Jennifer and Ken Armstrong were just 29 and 35 when they won the international competition to design it, in 1990. It was their first really big break. Seven years and pounds 20m later, both are back in London, their marriage and their practice ended, though they still appear as a double act to talk about the project.
It opens to the public on 24 September, and I'll put money on its being shortlisted the following day for the RIBA prize for the best building in the EU by a British architect.
The site, which was one of the last open sites on the river, was a tricky shape, like a guitar pick. Onerous rights of light and space applied to the adjacent 19th century building. So the architects made a scale model and simply filled the site with as big a structure as they were allowed by planning, with all the angles cut back to meet regulations. What they were left with was a very ugly structure, but in the right volumes. It persuaded them to put five storeys underground including the theatre, built in such a flexible, modular way that it allows for 16 different configurations. Seats tilt, floors swoop up in tiers, rotate and change levels when needed.
That rigorous honing and paring-down of the first volumes before finding the language to interpret the building influenced its simple purity. It is not a bit of Japan landed in a foreign country, but a softly diffused light box waiting for things to happen. That's why the building has a deliberately pale and restrained palette with glass and sycamore, beech and white. The lack of colour is important. Japanese graphics are so bold and strong.
Ken Armstrong is most pleased with their deliberate understatement. "We stood back and produced a design that is entirely appropriate. Young architects are tempted to put in too much, and looking at our work in the past, while we were never flamboyant, we have never been as restrained." But this isn't architecture with a straitjacket on it. There is a sinuous line to the flow, and the facade by night lights up as different glass bands, like a Noguchi lantern. They may not have wished it, but that is drop- dead fashionable.
So the building in a curious and unexpected way becomes a national symbol as well as a showcase for Japan. Cultural buildings are a bit of an anachronism now the world is a global village on the Net, but this purpose-built building has to expand to stage Sumo wrestling and Kabuki and Noh as well as show Japanese films and exhibit electronic wizardry.
Both Armstrongs immersed themselves in Japanese culture before they blueprinted their building. They experienced the old, and the ceremonial, the hot tub soaks, and Shogun spectaculars, tea ceremonies and the art of origami, ikebana and Noh and Kabuki theatre. Then there's Tokyo as click city, with robots and microchips and virtual reality. Both like the way Japan reinvents itself all the time: the centuries-old wooden temple that takes 20 years to build and no sooner has the last dovetail bolt clicked in and the porcelain cleared away after the tea ceremony than they knock it down to begin again.
"I find the Japanese approach to things utilitarian. It's hardly surprising when you get sushi in a black lacquer box with all its compartments, very compact and well designed. Subtle." So the cultural centre changes between night and day and between seasons. Ken Armstrong describes a Kabuki performance there. Weird instruments and gongs sounding, brilliant gowns and glitter, white faces, expressionless, and behind it all the backdrop of the pale woods - beech and sycamore - of an intimate theatre - intimate because they could configure it any way to arrange the audience in tiers across the stage or in lines around it, with the proscenium projected into the audience. "In the end this building doesn't compete with the spectacle within. The building needs the activities it was designed for to make it come alive."
That expressiveness was witnessed by Jacques Chirac, who believes the Maison du Japon is more than just a cultural exchange between France and Japan and the friendship between the two countries, but "an excellent synthesis of different cultures, of elegance in its construction and respect for the site in an accomplished project".
No wonder that in happier days when Armstrong Associates were in joint practice, their brochure cover showed a parrot fish in an azure sea with a little parasitical fish attached. It illustrates the synthesis that exists between architecture and its site, between architects and their clients and audience, engineers and builders