very little sun, and some of it is used by almost nobody. It is open to the sky and it counts, which is what the rule asks of it.
The second compromise is arithmetic. A smaller plate gives up a larger share of itself to structure and to vertical transportation as it rises, so the floors bought by standing on less ground are worth less than their area suggests.
None of the numbers here are mine to give. The percentages, and whether a given lot is subject to them at all, live in the zoning text and change with it, so a zoning consultant and land use counsel establish them for a site rather than anyone reading a map.
What I can describe is the control and the order it works in. The survey comes from a licensed surveyor, the reading of the text from counsel, and the plan from an architect who has been told both before drawing anything.
The ground a building does not stand on is as much a part of its design as the ground it does, and it gets decided first.
Michael Stern is the founder and chief executive officer of JDS Development Group.
Photos by Levi Arnold and Uri Pernalete on Unsplash
