Though Manhattan apartment houses are often known for the anonymity they afford, the six-story brick tenement on the northwest corner of 92nd Street and Amsterdam Avenue has for decades offered camaraderie instead. Residents hang the finger paintings of other tenants' children in the halls, cooks bake pies in neighbors' ovens, and novels are left on lobby mailboxes for the taking.

"Our friends have always said our building is like a kibbutz," said Marjorie Cohen, a 31-year resident, whose kitchen walls are adorned with neighbors' keys.

But this building of mostly rent-stabilized apartments differs from a kibbutz in a crucial respect: It has a landlord, a fact made abundantly clear in recent months.

In March 2005, the building and a contiguous sister building to the north were sold to a developer for $54 million. Rumors flew, but only in March of this year did tenants discover, on the Web site of the Buildings Department, that the new owners had received city approval to build nine stories on top of the existing six-story buildings — while the tenants are living there.

To hear them describe it, the specter of Upper West Side development now hovers above their heads like some mythical Condo of Damocles. (Such vertical enlargements of occupied apartment houses are not unprecedented; several years ago, half a dozen stories were added atop a six-story building at Amsterdam and 90th Street.)

"This has been very, very scary, particularly for older people, who are afraid they will be made to move somehow," Ms. Cohen said on Tuesday in her one-bedroom apartment, as she gave her baby grandson a bottle. "We have many people who have asthma and upper-respiratory illnesses or little babies, and they're very worried about dust."

According to tenants, Swig Equities, which manages both buildings, has rebuffed requests to meet with them.

Kent Swig, the president of Swig Equities and an owner of the tenements, said on Thursday that his staff had met with tenants, but that no plan had been shared because the engineering work was only recently completed. "The building is a very complicated structure," he said. "Until we knew we were actually able to do it, it wouldn't make sense to tell tenants or the world what we were doing."

The existing buildings will remain rentals, he said, and he expects to make a presentation to the tenants within 30 days.

Tenants' attempts to learn about the construction have also been stymied by procedures of the Buildings Department, under which approved construction plans are returned to the owner until the contractor applies for permits. Until then, only the project's outlines are available.

"Clearly, this is not a sensible procedure," said Scott Stringer, the Manhattan borough president, "to say to tenants one day, 'We are going to build on top of your head, and there are no plans for you to see at the Department of Buildings.' "

The department, said a spokeswoman, Ilyse Fink, has asked the architect of the rooftop addition to give it the building plans. She added that the agency would meet with tenants and officials and had put the project on hold until it re-examined the plans.

Still, not every renter wants the construction delayed. "It's a major improvement to the 90's to have this kind of development, in terms of the quality of people who are going to move here," Ephraim Nagar said as he sipped a three-olive martini at Talia's Steakhouse, a restaurant he owns on the ground floor of the northern building.

But State Assemblywoman Linda Rosenthal worried that renters would be priced out of the area by the arrival of luxury condos plopped on top of their homes.

Amsterdam Avenue has tall apartment houses, but it also has short tenements, and for longtime residents, that sawtoothed skyline is an ominous sign of its potential for developers. The new jostles with the old at sidewalk level, too, where delis plastered with cigarette stickers and dirty Corona banners share a streetscape with upscale Japanese restaurants.

"These have been solid middle-class buildings in the neighborhood for many years, and people who lived in buildings shorter than 10 stories felt safe about being able to afford their apartments," Ms. Rosenthal said on Wednesday. "But this kind of pressure from above calls into question all those assumptions. If the top nine stories are condo, who's to say the bottom six won't go condo, too?"