In a blow to New York "supertall" developers, the city’s Department of Buildings is threatening to revoke the permit for a 775-foot condominium tower on the Upper West Side, citing both a controversial design feature as well as safety concerns.

Developed by Extell, the proposed building at 50 West 66th Street was to have included a 160-foot “mechanical void,” or empty spaces between several floors that would allow penthouses to sit higher, thereby adding to their value. The building, which is slated to hold 127 condominium units, would be the tallest on the Upper West Side and boast commanding views of Central Park.

On Thursday night, the DOB explained the issue in a statement to Gothamist:

"DOB determined that the 160-foot void proposed for this building is not customarily found in residential buildings, and so is contrary to the Zoning Resolution. In addition, we have raised objections, which the developer has not addressed, that occupants of the building may not be able to get from one emergency stairway to the other, as is required, within the proposed void."

Extell, which is both credited and maligned for starting the supertall boom with its Midtown luxury building known as One57, now has 15 days from January 14th in which to respond to the DOB’s letter.

According to information provided by the DOB, the builder faces two choices: it can revise the design of the mechanical space or it can proceed to build a 25-story tower which had been the original plan the developer had filed with the DOB in 2017. At the time, the proposed tower's relatively modest height, which could have been taller as of right, drew suspicions. Later that year, residents' worst fears were realized when the new design was revealed. In a New York Times story, Gary Barnett, the company's founder, brushed off the concerns, joking that by today's standards, “A 700-footer doesn’t even rank anymore” as a super-tall tower.

The latest decision represents an about face for the DOB. Last year two neighborhood groups filed a challenge against the project, saying that it violated the city’s zoning codes. The measure had the backing of elected officials, including City Councilmember Helen Rosenthal and Borough President Gale Brewer. But in November, the DOB rejected the groups’ argument as invalid.

Back in 2017, when the new design was unveiled, Rosenthal promised her constituents that she would "fight Extell’s current plans with every tool at my disposal."

On Wednesday, Rosenthal celebrated the DOB’s latest decision.

"The Department of Buildings’ notice of intent to revoke the permits for 50 W. 66th Street is a critical first step in the process to overhaul the City’s approach to mechanical voids and ‘super-talls,'" Rosenthal said in a statement. "We are very pleased that the administration is taking this issue so seriously in terms of W. 66th Street, and we await a final decision. Extell now has a limited window of time to respond to the DOB.”

Extell did not immediately respond to a request for comment.

It's unclear what ramifications this will have on other buildings with mechanical voids. Early last year, the DOB greenlighted starchitect Rafael Viñoly's Upper East Side apartment building at 249 East 62nd Street which featured a 150-foot mechanical void. In May, Crain's reported that the Department of Planning was considering whether to such designs should be permitted.

"The notion that there are empty spaces for the sole purpose of making the building taller for the views at the top is not what was intended [by the zoning code]," Marisa Lago, director of the Department of City Planning said at a meeting.