Tribeca, a longtime artists' neighborhood turned real estate magnet for the rich and fabulous, doesn't wear its wealth on its sleeve. It wears it on its head. While Wall Street millionaires and media celebrities live in relative anonymity behind the area's unostentatious 19th-century commercial facades, TriBeCa's rooftops are being dressed up in remarkable fashion.

Even some TriBeCan old-timers, when they climb to a roof to survey the area, are surprised to discover how much of the neighborhood appears to have been furtively outfitted by a luxury haberdasher. From Canal Street south to Murray, from Broadway west to West Street, every third building seems to be topped with a fancy hat.

Set back from the facades of the buildings they sit on, and thus largely hidden from the street below, these private rooftop structures range from the quietly whimsical to the gleefully sybaritic: an exotic Moroccan tent in red, green and gold; a 75-seat outdoor movie theater with a bubbling hot tub; lush gardens evocative of the English countryside; a private lap pool set into a 2,200-square-foot terrace.

"They're these oases that nobody knows anything about," said Richard Reichgut, a software marketing executive who lives in hedonistic splendor with his partner, James Ronald Whitney, in a penthouse duplex on Duane Street. "Unless you or a friend has a rooftop, you would have no idea these spaces exist."

The past five years have seen dozens of additions to the neighborhood's rarefied skyline civilization. But because much of TriBeCa lies within city-designated historic districts and is controlled by a variety of zoning and landmark restrictions, TriBeCa's secret city in the clouds has burgeoned largely out of view of hoi polloi on the pavement below. And it is the hidden nature of these places, along with their sometimes flamboyant, over-the-top ostentation, that is perhaps their most defining feature.

"It's something new in New York," said David Garrard Lowe, author of the new book "Art Deco New York." "We used to build apartment buildings on Fifth Avenue or West End Avenue that made it clear to anyone going by that this was for upper-middle-class people or the upper class. This new building on top of roofs is, in a sense, a disguise of how much wealth there is. But at the same time, when you go there, they want to make it clear it's opulent."

TriBeCa's exclusive aeries, atop roofs that in the past were more likely to be decorated with water towers and lawn chairs, are also a telling sign that the city is increasingly home to large numbers of people for whom money is not only no object but practically an afterthought. "It's a very European sense of hiding your wealth, and it becomes very secretive and almost antisocial," Mr. Lowe said. "It's a society headed toward, as they used to say in Naples, public squalor and individual opulence. We don't have public squalor in TriBeCa, but I've seen a couple of those places and it's astonishing. It's a little like Xanadu."

A Theater, a Shrine, Even a Forest

When Mr. Reichgut and Mr. Whitney moved to 129 Duane from the Flatiron district in 1997, the spiral staircase in their penthouse loft led up to nothing more glamorous than an expanse of tar and some weathered parapets. Sitting on the roof on a carpet remnant and contemplating the hefty sums that a high-end rooftop addition would entail, Mr. Reichgut pronounced himself happy with the space just as it was.

"I, on the other hand, envisioned a big movie screen where we could sit in a hot tub and screen dailies instead of sitting in a boring, ugly editing suite," recalled Mr. Whitney, a filmmaker whose 2002 documentary "Telling Nicholas" included images of the World Trade Center collapse that were filmed from the roof. "I think the older you get, the more toys you need."

Three years and half a million dollars later, the couple's lavish rooftop playpen was complete. On a recent sunny morning, as Ashanti throbbed through 10 outdoor speakers, the pair enumerated the roof's creature comforts. The staircase now led to a set of white leather furniture and a Triton synthesizer inside a gleaming glass-and-steel solarium. Just outside, a steaming hot tub and sleek Philippe Starck chairs and sofas provided seating from which to watch the 15-foot-wide outdoor movie screen. Nor is the couple quite finished.

"See the plumbing there?" Mr. Whitney said, gesturing toward his rose garden. "That's where the wet bar is going."

The couple's raffish theater harks back to the grand rooftop gardens of the Gilded Age, with the exclusivity ratcheted up a notch. In the early 1900's, the city's most luxurious hotels, like the Astor on Times Square, had opulent roof gardens, and the New Amsterdam Theater and Madison Square Garden staged revues on their elegant rooftops. But while those gardens were public spaces at which society's nabobs sought to be seen, the invited guests at the Duane Street theater strive to be not seen, perhaps in part so they can indulge freely in what Mr. Whitney calls "naked Jacuzzi romps."

The sound of clinking glasses atop 129 Duane is sometimes borne on the breeze across Church Street and south to the rooftop hideaway that the architect Matthew Baird has designed atop his loft at 74 Reade Street. The seventh-story sanctuary has at its center a generous master bedroom encased in a box of glass and mill-finished copper. Surrounding the room on three sides is a 1,500-square-foot terraced garden of limestone and South American hardwood, planted with Japanese maples, lilac trees, Russian sage, hydrangeas and Himalayan birch trees.

Reclining on their outdoor teak daybed, which is edged with wild sea foam roses, Mr. Baird and his wife, Liz, can glimpse tantalizing bits and pieces of other rooftop oases, like the black-and-white desert tent across Church Street that Doug Kaplan, an executive with Sirius Satellite Radio, commissioned from a company in Morocco. "This is sort of the modern TriBeCa," Mr. Baird said. "You can imagine the lap dog in the tent."

Mr. Kaplan does not, in fact, have a lap dog, but he does have dozens of vibrantly colorful tropical fish swimming around the perimeter of his oasis in a dramatic pool, which is planted in summer with lotus, Egyptian papyrus and wild grasses. When the mood strikes, Mr. Kaplan and his girlfriend, Lauren Hyman, lay out rich Turkish rugs and cushions and relax in their tent by the light of a Moroccan stained-glass lantern, sometimes puffing apple tobacco through a hookah that was purchased, Mr. Kaplan said, from a Lebanese smoke shop "in the exotic West Village."

The playful extravagance of rooftop additions like Mr. Kaplan's tent is reminiscent of the whimsical follies built by European gentry on their country estates. A similarly fanciful confection stands at the edge of Laurie Weltz's rooftop garden on Reade Street, which features a recirculating stream running through a 400-square-foot lawn. Ms. Weltz's 10-year-old daughter, India, is fascinated with all things Indian, and as a result her family built a double-peaked structure that they call her shrine. Inside the shrine, which is made of gold-painted plywood and wooden latticework, India makes offerings to her little statue of Ganesh, the elephant-headed Hindu god of wisdom.

Up at 44 Walker Street, Mark Seth Lender and his wife, Valerie Pettis, have created an aerie with a very different purpose; they have planted their rooftop with 2,100 square feet of forest intended to provide habitat for birds and butterflies. "If I plant this right," said Mr. Lender, a conservation columnist for the Shoreline Times in Connecticut, "you could sit here and be very quiet and you'll see birds that'll knock your brains out." Scooping up a thin gray feather, he said: "See, we had a visitor. That's a mourning dove."

The Odd Tin Shack, Till the Cash Came In

The recent rash of outré mid-six- to seven-figure additions to TriBeCa's roofscape are descended from a far more humble, and sometimes oddball, lineage. The early ad hoc roof structures, built by the neighborhood's artistic pioneers before any of the area was subject to landmark regulations, included Quonset huts, idiosyncratic greenhouses and even a red London phone booth that stood on a North Moore Street rooftop as part of a hippie fantasy of urban freedom. "But as eyes became more and more focused on TriBeCa," said Bruce Ehrmann, chairman of Community Board 1's landmarks committee, "more and more things became codified."

By designating four historic districts in TriBeCa in 1991 and 1992, the city in effect placed much of the area under landmark protection, a move that curtailed the neighborhood's traditionally iconoclastic rooftop behavior. And as the recession of the early 90's gave way to a more robust real estate market, TriBeCa's rooftop additions went upscale in a hurry.

The 1996 conversion of the Dietz Lantern building at 429 Greenwich Street ushered in a wave of luxury conversions of commercial and industrial buildings. Ever since, according to Barrie Mandel, an executive with the Corcoran Group and a TriBeCa resident, developed rooftops have become de rigueur for new luxury-loft condominiums. Atop the nine-story Dietz Lantern building, for example, two stories were added to form a 50-window penthouse duplex with 5,300 square feet of interior space and a 2,200-square-foot private terrace with a lap pool. The duplex was resold in 2000 for $7 million.

In the Bazzini Building, a 1999 residential conversion of a nut processing facility at 21 Jay Street, the existing sixth floor was combined with a large rooftop addition to create two penthouse duplexes. In the new tower of the two-building River Lofts at 92 Laight Street, a 4,000-square-foot penthouse with a wraparound terrace was sold to Meryl Streep this year for $9.95 million.

"Virtually anywhere and everywhere it can be done, it's either being done, or wants to be done," said George Boyle, a local architect working on a hat trick of new rooftop structures along Greenwich Street for three clients. "The land's too valuable."

While many rooftops are being developed by longtime TriBeCans working within budgets, some of the most eye-popping additions have been built by more recent transplants from uptown. Wealth has been pouring into TriBeCa the last several years, transforming the area into what one local architect calls "the land of the $500 stroller." Since 1999, the average price of a loft has soared 77 percent to $1.6 million, according to statistics provided by the Corcoran Group.

TriBeCa derives much of its cachet from its heritage as a creative frontier homesteaded by artists, and this artistic pedigree does much to explain the over-the-top roof structures erected with the neighborhood's plentiful new cash. In sharp contrast to the Upper East Side or Westchester, TriBeCa is a traditionally iconoclastic place where status is conferred not on the basis of conformity but on individual, creative expression. As a result, an imaginative, or at least unconventional, gesture is all but expected of anyone who moves to TriBeCa and confronts the blank canvas of an empty loft or roof, especially if he or she has the blank check to go with it.

"It certainly sounds like new money, doesn't it?" said Stephen Birmingham, who has chronicled the ways of the wealthy in such books as "Life at the Dakota: New York's Most Unusual Address." "What better way to make your friends drop dead than to take them off this kind of scruffy street in TriBeCa and bring them up to this Hollywood vision?"

Wealth and daring do not, of course, always combine to produce inspired design. But zoning and landmarks restrictions have served to keep most of the rooftop aeries minimally visible and preserved the area's historic, architecturally cohesive feel. "The Landmarks Preservation Commission has done a very, very good job keeping these rooftop extensions within appropriate boundaries," said Mr. Ehrmann of Community Board 1. "Where they're not within appropriate boundaries tends to be as a result of developers trying to squeeze the last square inch out of a conversion project."

When it comes to winning approval for these rooftop extravaganzas, hiddenness has become a key issue. Though much of TriBeCa's city in the clouds is concealed, it occasionally offers up hints of its existence to earthbound sky gazers: a string of Chinese lanterns, a wayward juniper branch dangling over a cornice. Standing on West Broadway opposite Bouley restaurant, one can even spot, sitting atop a red brick building, the peaked roof of a gray clapboard cottage that one young Duane Street resident, Lucie Lagodich, described as the city home of the Three Bears.

Robert B. Tierney, chairman of the landmarks commission, said the criterion of minimal visibility had become critical to his agency's decisions as to which projects would be approved. "You can quibble perhaps over what minimally visible is," he said. "But believe me, our experience has been that it really means you can barely see it, or you really have to sort of strain to see it, or you can see it from maybe a 10-yard stretch on Church Street as you're walking north - that sort of thing - and even then you only see a tiny slice of it and only if you're really, really looking for it."

With outdoor space at a premium in Manhattan, the proliferation of rooftop havens shows no sign of abating, in part because of the financial incentives. According to Jonathan Miller, president of the appraisal firm Miller Samuel and author of the widely read Douglas Elliman Manhattan Market Report, a typical high-end developed rooftop in TriBeCa contributes $700,000 to $1 million to a loft's value.

Such appealing arithmetic has kept local architects busy designing ever more sumptuous rooftop environments. On North Moore Street, Joy and Leonard Toboroff, who already have an elegant 1,000-square-foot roof terrace, purchased the penthouse condo next door and are spending lavish sums to combine the two roofs into a palatial entertaining space, complete with a formal outdoor dining room, pergola, mahogany decking and lush plantings watered by a computerized irrigation system.

"It will allow for an elaborate, choreographed dinner party, fully staffed," said Stephen Corelli, their architect. "The design of the terrace provides seamless integration from predinner cocktails to dinner to after-dinner drinks and cigars in a finely articulated sequence of individual spaces." However, since the couple's primary residence is in France, the new space will be used mostly in late summer and fall.

Spurning the Stoop for Good?

Annie Nocenti, former editor of High Times magazine and a longtime TriBeCan, sees an insidious undercurrent to the wealthy removing themselves to such lofty heights. "When you see urban films or the photographs of Jacob Riis," Ms. Nocenti pointed out, "everyone sits on the stoops. Stoop sitting is a poor-person thing to do, and you're part of the street. If you're up on a rooftop, you're isolating yourself, and I have absolutely no interest in isolated utopias." Besides, she added, "most utopias turn out to be dystopias."

The closest to dystopia that owners of these luxe rooftop extensions would acknowledge are bothersome leaks or, in the case of the Duane Street rooftop theater, high winds that once loosened the big movie screen and threatened to send it crashing through the skylight above its owners' bed. Such occupational hazards of high living seem unlikely to diminish the number of TriBeCan penthouse owners seeking greater proximity to the heavens. At least three major new condominium developments are slated to come onto the market in the next three years, while more owners of existing penthouse lofts are likely to see their neighbors' Shangri-Las and conclude that their ancient ladder-and-hatch access to the old tar roof just won't cut it anymore.

Mark Winkelman, a TriBeCa architect, said that he has developed a simple standard for determining whether a client's existing roof space and access are in need of an upgrade. "I have the martini test," he said. "Can you make a martini in your kitchen and get it all the way up to the roof without spilling it?"