Following a growing public uproar over developers exploiting loopholes to build "supertall" towers, the Department of City Planning on Friday proposed a zoning amendment that seeks to discourage them from using mechanical spaces, also known as “voids,” to add extra height to buildings.

Last week, the Department of Buildings threatened to revoke the permit for developer Extell’s residential project at 50 West 66th Street, citing both the excessive height of its 160-foot tall mechanical void and safety reasons. Voids, which are essentially empty spaces between several floors, allow residential units above them to sit higher, making them more valuable.

Now, city planning officials are seeking to create a rule in certain residential zoning districts that would require voids taller than 25 feet to count toward a developer’s allowable floor area, thus limiting the number of dwelling units that can be built. The plan would discourage, although not directly outlaw, excessively tall voids.

“This closes one loophole in certain zoning districts,” said Sean Khorsandi, the executive director of Landmark West, one of the Upper West Side groups who mounted the effort against the project.

To prevent developers from simply stacking multiple 25 feet voids on top of one another, City Planning is also proposing that buildings that have more than one void within 75 feet of each other would also have to count any height in excess of 25 feet toward the allowable floor area.

Finally, mixed-use buildings, which might consist of commercial and residential spaces, would be subject to the same rule should non-residential spaces make up less than 25 percent of the building.

George Janes, a planning consultant who was hired by Landmark West to mount a challenge against Extell's project, praised City Planning's response. He said the amendment should directly address contested buildings in the pipeline now.

Sara Kamillatos, a preservation associate at Friends of the Upper East Side Historic Districts, a group which lodged a similar zoning protest against 249 East 62nd Street, said she was “heartened” by City Planning’s proposed amendment. She said she hoped that the DOB, which rejected their challenge, would similarly move to revoke the permit of the Upper East Side project.

However, asked about that project, a DOB spokesman on Friday said the designs for the two buildings were different, citing the fact that the mechanical void in the Upper East Side project is exposed to the outdoors.

"The design for 249 East 62nd Street calls for outdoor space that is open to the air, along with structural outriggers," the DOB spokesperson said in a statement. "This design has previously been used in other buildings around the city, for example at the Citicorp Building at 601 Lexington Avenue. Under the Zoning Resolution, outdoor space is not governed by the same rules as fully enclosed mechanical space, such as what is proposed for 36 West 66th Street."

With regards to Extell, the DOB said it had received a letter from Extell "indicating that they intend to resolve the objections raised" by building officials of its mechanical void design. The developer has yet to submit revised plans for 50 West 66th Street, according to the DOB.

On Thursday, the NY Times reported that Gary Barnett, the Extell founder credited with reshaping the skyline with supertall luxury towers, intends to sue the city for retroactively applying the new zoning rules to his project.

Extell's public relations firm, M18, did not respond to a request for comment.

Janes cautioned that it may still be possible for developers to use other non-dwelling spaces to add height.

"I wish it would have gone a little further," he said. "It invites other ways to use this. It really only addresses mechanicals."