For a famously down-to-earth architect, Tadao Ando has a celebrity-studded client list. The former boxer with the Osaka accent and the trademark haircut is currently working on a studio for Damien Hirst in Mexico, a ranch for Tom Ford in Santa Fe, and an exhibition space for Giorgio Armani in Milan.

“It’s interesting working with creative people,” he says. “They have their own ideas.” What they are paying for, of course, are Ando’s ideas and his personal architectural style – a concrete minimalism that has come to define contemporary Japanese architecture.

Ando is one of architecture’s most understated superstars. While his contemporaries have passed through postmodernism, new modernism and all the other -isms, Ando has resisted fads and maintained his own clear vision. It will soon find its newest expression, in the redevelopment of one of Tokyo’s biggest and busiest subway stations at Shibuya.

Ando’s extraordinary underground pod is part of a huge urban planning project that involves the construction of a new subway line. Known as “chichusen” (underground space station), the pod will act as an interchange between the new line and the dense transport network that already exists under the busy station. It’s an impressive piece of engineering, “like squeezing a ship into a bottle”, in the words of Tatsumi Nakatomi, the man who is responsible for its construction.

The armadillo-like shell in glass fibre-reinforced concrete is already taking shape; it should be in place by late October and will open along with the new Fukutoshin subway line next June. Born in Osaka and raised by his grandmother, Ando never received any formal training. He travelled widely in his twenties, studying the works of the great modernists at close quarters before setting up his office in 1969. Today, he is one of the world’s best-known architects. He’s won almost every prize in architecture, including its top gong, the Pritzker, and holds a chair at Tokyo University – the first professor emeritus at the prestigious institution without a degree.

Ando’s office, tucked away on a quiet street in his home city of Osaka, does nothing to suggest his A-list status. It’s a slender building on the site of one of the first houses he designed, which he remodelled over the years and finally rebuilt in 1992. There’s no sign outside, to keep the architectural pilgrims at bay, and inside a monastic silence rules while Ando and his staff work in the open-plan space.

Off the top of his head, Ando says he’s working on projects in the US, Mexico, Switzerland, France, Italy, Taiwan, Korea and China. Several are large-scale projects such as the Basel headquarters of Swiss pharmaceutical giant Novartis (see Monocle issue 2), and Shanghai’s new International Design Centre, due for completion next year. Yet for all his workload, Ando has a staff of only 30. “I like to keep it that way,” he says, laughing. “Renzo Piano has 1,000 people working for him, and Norman Foster probably has 2,000 around the world. They’re both friends of mine and they often tell me that they’d love to come back to a small scale again.”

These days, high-profile projects seem to be shared out among a small group of international architects of which Ando is one, and which includes Frank Gehry, Herzog & de Meuron, Jean Nouvel, Zaha Hadid, as well as Foster and Piano. “It’s the era we live in,” says Ando. “There’s a big competition for every project.”

Ando and Hadid recently went head to head in Venice, both designing museum proposals for the Punta della Dogana, a 17th-century building on the Grand Canal. Ando was working for French luxury goods magnate François Pinault, who is looking for a home for his contemporary art, and Hadid for the Guggenheim. Ando won, and the office is now working to complete the project in time for the 2009 Biennale. At Saadiyat Island, Abu Dhabi’s €20bn bid to turn itself into the cultural centre of the Middle East, several of the big names have been commissioned for different parts. There will be outposts of the Louvre, the Guggenheim and the Sorbonne, and Ando has designed the maritime museum.

“I don’t know if it will ever be built or not,” he says. “We’ll see.” More and more Ando is asking whether so much building is a good thing anyway. “There is so much building in places like China and Dubai. The number of new buildings has got completely out of hand,” he attests.

Ando has often said that the problem with urban Japan is the lack of planning and the paucity of open spaces around buildings that are so jammed in that they can hardly breathe. Many of his plans seem to involve building as little, or as discreetly, as possible. “We have to take care of the planet or it will collapse,” he says.

His response to Ground Zero, was characteristically minimal. Instead of joining in the competition to build a new tower, he came up with a memorial park dubbed the Garden of Nothingness, that didn’t make it past the drawing board. “It seemed wrong to put another building in this place where so many people had died,” he says. 21_21 Design Sight, the design centre he built this year at the new Tokyo Midtown development with Issey Miyake and Naoto Fukasawa, is largely constructed underground. Chichu Art Museum, built as part of Benesse Corporation’s art project on Naoshima island, is also partly beneath the earth.

At this stage in his career, Ando has nothing to prove architecturally. Having made a fortune, and a reputation, from Japan’s mania for ripping down and rebuilding, he wants to project an image of environmental responsibility. There are schemes to create urban parks and to plant a million trees in the islands of the Seto Inland Sea. “It’s not strictly architecture, but even planting a tree is an act of architecture,” he says. He has also been key to the reconstruction of the waterfront in Kobe, which was destroyed by the 1995 earthquake.

Ando is working with Tokyo’s Governor Shintaro Ishihara on a 15-year project to make Tokyo greener. Their plans include burying overhead electric cables, planting trees, grassing over school yards and creating a network of open spaces to counteract the stifling summer heat.

He feels that the post-war reconstruction of Japan was impressive in many respects, but it ignored the environmental consequences. “The Japanese have always had a strong sense of responsibility towards the beauty of objects. They made things with care. We’re in danger of losing that. Progress was so quick, particularly during the economic bubble and the impact on nature was very harsh,” he says. “Our vision is to make Tokyo a garden city. [Ishihara] is a strong person so he can make these ideas a reality. I want to contribute my knowledge and my skills as an architect.”

One of their more ambitious schemes is to turn a landfill island in Tokyo Bay into a forest – the Umi no Mori (Forest on the Sea). “We’re collecting funds now, ¥1,000 (€6) from each person,” he says. “Our target is 500,000 people. The point is to involve Tokyo citizens in the project. It’s not enough to build it – you have to maintain it.”

They’re also working together on an unusual plan for Tokyo’s bid to host the 2016 Olympics. Their ideas would be an antidote to the orgy of construction currently on display in Beijing. It will be a spectacular sight if it is ever built, a grass-covered volcano with a stadium built into its crater. Otherwise Ando’s main plan for the Olympics is to recycle existing buildings. “We’d like to do something environmental, that involves making the most of the existing infrastructure.”

Ando often works on ideas for himself, with no competition in mind, just for his atama – to exercise his mind. The office is filled with models of his own buildings but also of buildings he likes – by architects such as Louis Kahn and Norman Foster – and wants his architects to look at. He is thinking about his legacy. “I want my buildings to last for two hundred years.” Unthinkable for many concrete buildings but maybe not for those built by Ando, who has perfected the use of a frequently unloved material.

“Our target was to make buildings in concrete as beautiful as those made with Portland stone,” he says. “It was one of the big challenges of the 20th century and I’d have to say that we failed.” Not that he’s speaking about himself when he talks of failure. Ando’s style is much imitated but rarely matched and in his hands concrete is a beautiful material. “People have always been fascinated by my use of concrete. The big difference between stone and concrete is that stone has to be cut in pieces but with concrete you can make almost any shape you like.”

Tadao Ando has many admirers but none more dedicated than Yoko Ito. Thanks to Ito, a gallery owner from the Tokyo satellite city of Chofu, Ando has been able to realise what for many architects would be a vain fantasy – an entire street of buildings designed by him.

The project arose from the announcement by the local authorities that they were going to build a 16-metre-wide road through a piece of land Ito had inherited. She was determined to turn what was left into a development that would benefit the whole community. As a fan of Ando’s architecture, she asked him to take on the task of designing the buildings on both sides of the new road; after visiting the site in 1994, he agreed.

Trying to stick to her high ideals for the project while negotiating inheritance tax laws and resisting the temptation to sell the land off was a challenge for Ito. At one point, problems seemed insurmountable and the whole scheme looked doomed. “All those who were involved in this project, including myself, were now half-resigned to abandoning the plan,” says Ando. “It was Ms Ito alone who did not give up and continued to pursue her dream.” She roused the interest of private developers and also got the local government involved. Soon the Sengawa Street project had grown into a more ambitious project and construction was underway.

The first group of buildings which includes the small Tokyo Art Museum, a shop and apartment block were completed in 2004. A community hall and nursery were opened this year. The final structures will be finished in two years’ time. “In the end,” says Ito, “the completed street landscape turned out to be far better than originally envisioned.”