You have to have a little sympathy for Japanese architects. Virtually every time a westerner interviews them they get asked about the influence of traditional Japanese architecture. Are western architects visiting Japan, I wonder, asked about how half-timbering has impinged on their aesthetic? Or whether their work is related to gothic or Renaissance precedents? On the other hand, I think to myself, perhaps it is something like the club sandwich test for a hotel: if they can provide a convincing answer, they probably do the other things pretty well too. So I ask anyway.

The architects Kazuyo Sejima and Ryue Nishizawa, working together as SANAA, are designing this year’s Serpentine Pavilion in London, a temporary folly in Kensington Gardens that has become one of the most awaited and intriguing events in contemporary architecture. They follow Frank Gehry’s collapsing, disjointed forms of last year and other, never less than arresting designs by architects from Zaha Hadid to Oscar Niemeyer.

There is a pause when I ask the Japanese question – it is the least I would expect – but the answer, when it comes, is gracious and considered. It comes from Nishizawa, the male and younger member of the duo.

“We grew up in Japanese culture so, of course, we are Japanese in how we build.

“For example the idea of being surrounded by a stone wall doesn’t sound so comfortable in Tokyo. The climate is wet and hot. We like lightness. The structure is not made by the walls, they often disappear, with sliding doors or terraces. The structure is made by columns.”

I meet the duo on the terrace of a café in Venice, where they are about to take part in a symposium on architecture and art. They are in Europe fleetingly and this is my one chance to catch them but even when they are right in front of me they remain pretty hard to catch. Unlike most architects, who can barely stop talking about their work, they are happy to let their buildings speak for themselves. There is no justification or post-rationalisation; their explanations are brief and polite. Sejima looks grateful to be puffing on a cigarette while we drink a mild Venetian version of English tea, weaker than the water that surrounds us. After a drag and a sip, Sejima adds to her answer “Also maybe the roof is more important to us than the walls. It is more visible, in history it is more decorated.”

That, perhaps, explains everything. SANAA’s Serpentine design is all columns and roof, a thin, amoeboid splodge wobbling and bulging, dipping and soaring its way through the trees and shrubs on the site. “The roof is very polished aluminium and it undulates,” Sejima says, “so people can see the underside, and sometimes the top.”

A wafer-thin metallic membrane, supported on the spindliest network of columns, it appears in the park as a thing of genuine delicacy and beauty. If there is a criticism of the Serpentine’s pavilion programme it would be that it has become almost too ambitious, and architects have begun to compete with each other (even if, of course, they profess they don’t) to design the most expressionistic, sculptural and impressive of mini-buildings. Gehry’s pavilion last year exemplified the trend: impressive, dynamic but, perhaps, too chunky. I ask the architects about the shape of their design, suggesting it resembles the kind of diagram architects used to use in the concept stages to suggest where things might be in a building, way before the architecture began to take shape. “It was decided by the trees and the landscape,” Sejima says, “not by the programme.”

SANAA’s effort sees a return to the light touch that characterised earlier efforts. But it also presents a wonderful twist to their extraordinary oeuvre, a body of work that has seen them rapidly and stealthily ascend to a position among the most admired architects working today.

Their best-known building so far is New York’s New Museum. It is a serene stack of shifting boxes that manages to refer to the ziggurat setbacks so characteristic of Manhattan skyscrapers while also appearing as an architectural apparition so light, so loosely stacked that the blocks might topple over at any minute. The expression of the galleries as individual boxes allowed them to articulate and naturally top-light each gallery separately. It is a wonderfully simple idea, even if the slightly neutered galleries themselves fall a little short of the conception. The New Museum is walled off from its gritty neighbours, the flophouses and pawnshops of the Bowery, by a net curtain of industrial mesh. And it is with this idea of the superfine membrane, the finest of veils, that we see SANAA working to create buildings of ethereal beauty: the exquisite, almost invisible architecture of the Glass Pavilion at Toledo’s Museum of Art and the shimmering net-curtain walls of their Dior store in Tokyo’s Omotesando.

In the light of these buildings, I ask, is the Serpentine Pavilion the natural conclusion of their efforts towards the ultimate disappearance of walls? Pause. Vigorous nodding from both of them. Nishizawa “Errr. Yes.” Shorter pause. Both of them laugh. It is the nervous laugh of a pair slightly uncomfortable in English but not that uncomfortable. Nishizawa continues. “Something we always look at is how to open the building up.”

Another pause. “We wanted to avoid the pavilion being a box in a park. We wanted to make it a park within a park for the three months. So the roof is like [the canopy of] trees and its shape reflects the trees.”

With an architecture as ethereal as theirs, so Japanese in its lightness, how, I asked, had they been able to respond to London, to the site? “We visited the site,” Sejima says, “and the park seemed such an important thing in people’s lives. It’s such a rare situation, a site in the city but not in the city, in this extraordinary frame. We wanted to make this a place for people to enjoy, to continue what they do in the park, eating, sitting, sleeping.”

The pavilion is also an unusual situation, an adjunct of a gallery but not itself used for displaying art. Is it harder to design an object when you know the attention will be on the form more than on its content? “We didn’t want to create an object but a place for people to enjoy.”

As well as buildings, Sejima has also become known for her quirky product designs. She designed the spindly Pica Pica table, so minimal it’s barely there, the Hana Hana flower stand, a surreal tree-shaped vase, the squashy doughnut-shaped Marumaru chair and the Rabbit chair, with its back composed of two big ears. I ask whether this pavilion, which shares the wiry, ethereal language of the furniture and which is, in effect, a demountable summer house designed to be sold on to a private client after the event, is as much product design as architecture.

Nishizawa replies: “Architecture is big. Products, though, are small.” I search for the Zen wisdom here but it passes me by. After a long pause, he expands. “We didn’t want to make a product, or a building that looks temporary, that looked like it could be sub-divided into different pieces. It is not a temporary structure but a building designed for re-use. Architecture,” he continues, “is about people sharing space together.”

I ask whether they’ve thought what furniture will be used in the pavilion. “The Rabbit chairs,” Sejima giggles. “I can imagine people wanting to sit in them.” With their characteristic blend of playfulness and elegance, frankly, so can I.

The Serpentine Gallery Pavilion 2009, London, July 12-October 18, www.serpentinegallery.org