New York City is known for its skyline that is always seemingly changing and growing. Dozens of taller residential buildings have been built since the turn of the century in two neighborhoods in particular: the Upper East Side and Upper West Side.

For a long time, construction along Second Avenue by 86th Street was focused underground, as part of the city’s newest subway hub. But now, the work is taking place much higher.

“Those corridors are evolving with many new developments,” said Dan Oelsner, who is the second-in-command at the U.S. branch of Izaki Group Investments, a private real estate company.

The firm’s latest project in Manhattan is The Harper, a 21-story condominium which used to be a three-story rental.

“We are here to create value again and create something for the neighborhood and we look at it as if the neighborhood gains form it. Also us," Oelsner said. "So it’s a win-win situation for everybody.”

The Harper is part of a trend on both the Upper East Side and the Upper West Side. Developers are building taller.

NY1 dug into the data on buildings higher than 328 feet that have been built since 2000 or are under construction. There have been 27 built in each neighborhood, according to the Council on Tall Buildings and Urban Habitat, a nonprofit that tracks how cities are changing.

The buildings make up a total of more than 23,000 feet in the air, or more than 13 One World Trade Center buildings combined. Almost all of these buildings are residential.

“I’m getting people from Brooklyn saying I’m back to the Upper West Side because I can get more for my money up here,” said Brian K. Lewis, a real estate agent with Compass, a real estate platform. “I find that astounding.”

Lewis told NY1 that he remembers when people were fleeing the Upper East Side and Upper West Side for Brooklyn. And he added that Manhattan developers can create even more apartments.

“New York is about height, right?” Lewis said. “We don’t have that much room sideways. We have all the room up and down.”

Developers often need to be creative. The developers at 200 Amsterdam Avenue bought plots all around the building in order to combine air rights, allowing them to construct what is now the tallest building on the Upper West Side.

“So they’re able to go to the [city Department of Buildings], get their permit and then suddenly the neighbors turn around and there’s an enormous hole in the ground,” said Elizabeth Goldstein, the president of the Municipal Art Society of New York, a nonprofit that in part advocates for thoughtful city planning.

The transfer of air rights is something Goldstein said she worries about.

Her nonprofit compiled a map that shows available air rights around the city. NY1 went through each plot on the Upper East Side and Upper West Side.

The data shows that there are 33 plots with unused air rights that could yield buildings taller than 400 feet. And there are 62 parcels on the Upper West Side that could do the same.

And Goldstein said she believes more of these buildings will get constructed.

“Because of changes in construction, we can literally build higher than we used to be able to do,” Goldstein said.

Sources in the real estate industry noted there are restrictions for some buildings, such as buildings with landmark status.

Goldstein said she’s fine with development. She just wants to maintain the character of city neighborhoods.

Oelsner said his company has tried to have The Harper blend in with the area, making the exterior limestone.

“They want to be part of the neighborhood to make sure the neighborhood accepts those buildings,” Oelsner said.

He said he hopes to keep building in the neighborhood. The only questions now is how high and when?