With the art market flourishing, several Chelsea galleries are expanding into larger and more dramatic spaces, creating a new species of mega-galleries and accelerating the upscale transformation of the neighborhood.

Two galleries known for their highselling artists, David Zwirner and Gagosian, lead the pack in physical growth.In the most extreme expansion, Mr. Zwirner has tripled the size of his gallery by acquiring the two adjacent buildings on West 19th Street. He now has a rambling 30,000 square feet of exhibition space. Meanwhile, Gagosian, which has 26,000 square feet of gallery space on West 24th Street, is adding a new 9,000-square-foot showroom on West 21st Street.

Down the street from Gagosian on West 24th Street, Marianne Boesky built a new building, designed by Deborah Berke & Partners. In addition to a 6,000-square-foot gallery on the first floor, the building has two apartments — one for Ms. Boesky and one for an artist in residence –– and a terrace with a view of the High Line. The façade, made of corrugated galvanized steel, white brick, and customized concrete block, looks both strikingly new and in keeping with the character of the neighborhood. Last week, Ms. Boesky opened the gallery with a show of monumental paintings by Barnaby Furnas.

Other galleries, like Andrea Rosen, Luhring Augustine, and Zach Feuer, have recently completed renovations, which, while not dramatically increasing square footage, reshaped gallery spaces to accommodate either more art or a growing staff.

The pace of new construction reflects the strength of the art market, insiders say. “The gallery expansion is a bellwether of the current bull market,” an art consultant and the director of Art Ventures International, Avi Spira, wrote in an e-mail message. “And because the ability to show more art/artists, do more shows, attract more collectors, etc., it’s all just adding tremendous fuel to a very strong time in the art world.”

The expansions and renovations also reflect the pace of change in Chelsea, where what was once a neighborhood of warehouses first became, around 2000, an edgy outpost for the art world. Now it is quickly becoming upscale, with galleries outnumbering factories and chic restaurants cropping up on 10th Avenue, where 5,000 new apartments are set to be developed in the next five years.

“Galleries are flourishing in Chelsea,” said the president of Whitehall Storage, Jack Fuchs, who owns two buildings on West 25th Street that house about 35 galleries. “What’s available that’s not galleries will be turned into galleries in the near-term, unless it’s on 10th or 11th, in which case it will be turned into apartment houses.”

The managing director of David Zwirner, Amy Davila, said that the gallery had taken on eight new artists in the last two years, making expansion necessary. (Among the famous names who have joined Zwirner recently are Chris Ofili and Lisa Yuskavage, who defected from their previous dealers, Gavin Brown and Marianne Boesky, respectively.)

“You show an artist about every two years,” Ms. Davila said. “We have 29 artists.We had one space.Well, you do the math.”

Besides that, the opportunity of adjacent buildings coming available was one Mr. Zwirner couldn’t pass up, Ms. Davila said. The building at 533 W. 19th St., formerly occupied by a film company, came up for sale, and Mr. Zwirner purchased it. Ms. Davila declined to say how much Mr. Zwirner paid, but Mr. Fuchs estimated that a

building of that size would cost around $6 million or $7 million.

The space to the east, at 519 West 19th St., which Mr. Zwirner is leasing, was formerly occupied by a factory, Ms. Davila said. Mr. Zwirner has chosen to leave it essentially raw and use it for large installations, like the current show, Yutaka Sone’s “It Seems Like Snow Leopard Island.” For the next show, an exhibition of outdoor sculpture, the gallery will lay down grass on the floor. “We can be a little rougher and dirtier in this space,” Ms. Davila said.

Some say, though, that gallerists’ desire to build beautiful new spaces is not based only on programmatic needs. Ms. Berke, who designed the new Boesky gallery, said that galleries are under pressure to keep up with the high-octane architecture of museums. “The model of the old garage warehouse is over,” Ms. Boesky said. “There’s a real pressure on the galleries to update their space.” Museums are “on a building boom,” she added, so both they and collectors expect more from gallery architecture. “Galleries have to stay current because their clients are moving ahead,” Ms. Berke said. “It’s less about program and more about the character of the space.”

But not all galleries want to join the supersizing trend. Andrea Rosen and Luhring Augustine, which occupy the same building on West 24th Street, underwent a renovation this summer that was partly functional: The galleries suffered flooding in 2004, so they raised the sidewalk to create a downgrade toward the street, and installed a system of pumps and collecting tanks below ground.

Both galleries also shifted their space internally, mostly to create more work space for their growing staffs. (Andrea Rosen’s staff has increased to 16 from six in 1998, while Luhring Augustine’s has grown to 10 from six.)

Ms. Rosen said that she did not want to drastically expand her exhibition space in the recent renovation. “We really love how concentrated our gallery spaces are,” she said. To Ms. Rosen, massive expansion seems counterproductive. “I have personally made a choice across the board — a philosophical, even political point of view — which is that evolution is not about this sort of exponential growth, it’s about concentration and depth and nuance and focus.”

In an e-mail message, the owner of Feature, Inc., Hudson (he goes by one name) expressed a similar opinion. “Being or even having the Home Depot of the art world isn’t my idea of a good time,” he wrote. “I’ve opted to grow in terms of concentrated exhibitions and additional publications and smaller nurturing things for artist and clients.”

Whether you approve of huge galleries or not, the new Chelsea certainly has its benefits. Going to visit the galleries used to feel like a pilgrimage into a no-man’s-land.Today, other than the distance from the subway, it’s an extremely welcoming neighborhood, with numerous options for dinner after an afternoon of looking at art — that is, if you can get into the Red Cat or Cookshop or Tia Pol, which are often packed.

The increasingly upscale character of the neighborhood raises the question of whether galleries someday will be priced out of the neighborhood, as they were priced out of SoHo in the late 1990s. Mr. Fuchs said ground-floor rents in Chelsea have increased to $70–$100 per square foot, from $30 per square foot six years ago.

“I think that’s inevitable,” said Sundaram Tagore, who recently moved his gallery to West 27th Street from SoHo. “The process of expansion and growth is getting exponential. What took 20 years to develop in Soho has taken five years in Chelsea.” But, he added, “That’s what makes New York interesting.”