Concrete couture: A former chocolate factory in Milan's industrial quarter has been magically transformed into a setting for the spectacle and business of haute couture.

Milan is reverting to what it must have been in the centuries before Italian unification, a city state of private wealth and public squalor, in which ubiquitous graffiti and crumbling services crowd the privileged enclaves of fashion. One imagines that today's merchant princes have as little contact -- or concern -- with urban degeneration as their aristocratic forebears, but the art of patronage still flourishes. Giorgio Armani commissioned Tadao Ando to turn a former Nestle chocolate factory into his world headquarters, with a theatre for fashion shows and other events. Two masters of rigour and restraint -- one tailoring cloth to flatter the figure, the other deploying concrete masses in light as a setting for human activity -- have found common ground. It was Armani who said, 'I want to create something as simple as possible but of lasting value, and I have always loved and admired the Japanese spirit in buildings', but the words would be equally apt for Ando. The product of their collaboration is a trium ph of refined austerity.

The low, greenish brick building is located in the Porta Genova district, which is slowly mutating from industry to arts, despite the proximity of a muck-filled canal. In a country fiercely protective of its past, even mundane facades are sacrosanct and the only outward evidence of transformation is a modest sign and some inset panels of translucent glass. One of these slides back to reveal a corridor, 100m long, that slopes gently up to the rear of the building. Polished stone payers, Ando's signature concrete walls, and a white plaster ceiling seem to float clear of each other, held apart by narrow strips of concealed lighting at top and bottom. These are augmented by uplights in the row of slender free-standing columns that bisect this axis.

The first hint of the offices and showrooms concealed behind the walls comes near the end of the corridor where an expanse of glass frames a narrow interior court with a shallow pool reflecting light into the spaces that surround it. A dining room that doubles as a gallery and reception hall extends along one side of the court and this is lit from a long low window that draws the eye down to the water and the sandstone floor.

A cove-lit ceiling vault braced with polished steel rods is dramatically cut away to reveal a steel-framed butterfly roof and clerestory that survive from the original factory.

A curved and tilted concrete wall, washed by light from a recessed ceiling, defines a foyer at the end of the corridor and conceals the cloakroom and lavatories. Three blocks of backlit glass serve as reception desks. An etched steel door opens onto the black-box theatre, with its 682 upholstered armchairs mounted on dark wood bleachers flanking a polished metal runway that is intensively lit from above and below. During a fashion show, photographers are clustered on tiers beside the door facing the models as they emerge at the far end. Bleachers can be lowered and the seating reconfigured to accommodate conferences, performances, art installations and movies.

Ando has created an enduring frame for an icon of fashion and a stage for its votaries, while cleverly concealing the private domain of staff and buyers. Indeed, the whole complex is like a theatre, with an auditorium and expansive backstage area, and a corridor serving (like the grand staircase of La Scala) as a processional route on which the glitterati can preen and strut. It also demonstrates how this architect has reinvented the traditional Japanese aesthetic of light and shade, offering linear progression through a walled labyrinth, guiding foot and eye, concealing and selectively revealing to build anticipation for the drama to come. Materials are plain, forms simple, but the effects are thrilling. From Buddhist temple to European fashion house, Ando finds a common thread between diverse cultures and patterns of human behaviour.