Without formally voting, the commission declined to approve the addition or support a zoning waiver, two steps that would have been essential for the project to go forward.

The battle may not be over; the commission encouraged Mr. Rosen and Mr. Foster to come back with another proposal for an addition to the building, the former Parke-Bernet Galleries, at 980 Madison Avenue, between 76th and 77th Streets. But several commissioners made it clear that they would approve only a very modest addition of perhaps a few stories — nowhere near 30.

“We will ask you to rethink, restudy, in light of everything that’s been said, and revisit this at some point down the line,” the commission’s chairman, Robert B. Tierney, told Mr. Rosen and Mr. Foster at the end of a public meeting at the Surrogate’s Court building in Lower Manhattan.

In an interview afterward, Mr. Rosen vowed to continue pursuing the project. “Maybe it can come down in size, and we’ll study that,” he said. Mr. Foster, who said he gained “many valuable insights” from the meeting, said, “To be an architect, you have to be an optimist.”

At the start of the session, Mr. Foster offered a passionate defense of his design in a presentation before the commission, responding to objections raised by opponents of the project at a public hearing on Oct. 24. “The most appropriate, delicate intervention is to contrast a slim, vertical tower against the original, very horizontal base,” he said.

He said he had adjusted the color of the building’s skin — less silver and more bronze — but defended the juxtaposition of modern glass and steel additions on older stone structures, citing additions to the Metropolitan Museum of Art by Kevin Roche and to theMorgan Library & Museum by Renzo Piano.