In a sharp turn of events, Billionaires' Row developer Extell has been granted permission to proceed with building an Upper West Side residential skyscraper that is set to include a controversial height-boosting feature known as a “mechanical void.”

Back in January, the design of the 775-foot-tall tower known as 50 West 66th Street appeared to be in jeopardy after the city’s Department of Buildings threatened to revoke the project’s permit over concerns that the roughly 160-foot void posed fire safety hazards.

Voids are non-dwelling spaces used to house a building’s mechanical equipment. Because, by city zoning law, voids don't count toward a building's maximum floor area, luxury developers have recently begun increasing the height of these spaces to allow the units above them to sit higher and fetch higher prices.

Following a review by the FDNY of a revised void design submitted by Extell, the DOB signed off on the plans last Thursday. The previous plans had called for the aforementioned 160-foot void, plus two additional 16-foot mechanical floors, for a total of 192 feet; the new void will measure 176 feet in height, but broken into two 64-foot mechanical spaces plus a 48-foot void.

A DOB spokesman issued the following statement, “Safety is our highest priority, for residents and first responders alike. DOB has approved amended plans for the building’s mechanical floors that satisfy safety-related objections and bring the project into compliance with the city’s Zoning Resolution.”

In addition to reallocating the void space so no individual floor will be over 64 feet tall, Extell created new access points where residents will be able to go from one emergency stairwell to the other, and added elevator stops to each floor to allow FDNY access.

For opponents of the project—who argued against what they say is a flagrant abuse of voids to create a project of unprecedented height for the Upper West Side, where the tallest building currently is the 583-foot Trump International hotel—the city’s decision sparked questions about how zoning regulations were being applied and to whom. The site lies within the Lincoln Square special zoning district, where city officials had sought to limit buildings to 25 to 30 stories. However, Extell argued that the rules, which say that 60 percent of the total floor area of new developments should be no taller than 150 feet, still permit a slim tower of unlimited height.

“The question is what rules are we following?” said Sean Khorsandi, the executive director of Landmark West, one of the groups who have been fighting the Extell project. “The city is picking and choosing when to apply standards.”

Khorsandi said Landmark West is currently weighing its options. It can elect to revive an appeal it had filed before the Board of Standards and Appeals, which was put on hold when the DOB suspended the project for review.

Ironically, the City Planning Commission is set to vote today on an amendment that would require voids taller than 30 feet to count toward a developer’s allowable floor area, thus discouraging the expansion of voids. But since Extell has already been granted a building permit, the new rule is not expected to apply to 50 West 66th Street. George Janes, a New York City planning consultant, told Gothamist that a project with a foundation in place and a valid building permit is typically allowed to build under old law.

The outcry has prompted Assemblymember Linda B. Rosenthal and Senator Robert Jackson to introduce legislation at the state level. According to their proposal, developers would have to limit void space to 20 feet for the first void, and 12 feet for additional ones.

A spokesperson for Extell did not immediately respond to a request for comment. In a January New York Times story, Extell head Gary Barnett said he was not opposed to the city regulating voids but did not want to be punished retroactively.

On the Upper East Side, a similar drama is playing out with a Rafael Viñoly-designed project at 249 East 62nd Street known as the "building on stilts" that is set to feature a 150-foot mechanical void. Similar to Extell, developers Real Estate Inverlad and Third Palm Capital were recently forced to suspend work so that the FDNY could review the plans for safety. But neighborhood residents have been frustrated that the city has not deemed the void design illegal.

In a statement, City Councilmember Helen Rosenthal, who represents the Upper West Side, said: “I am deeply disappointed by the Department of Buildings’ decision regarding 50 W. 66th, but we always knew this was a possibility. More importantly, our community is completely resolved to fight on. For over two years, we’ve been pushing the developer, Extell, to be transparent about their plans. Despite initially filing plans for a 25-story building in 2016, they have now received approval for a 775-foot building which absolutely does not conform with the Special District where it is located.”

UPDATE: An earlier version misstated the total new void height. It is 176 feet.