It's an unlikely location, perhaps, for the creative hub of one of the planet's most cutting-edge architects: a dockside warehouse near Tokyo Bay hemmed in by dull concrete apartment blocks. Behind its cinder-block walls, young people toil over computers and diagrams in a cavernous open-plan office. Paper cut-outs crowd every desk. Ignore the atmosphere of beetle-browed intensity and it could be a giant origami convention for children. 'Oh, please don't show this - it's such a mess,' pleads Kazuyo Sejima, one half of the acclaimed duo behind Sejima and Nishizawa and Associates (Sanaa). 'People will think we're chaotic.' A slight, wiry figure dressed in black, Sejima orchestrates Sanaa's swelling global empire of projects from behind her desk in this warehouse. She is just back from New York with partner Ryue Nishizawa to pick up the US$100,000 Pritzker Prize, the industry's equivalent of the Nobel, given to architects whose work has made 'consistent and significant contributions to humanity'. Drawing on a string of innovative projects, perhaps the most famous of which is the New Museum of Contemporary Art in Manhattan, Pritzker singled out a collaborative process that is both unique and inspirational: 'It is virtually impossible to untangle which individual is responsible for what aspect of a particular project,' the judges concluded. 'Well, we've been together quite a long time,' says Sejima. She laughs at suggestions the pair have developed a sort of professional telepathy. 'Of course, there is sometimes friction and differences of opinion, although nothing like the fights we had before. I don't have the same energy.' Asked why they've lasted together for 15 years, the boyish Nishizawa says simply, 'Her charm as a person.' Like their buildings, the relationship is more complex and finely balanced than its deceptively simple exterior. Both have swatted away speculation that their union is anything more than professional. Nishizawa, the younger of the two (and at 44 the youngest architect ever to win the Pritzker) is technically the junior partner having begun working for Sejima (54) while still a graduate student in the late 1980s. But the former prot?g?, who runs a separate practice within the same warehouse, reportedly generates most of Sanaa's creative ideas. Observers say he brings a vital logical and critical perspective to her more intuitive thought processes. Ideas are bounced around the warehouse before Sejima pulls the trigger on the best. 'She is somehow the Zen master who has the intuition to pick the right one,' explains Thomas Daniell, author of the 2008 book After the Crash: Architecture in Post-Bubble Japan. In many cases, she must see dozens of paper models before she makes a final decision, which often comes from the gut after the rulebook has been cast aside. 'The company is confident enough to make buildings based on intuition,' says Irish architect Jack Hogan, who works for Sanaa. 'It's not really rationalism. The theory comes afterwards.' That intuitive approach has helped guide the company through a run of striking, widely praised projects that create a 'subtle synthesis' of lightness, transparency, materiality and space, said the Pritzker judges. The Christian Dior building in Tokyo's Omotesando - their first major project in the capital - is shielded behind a curtain of translucent glass that makes the building appear to be floating. The circular 21st Century Museum of Contemporary Art in Kanazawa, Japan, with its naturally lit interior and glass perimeter, has become a model of how to achieve a balance of opacity and transparency in modern construction. The New Museum of Contemporary Art looks endearingly like a child's unsteady attempt to stack half a dozen rectilinear blocks in the middle of Manhattan. 'What's mysterious about their buildings is that they don't look like buildings,' says Daniell. 'They're translucent, lightweight and use a lot of glass, not much colour. When you make a building it is such a heavy, solid, opaque thing, and there is a magic to making it appear like it has no weight.' Like many industry observers, he praises Sejima and Nishizawa's ability to turn a very simple diagram into a real object without losing any of its simplicity. 'It is almost childlike - that simplicity and beauty becomes a solid building that you can use.' The adjective used most often to describe the result is minimalist, but while Sejima doesn't reject the description she says that's not the intention. 'We absolutely don't set out with simple or complicated in mind. We just want to create buildings that draw people to them. That's our starting point: making structures that are enjoyable and easy to use. The implication of simple is that we're trying to repress our creative instincts, but we're not.' Their dedication to thinking deeply about how people use their buildings quickly won them acclaim in Japan (Nishizawa was the country's young architect of the year in 1992). But it took longer for Sanaa, set up in the mid-1990s, to win the same attention elsewhere. The Glass Pavilion at the Toledo Museum of Art in Ohio, which opened four years ago, was many people's first encounter with their wry, low-key oeuvre - a point noted by the Pritzker judges who praised its direct contrast with the 'bombastic and rhetorical'. Still, the award raised eyebrows in Japan, where longer-established names such as Kengo Kuma have yet to win the same international recognition. Sejima and Nishizawa claim to have been 'astonished' by the Pritzker. 'When we started our company, we didn't even know if we could win foreign contracts. And when we did, it was very unexpected and we would go: 'Wow, now we have to build it,'' Sejima recalls, laughing. 'We sweated so much over each project, so we were so surprised to be recognised for what we did.' A self-confessed workaholic, Sejima rarely leaves the warehouse until after midnight. The company is halfway through building a branch museum of the Louvre in the French city of Lens. It is redesigning the 19th century art deco landmark La Samaritaine in Paris, a project that has sparked protests from conservationists ('Very unpleasant,' she notes). There are other sites in Switzerland, the New York suburbs, Connecticut, and houses in Berlin and Tokyo. She is curator of the Venice Biennale's architecture exhibition, too, and teaches a university course. All of this keeps her away from home for a third of the year, she estimates. Earlier this year she found herself on a whirlwind tour of six great cities. 'I was in Rome, then Berlin, London, Paris, Tokyo, then New York - all in about 10 days. I didn't have much chance to linger but I was so moved because the character of each city is so completely different. It's amazing to see so many people living and working in such completely different architectural environments.' When pressed, she admits that Rome is one of her favourite cities, or Manhattan for walking. Tokyo is a hard city if you're poor or old, she says. 'In Tokyo, you're always directed into shopping streets. It would be nice to be able to enjoy the city without having to shop and spend money. I had a friend who lived in Italy and when she came to Tokyo she said 'The poorer you are the more enjoyable life in Italy is.' In Tokyo, if you're poor, you're poor. I think it's important to create a public space that can be savoured without money.' Is China making the same mistake? She says the 'energy' of Chinese architects is phenomenal. 'China is in a huge hurry to get where it's going; they're building a lot and for young architects it must be very exciting. I don't want to be harsh or critical but design should be contemplative, so it might be better if they took a bit more time. Once you've built something, you can't easily change it. Buildings have a permanence unlike anything else.' As the interview finishes she again declines to be photographed among the origami clutter of the warehouse. 'People don't want to see a mess,' she says. 'It's appearance that counts.'