21st-century design: The ultra-cool world of Rem Koolhaas

His debut is this summer's pavilion for the Serpentine Gallery in London's Kensington Garden for which he has been paid nothing, despite a technically demanding design that has taxed his engineering partner, Cecil Balmond of the Arup design consultancy.

The centrepiece of the structure, which wasopened last night with a party, is a balloon - or "ovoid-shaped inflatable canopy" as the architectural jargon puts it. In good weather, it is raised to float above an amphitheatre beside the gallery and, in bad, lowered to enclose it.

Even Koolhaas, a man prone to elliptical and qualified responses, seemed content with it. "I think it will be beautiful," he said, as engineers attended to the finishing details.

Rem - short for Remment - Koolhaas was born 62 years ago in war-torn Rotterdam. He began working as a journalist and scriptwriter, before training at the Architectural Association in London, and now combines writing on the subject involumes such as S, M, L, XL with building his practice, the Office for Metropolitan Architecture (OMA).

His buildings elicit superlatives. His Seattle Public Library was described by The New York Times as "a blazing chandelier to swing your dreams upon". The Casa da Musica hall, which opened in Porto last year, is acclaimed as one of the most important concert halls in the world. And in Asia, favourable comments garland CCTV, a giant headquarters for China's national broadcaster which will be completed for the 2008 Beijing Olympics.

Yet for a long time, Koolhaas was almost as well known for buildings he did not build as those he did. He parted company with projects rather than compromise. Both the Whitney in New York and the Los Angeles County Museum of Art, for instance, replaced his grand suggestions with understated alternatives.

Vicky Richardson, editor of the design journal Blueprint, said: "He's like the pop star of the architectural world." Part of that "cult status", she said, is because nobody can fully figure him out. "He's constantly changing and rethinking what he's doing, which means he's so exciting to follow. His lectures are always sold out."

His refusal to compromise is another factor in a business where many major players toe the line. "I don't think he's a big-headed egotistical architect. He's a team player. But he has a great commitment to ideas. In history, he'll go down as an incredibly important architect... And there's no doubt he's produced iconic buildings," Ms Richardson said. Julia Peyton-Jones, director of the Serpentine, was convinced he was an ideal candidate for her annual pavilion commission. She inaugurated the event in 2000 to showcase architects whose work had not been seen in Britain before. She says what was special about him was "a very rigorous intellect and an ability to really say things in a new way. It's almost as if he turns everything on its head".

Hans Ulrich Obrist, the Serpentine's new programme head and a friend of the architect, added: "It's incredible to think this is the first building of Rem Koolhaas's in the UK. He has lived here for more than 30 years."

Koolhaas said yesterday that "for a while, it was actually great not to work in England". Instead he used it as a "privileged terrain" where he could explore the engineering aspects of his work with Cecil Balmond.

Now, the pavilion will be a curtain-raiser for his permanent arrival on the British landscape. Although it will be dismantled in October and sold for £750,000 to help cover costs, other projects are under way in the city he calls home. He is working on a masterplan for White City in west London and is also designing a new headquarters for NM Rothschild Bank in the City.

Both are likely to be informed by his strong sense of social mission. Koolhaas said that what convinced him to accept the Serpentine project was that he could "make a contribution". Instead of being ornamental, he likes the way the pavilion will be used for film screenings and talks, including two 24-hour interview marathons, on themes close to his heart such as globalisation. "It's a studio," he said.

The social implications of architecture matter to him. "I'm interested in the effects of things on the larger system. I like the idea that what we do has an impact beyond shape," he said.

His commitment to the bigger picture is such that, for the past five years, OMA has had its own think-tank, focusing on social, economic and technological issues such as "how urbanism evolves". With an audacity which further enhances his cult status, the think-tank ponders areas where Koolhaas has not been asked to intervene, such as European identity.

The big thinking is not confined to theory. A tethered balloon might seem a straightforward proposal, but this one is technically challenging. It is the size of the dome of St Paul's and, filled with 6,000 cubic metres of helium, it has the potential to lift three small cars. Chris Carroll, a director at Arup, said: "This structural form has no precedent."

Yet it took just four months to design and four weeks to construct in what is accepted as an extraordinary collaboration. Koolhaas himself said the process was "unbelievably enjoyable because architecture is [usually] so incredibly complex and laborious".

In place of confrontation, this has been blame-free and effortless. "Maybe because there's no money involved. All of us are tendering our services for free. It's asking a lot, but it's also wonderful to be generous. The generosity of the whole thing maybe inspires all the parties."