Last Thursday evening, as David Sellers walked into the University of Vermont’s Robert Hull Fleming Museum, a jazz band began playing “Somewhere Over the Rainbow.”

The song was oddly apropos. When Sellers moved to Vermont from Connecticut in 1964 with a few friends, his architectural fantasies were just that — over the rainbow. Forty-four years later, they are the subject of a major Fleming retrospective, “Architectural Improvisation: A History of Vermont’s Design/Build Movement 1964-1977.”

Attempting to capture a quirky architectural movement in three rooms is, by definition, a tight squeeze. (“That’s the trick,” explains designer/preparator Perry Price. “We still are a museum.”) Nevertheless, the Fleming show’s interactive listening stations, scale models of design/build landmarks and large color photos of the uniquely shaped, improvised structures built by Sellers and his fellow pioneers of the movement — think free-form jazz meets the hardware store — upend a visitor’s visual perspective in a delightful way.

Entering the gallery last week, Sellers, who has unkempt white hair and bright eyes, saw a photo of “Tack House,” one of his first design/build projects. On a far wall, he saw “Dimetrodon,” a multistory complex whose labyrinthine balconies, towers and curvy windows pushed the limits of modern design, and the experimentally windowed Goddard College Design Center, which Sellers and John Mallery built in the ’70s with $40,000 and the elbow grease of 40 untrained undergraduates.

Despite their indoor location, the photos — most of which were snapped outdoors on crystal-clear days — lend the Fleming show an outdoorsy feel. With a little imagination, in fact, a visitor to the museum last Thursday could have imagined Sellers, now 70, strolling the fields and meadows near his Warren workshop as a younger man.

After looking at a photo of “Dimetrodon,” Sellers turned the corner. There, in a far corner, stood an 8-foot-tall arch created for the Fleming exhibit by builder/landscaper and Yestermorrow instructor Erik Hegre as a monument to Vermont’s design/build movement. The contrast between the sculpture’s exterior “wacky wood” — a high-tech, bendable material — and its simple plywood guts suggests a construction aesthetic that is at once far-out and seat-of-the-pants.

“This is a good show,” Sellers declared.

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In 1964, when Sellers and his friends left the Yale School of Architecture, they talked about finding a “natural valley the size of Manhattan” in which to build homes. Oceanfront property was promising but expensive. So they looked at 450 acres of meadow and woodland in Vermont’s Mad River Valley. Sellers put a $1000 down on the property, which was sited on a place they named “Prickly Mountain,” and they set to work.

Despite their Ivy League degrees, the Prickly Mountaineers took an unorthodox approach to construction: Rather than build according to preexisting designs, they decided, why not design while they built? “We thought no one would hire us,” Seller recalled, “so we said, ‘Let’s just make something and see who shows up.’”

Back then, before the passage of Vermont’s landmark 1970 land-use law, Act 250, Prickly Mountain design/builders could do almost anything they wanted — albeit within the confines of a shoestring budget. But their freewheeling approach carried practical limitations. For example, while design/building Tack House — for $6000 and with salvaged materials — Sellers and his partners had to leave a refrigerator’s cold rear protruding from the exterior of a cramped kitchen.

For the next six years, Sellers and his design/build buddies worked from morning to nightfall nearly every day on Prickly Mountain, stopping only for the occasional swim or game of touch football. “This was pre-drugs — no dope, not even beer,” Sellers recalled. “We were intoxicated with the freedom to make stuff!”

They did, to the point where geometry itself might have tired from the exertion. In addition to Tack House, Sellers and his friend Ed Owre (later an art prof at UVM) design/built “Bridge House,” a boxy, primary-colored dwelling with a 100-foot bridge leading up to its entryway. Sellers and fellow architect William Reineke built “Pyramid House,” a wooden mass of triangles that suggests a Cape Cod rental whose water pipes have been laced with LSD. By the early 1970s, Sellers and friends Barry Simpson and Fred Steele were also establishing eco-conscious companies in a converted Warren textile mill. (Some, including Vermont Castings and Dirt Road Company, are still in business.)

The early success of the Prickly Mountain projects inspired more conceptual innovation. In 1971, on Sellers’ invitation, University of Pennsylvania architecture scholars William Maclay, Jim Sanford and Richard Travers began construction on an eco-friendly house they hoped would take its cues from Vermont’s nascent “back to the land” movement. The result of their process was Dimetrodon, a roughly 20,000-square-foot, multifamily dwelling that could pass as a spaceship for Luddites.

By the mid-1970s, structures like Tack House and Dimetrodon had inspired articles in publications as diverse as The New York Times and Glamour. Sellers said it wasn’t uncommon to see nosy reporters showing up unannounced at Prickly Mountain. One day, he found two Japanese photographers in his yard and invited them to dinner. “One guy didn’t speak any English, and the other spoke about 10 percent English,” he recalled with a chuckle. “But they had fabulous cameras!”

Some Prickly Mountaineers eventually switched professions or took jobs in traditional architecture firms, but traces of their design/build legacy remain all around Vermont. The clearest example, Yestermorrow Design/Build School off Route 100 in Waitsfield, is a quirky village featuring a wheelchair-accessible tree house, a solar-powered shower and a Hobbit-like wall made of Cob — a mixture of straw, clay, sand and mud. Buildings such as Maclay’s NRG Systems production facility in Hinesburg and Reineke’s Montpelier Police Department are mature riffs on early design/build themes.

“Like the commune movement, the social ecologists and the organic farmers,” writes Danny Sagan, a Norwich University architecture professor and guest curator of the Fleming retrospective, in the exhibition catalogue, “the architects of the design-build movement have helped to make Vermont a place that thrives by continual improvisation . . . in creative ways that address need and strengthen community.”

Vermont’s design/build movement also spurred institutional changes across the country. After Goddard’s “Design and Construction” program closed in 1977, similar ones popped up at such schools as Yale, MIT, Norwich and the University of Washington. By 2007, the only missing chapter in Vermont’s design/build Cinderella story was a funky museum retrospective.

Enter Sagan. A few years ago, he mentioned to Fleming Museum Executive Director Janie Cohen that he wanted to interview David Sellers and other design/build legends. Did she know of any donors who would be interested? Cohen, who had seen Sellers’ Tack House when she came to Vermont in the early ’90s, made a few calls. Last year, with cash in hand from a Chicago-based foundation, she and Sagan finally set to work on an exhibit.

In recent months, the Fleming’s new curator, Aimee Marcereau DeGalan, offered Sagan conceptual and art-historical advice. It wasn’t easy: Since the Prickly Mountain design/builders resisted naming their influences, such as U-Penn architect Louis Kahn, Marcereau DeGalan had to pry the details of the creative process out of them. And, since there’s no Prickly Mountain database, the curator — who specializes in 18th-century European painting — had to find some design/build landmarks using Google Earth.

As Marcereau DeGalan worked out conceptual kinks and Sagan prepared his guest curator’s essay, exhibition designer/preparator Perry Price built plywood cases to display the photographs and historical documents about the movement that Sagan had collected. Price, 27, also painted display stands and the gallery entrance grass-green to illustrate a theme of the exhibit.

“Building green is a buzzword,” Price said, and Vermont’s early design/builders “did it before it was in vogue.”

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Not every Prickly Mountaineer is pleased with how the group’s legacy has been interpreted. Take Jim Sanford: He thinks early innovations have been watered down by so-called “design/build” companies that separate the two spheres in practice. Sanford also resents craftspeople who call themselves design/builders but produce work that, in his view, looks calculated and dull.

At last week’s opening, Sanford, a slim man with an angular face, illustrated his point by critiquing Erik Hegre’s installation sculpture. The roughly 8-by-12-foot arch, he pointed out, has a line of regularly spaced screws connecting its porous plywood boards. But Sanford, ever the design/build aesthete, claimed that Hegre’s screw placement shows a lack of creativity.

“If I were doing this? I would never do this!” Sanford declared as guests filed out of the Fleming. “I would design this; I would have six screws going down there; I would make something out of this joint!”

Sanford’s critique, however nitpicky, points to a rift within design/build circles. For some veteran design/builders, such as Sanford, Sellers and Yestermorrow founder John Connell, design/build is an architectural expression of a “fine art” aesthetic. (As Sellers told his friends at the opening, “It’s the highest order of being, an architect.”) But for others, design/build means any union of the two concepts, no matter how tenuous or un-artsy.

“It’s a squirrelly one,” admitted Connell, who conceived the idea for Yestermorrow while studying architecture at Yale in the late 1970s.

According to Connell, who is also the director of a related Warren design/build firm called 2morrow Studio, Yestermorrow began as a “direct extension” of a Prickly Mountain aesthetic. In those days, Connell recalled, Yestermorrow structures weren’t just physical shapes but “art objects” whose aesthetic properties were no more important than the creative processes from which they emerged. “As the years have gone by,” he lamented, “that’s not so much the case anymore.”

Of course, other factors besides aesthetic taste have caused changes in Vermont’s design/build culture. One is consumer preference. Michael Wisniewski, a founder of Duncan-Wisniewski Architecture in Burlington, said his clients — who include homeowners, businesses and nonprofits — are less inclined to finance design/build-inspired architectural gestures than they used to be. And besides, added Wisniewski, no matter how progressive Vermont’s politics, many left-leaning locals have a soft spot for classic New England architecture.

Then there is the small matter of legality. When design/builders like Sellers and Sanford were building crazy structures on Prickly Mountain in the late ’60s and early ’70s, Vermont building codes were lax or nonexistent. As Wisniewski, who attended last week’s opening at the Fleming, explained, “People were willing to take chances back then, but now you can get sued so easily that’s it’s very difficult to advance the art.”

Connell suggests the design/build mantra has been usurped by younger Vermonters who build folksy-looking “vernacular” structures such as yurts out of straw and Cob. However, he said, today’s natural builders aren’t modern-day “equivalents” of design/builders like Sellers and Sanford. Whereas he and the Prickly Mountain crew once channeled “cultural” ideas absorbed in prestigious graduate schools, Connell claimed, today’s builders create “natural” aesthetics that are not anchored in the highest ideals of Western art.

As for young architects emerging from top graduate programs now, Connell said they have bigger student loans than did their Prickly Mountain predecessors, and that alone might make them less likely to embrace risky design/build tactics. And since building codes have tightened since the ’80s, he added, it’s almost impossible to do experimental design/building anywhere in the United States. That’s why, when aspiring design/builders ask him for advice, Connell tells them to go abroad.

At least two young natural builders are resisting Connell’s suggestion. One is Ben Falk, design director of Whole Systems Design in Moretown, a firm that designs landscapes and homes based on such disciplines as ecology, landscape architecture and agriculture.

Falk, whose 2001 UVM Master’s thesis explored Vermont-centric design strategies, observed that yurts, straw-bale houses and other humble structures are a necessary — not a culturally inferior — response to contemporary ecological challenges. What’s more, Falk said, his design/build aesthetic is only a part of a larger approach to landscape grounded in the teachings of Berkeley architecture professor Christopher Alexander and Vermont eco-designer John Todd.

“The design/build movement was born out of a response to need, but it was also a creative, exuberant, imagining process,” he said. “Now, I think the process is driven more by the urgency of sustainability issues.”

David Sellers, for his part, said that some natural builders — like his protégée Ben Graham, a Rhode Island School of Design graduate who now runs Natural Design/Build in Plainfield — are doing a good job of bringing “high-end” architectural ideas to natural building. However, Sellers said, he is disappointed to watch John Connell’s Yestermorrow Design/Build School move toward a do-it-yourself, “back to the earth” aesthetic.

It’s not that Sellers doesn’t understand the relevance of ecological design — his firm, after all, uses natural materials for projects all the time. He just doesn’t think green building is an end in itself. Ten years from now, Sellers believes, an eco-innovation like a grass roof will seem “ho-hum” to avant-garde design/builders. “It’ll be another color on their palette,” he predicted.

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Technical squabbling aside, it’s clear that the Fleming Museum’s green-themed design/build retrospective highlights important generational links. Just ask Barry Simpson, a Prickly Mountain pioneer who moved to Vermont in the early 1970s. At the opening last week, Simpson said the connection between early design/builders and today’s environmentally conscious draftspeople reflects cyclical trends in American culture.

Simpson, a quiet man whose brown facial fuzz has morphed into a Rasputin-like white beard, was standing near a color photo of David Sellers’ Tack House. As guests meandered through the gallery, stopping to admire photos or walk through Erik Hegre’s sculpture-arch, Simpson recalled that his early days on Prickly Mountain were overshadowed by war in Vietnam and concerns over rising energy costs. For that reason, he thinks today’s “cultural contusion” — Wall Street’s meltdown and the Iraq War — may be sparking a similarly creative “flowering.”

Simpson stood near Russ Bennett, a designer who owns NorthLand Visual Design in Waitsfield and an art director who has worked for Phish and the Bonnaroo Music + Arts Festival in Manchester, Tennessee. Bennett, who moved to Vermont in the early 1970s, agreed with Simpson’s observations, noting that some intrepid twentysomethings from the Mad River Valley recently built a straw-bale post office at Bonnaroo.

“It’s like leapfrog: Things go here, then they leap over a generation or two,” suggested the white-haired Bennett. “I think there’s a dialogue going on all the time.”

Danny Sagan, who curated “Architectural Improvisation,” puts those comments in architectural perspective. In his catalogue essay, Sagan suggests the design/build pioneers on Prickly Mountain were motivated by a desire to engage the world on their own terms. In some ways, he says, today’s green builders are channeling the same rebellious spirit by building homes out of natural materials instead of 2-by-4s.

Not that he thinks Prickly Mountain is reproducible. Like John Connell, in fact, Sagan thinks true design/building pushes aesthetic boundaries, and that young natural builders are less intellectually rigorous about their process than their predecessors were. Whereas today’s natural-building phenomenon is a “subculture,” he says, Prickly Mountain was a “revolution.”

But in the end, Sagan says, it doesn’t matter, because the design/build movement has never been “ideological.” “There is nobody sitting around saying, ‘There is one right way to do design/build,’” he explains. “If they are, then they’re missing the point.”