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One half of Foreign Office Architects is missing. "Alejandro is on his way, but don't worry, we can start without him," says Farshid Moussavi, whose delicate Persian features mask a feisty businesswoman clearly used to holding court on her own. Moussavi is part of one of the most successful partnerships in the business. After 10 years together, Moussavi and her Spanish husband, Alejandro Zaera-Polo have established themselves as "the coolest architects in the world" (according to The Times), with a portfolio that includes the acclaimed Yokohama International Ferry Terminal in Japan and the masterplan for London's bid for the 2012 Olympics.

So how did it all start? Moussavi, 38, left a still turbulent, post-revolution Iran in 1979 to join her brother in England. "Because of the upheaval and turmoil of the revolution, my parents weren't sure of the educational opportunities in Iran," she says. So Moussavi had a very proper British education, attending boarding school in Brighton before studying architecture at Dundee University and The Bartlett School at University College London – although none of these institutions softened her accent much.

The pair met while studying at Harvard but did not become a couple until they went to work for Rem Koolhaas at the Office for Metropolitan Architecture in Rotterdam, in 1991. After two years at OMA, Moussavi and Zaera-Polo's ambition was to go it alone. "Some people know that they like to be independent," says Moussavi. "We realised we worked well together and had common interests. OMA was a very demanding office that required full commitment, but we knew at that stage we wanted to give priority to our own work."

In 1993, the pair married, moved to London and set up Foreign Office Architects in Pimlico. Both taught at the Architectural Association for eight years, combining FOA projects with teaching, which remains an integral part of their lives and the life of the practice.

Zaera-Polo, 39, is currently Dean of the Berlage Institute in the Netherlands, and Moussavi is a professor at the Academy of Fine Arts in Vienna. "We also encourage other members of FOA to go out and teach as much as possible," says Moussavi.

In 1996, FOA won its breakthrough project when it was commissioned to design the Yokohama International Ferry Terminal in Japan. Completed in May 2002, the £130m scheme has secured FOA's reputation as a practice able to deliver innovative design on a grand scale – it also earned the practice the honour of representing Britain at the 2002 Venice Biennale. Moussavi still seems surprised that they managed to pull it off: "The Japanese took a big risk by entrusting a 50,000 m² site to a very young office with no experience on that kind of scale. Japan was a fantastic place to work and we'd like to do more projects there."

Zaera-Polo arrives and he sits silently nodding in agreement as Moussavi describes how offers of work have flooded in since Yokohama. "We don't go looking for projects, they come to us," she says. Among these importunate schemes are the Zona Franca Office Park in Barcelona, a private sports harbour in the Netherlands and what the practice CV describes as a "publishing headquarters" in Paju City, Korea. At mention of this project, Moussavi laughs and displays just a touch of humility: "Headquarters makes it sound so grand – it's actually quite a small project."

After Yokohama, does the practice feel the pressure of delivering even bigger and better schemes? "Absolutely," says Moussavi. "We were very worried that we would be labelled by large infrastructural projects but we were determined not to live under the shadow of Yokohama. Luckily we've been approached by clients for small and medium-scale projects, so we've built up a very mixed portfolio, which is great because we're not interested in developing a signature style."

Moussavi uses the examples of Yokohama and the newly completed Villajoyosa police station near Alicante in Spain to illustrate the practice's range. "Whereas Yokohama is all about surfaces and is extreme in its publicness, this police station needed to be very private. The only light that comes through is from the windows on the roof. We used local stone and concrete to reflect local agricultural sheds so that the station just disappears into the landscape – the total opposite of Yokohama."

Talking of pressure, how does the husband-and-wife partnership operate in the office? "Initially, we used to share everything at all levels," says Zaera-Polo, who is gradually beginning to stir. "But as more projects came in, that was no longer feasible. We still try to work on projects together in the office phase but then we tend to split the management of clients between us." So it makes sense that Zaera-Polo oversees most of the practice's work in Spain where, he feels, the architect has greater input into projects than in the UK. He says: "Here, an architect's job is done after the handover of designs but in Spain you can make amendments by scribbling a sketch on a wall."

And how about working with your other half? Moussavi opts for abrupt but jocular honesty: "The disadvantage is that you have no free time from each other," she laughs. Zaera-Polo takes a more diplomatic route. "When you're working to such intensity, like on Yokohama, nobody else would understand that you have to work every hour until 2am. So that's clearly an advantage."

But mixing marriage with business will inevitably lead to disagreements. "Of course we disagree on projects!" says Moussavi. "Ultimately what's best for the practice is not to impose our aims on each other but to fish out the best idea, no matter who puts it on the table." Zaera-Polo smiles, clearly proud of the diplomacy the practice adopts when disagreements arise. He says: "We regularly get the office to vote on a contentious idea," but he is interrupted by his wife's laughter. "But that doesn't mean once you get the vote it goes your way," she says.

Of course we disagree on projects! But what’s best for the practice is to fish out the best idea, no matter who puts it on the table

Although Moussavi appears fairly used to getting her own way these days, it was not always so. "At school, you are equal with the men, but when you go out to work it is not the same," she says. Yokohama was a testing environment for Moussavi, who feels that women are forced to work harder than men to gain a client's trust. "When I went to Japan, I experienced this acutely," she says. "They were not used to vocal women and I think I broke their protocol constantly – especially when I was supervising the site pregnant!" she laughs.

True to its name, the practice has worked all over the world, from Japan to the Netherlands – but where to next? "Personally, I would love to work in China," says Zaera-Polo. "The sheer scale of the country and recent development means that urban problems will soon arise that need interesting new solutions." Moussavi turns to her husband. "What about England?" she asks, referring to the fact that despite the practice's London base, it has completed only three projects in the UK – namely the Belgo restaurants in London and Bristol in 1999 and the Bermondsey Antiques market back in 1997. But this may soon change. The practice is shortlisted to design the perpetually delayed music centre for the BBC at White City and is up against Foster and Partners and Lifschutz Davidson for the proposed redevelopment of Pelham Square in Hastings.

High-profile competitions for landmark buildings are what FOA do best. The practice is in the running to design the Pompidou Centre in Metz, France, and was last year shortlisted in the design competition for the World Trade Centre, eventually won by Daniel Libeskind. South Bank Centre chief executive Michael Lynch has also been in talks with the practice regarding the potential redevelopment of Jubilee Gardens and Hungerford Car Park. "Anyone would think we have some big PR machine working for us, but we really don't," says Moussavi. "We're too small and too busy – we've just been really lucky in having the right exposure."

Last month, FOA bagged a competition win that may eclipse even Yokohama. It is part of the consortium led by EDAW and HOK Sport appointed to draw up the masterplan for London's 2012 Olympics bid, mainly based in the Lower Lea Valley. FOA currently has two people working full-time with the consortium at its project office.