Why a Landmark Rooftop Addition Is Shaped by the Sidewalk

By Michael Stern, JDS Development Group

Put two of our buildings beside each other on a single variable, which is how much of the new work a person standing on the sidewalk can actually see. The Brooklyn Tower rises from the landmark Dime Savings Bank, and everything added there is visible from several neighborhoods away. Walker Tower is a Ralph Walker Art Deco telephone building conversion that added roughly 50,000 square feet, and almost none of that area announces itself to someone on the street below.

That difference is not a matter of taste. On a designated building, new work above the existing roof is reviewed for what can be seen from the public way, so the shape of a rooftop addition is generated by lines of sight rather than by the floor plate that wants the area. This is for developers looking at a designated building with unused area above it, and for the architects and preservation consultants who will have to draw the thing.

What a Rooftop Addition Is Measured Against From the Street

The measurement starts with a set of points on the public way. A sidewalk across the street, a corner two blocks down, the middle of a bridge, the far side of a park: each one is a place a reviewer will stand, and each one generates a line from roughly eye height past the top of the existing parapet.

Everything above those lines is visible and everything below them is not. The buildable volume for the addition is what remains after all of those lines are drawn at once, which on a corner site with long views in two directions can be a surprisingly small wedge sitting well back from the street wall.

The lines are not the same on every street. A narrow side street puts the reviewer close to the base and steep, so the visible zone starts just behind the parapet and the addition can stand nearly full height a few feet in. A wide avenue, a plaza or an open park flattens the angle and pushes the visible zone deep into the roof, which is why the same addition is buildable on one frontage of a corner building and not on the other.

That wedge is the starting geometry. The floor plate comes out of it, the structure comes out of the floor plate, and the unit plan comes last. Work the sequence the other way and you draw a building that has to be cut down later, which is the same mistake in reverse as treating a landmark base as a smaller version of a clear site.

A rooftop addition is drawn from the points where it has to vanish, never from the floor plate that wants the area.

The city agency that reviews work on designated buildings in New York is the body that establishes which of those points matter on a given block, and the Landmarks Preservation Commission publishes how its review proceeds. Which points it asks for varies with the building and the street, and nobody should assume the list from the last project carries over.

Why the Vantage Points Are Fixed Before the Massing

Choosing the vantage points early is the whole discipline. They are an input to design rather than a test applied to a finished scheme, and a project that settles them in the first weeks saves itself a redraw it cannot schedule.

This is the same logic that governs the support of an excavation beside a designated building, which I have written about in the sequencing of underpinning next to a landmark. In both cases the old building sets the terms and the new work is designed outward from them.

On a designated roof, design management means holding the vantage points as fixed while everything else moves. The setback distance, the height of the addition, the slope of its roof and whether it reads as an attic or a volume are all variables inside a constraint the design does not control.

The national framework most review bodies in the United States work from is published by the National Park Service, and the Secretary of the Interior standards for the treatment of historic properties set out the principle that new work should be differentiated from the old and compatible with it. A local body applies that principle through its own procedure, and a preservation consultant is who reads the two together for your lot.

Why a Set Back Addition Turns Floor Area Into Terrace

Here is the compromise, said flatly. The floors that get set back the most are the floors that would have been worth the most, because a volume pulled away from the street wall on all sides turns usable area into terrace.

A designated building therefore tends to give back its best space at exactly the level where a new building would have taken it. I have never found a way around that, and I have stopped looking for one.

What a Mockup Settles That a Drawing Cannot

Visibility is judged by people looking, which means a section through a sightline is an argument rather than a proof. On an addition of any consequence the review ends up asking for something physical: a story pole, a scaffold mounted silhouette, a sample of the cladding held up at height, photographed from the agreed points.

Those mockups are slow, they arrive late, and they can send a scheme back for a revision nobody budgeted time for. They are also the only instrument that answers the question honestly, because a rendering can be drawn from a vantage point that flatters it.

The useful response is to buy the mockup early rather than to argue with the request when it arrives. A silhouette raised while the massing is still soft is cheap information, and the same silhouette raised after the structure is coordinated is a schedule problem.

Practitioners who do this work as a discipline rather than as an occasional task gather at the Association for Preservation Technology International, and the documentation standards they argue about are what a good submission looks like.

Why the Sightline Study Comes Before the Structural Scheme

I treat the sightline study as the governing design document on a landmark roof, earlier in the sequence than the structural scheme. A structural engineer could tell me that is backward. An addition pulled deep behind the parapet concentrates load inboard, over spans the existing frame was never organized around, and finding that out after the envelope is fixed means transfer structure that a different massing would not have needed.

That objection is correct about the consequence and I still think the order is right. The visibility envelope is not available for negotiation by anyone on the design team, while load paths can be solved in more than one way at a price in steel and in depth. Starting from the constraint that cannot move costs a cycle; starting from the one that can costs the scheme.

The people to retain before any of this is drawn are a preservation consultant who knows how the local body behaves, a restoration architect for the junction between new and old, land use counsel for what the designation actually covers, and a structural engineer early enough to say what the existing frame can carry. Nothing above is legal or engineering advice and none of it substitutes for their reading of your building.

The addition that gets built on a landmark roof is the one whose designers accepted that the street decides its shape before anyone inside the building does.

Michael Stern is the founder and chief executive officer of JDS Development Group.