Architecture: Putting Geffrye on the map

THE GEFFRYE Museum, where interior designers and furniture makers get all their ideas, used to be the best-kept secret in London. No more. On Wednesday Prince Charles presided at the dedication ceremony of the new wing of the Geffrye Museum - an astonishingly stylised Arts and Crafts addition, by Nigel Coates and Doug Branson, to the old almshouses, built in 1751.

Prince Charles hesitantly called the new building "a modern dynamic extension to complement the existing architecture" and revealed his admiration for its "wonderful sense of atmosphere and place". The building offers 1,900 square metres of new space, almost doubling the public areas of the museum, and nestles behind the sober almshouse facade so discreetly, you would never know it was there.

Ever since 1914, when a band of Arts and Crafts enthusiasts petitioned for the Geffrye to become a museum, period roomsets have lined the long corridor that runs the length of the facade.

The museum, which aims to illustrate middle-class urban fashions in furnishings from 1600 to 1900, ended in a bit of a cul de sac until this new building sprang out of the ground. Now, the tour of the period rooms breaks out into a great brick loop shaped like a horseshoe. The space in between is sheltered by a floating latticed grid of roof, painted "sicko primrose" in the words of Doug Branson. Two gabled brick buildings at either end of this horseshoe house the kitchen and the bookshop, with exhibition rooms for the 20th century in the curve. A central staircase leads to the education rooms, temporary exhibition gallery, design centre and collections stores below.

The total cost is pounds 5.3m, of which pounds 3.75m was provided by the Heritage Lottery Fund, which is why Chris Smith was at the opening to congratulate everyone on the stringent controls and budget restraints.

Visitors will always enter the new building from the existing 18th-century buildings. It has no entrance as such, except the roofed transitional area between the two gabled walls of the new galleries and the corner of the old building, which becomes a restaurant. The glazed overhang outside, almost a verandah, is supported by a forest of treetrunk-like steel girders.

Heavyweight old-fashioned windows that open by hand, and the latticed roof that the architects call a diagrid, give the restaurant a conservatory feel that is anything but conservative in taste. Nigel Coates insists that the building is "crafted, but not Arts and Crafts", because he doesn't like the Gothic spin those words carry with them. His work is more Frank Lloyd Wright than Art Nouveau, with big columns that open out like lotus leaves reminiscent of those in the Johnson Wax factory.

"This is our interpretation of that moment of transition from 20th century to the 21st," says Coates. "Arts and Crafts was the 19th- to 20th-century style statement. We gave the Geffrye the intimacy of an Italian hilltop town, not a grandiose colonnaded cafe. We just extended the mood and vocabulary of the place, like writing another act for an opera."

The Geffrye Museum offers a glimpse of other times, from the throne-like seats in the Stuart rooms (all the rage after King Charles II's exile in France introduced Continental flourishes to plain English furniture) to loft living in the 1990s, with everything happening in one open-plan space: the futon on the mezzanine floor; no dining-room but a breakfast- bar stool at a self-assembly IKEA kitchen customised with granite tops; no fireplace, but the focal point a telly with Matthew Hilton's stretch- limo club chairs pulled up alongside it. Wallpaper*, The Independent and last week's Economist, as Chris Smith observed in his opening address, were chosen to represent late 1990s reading tastes.

What makes the Geffrye Museum's displays invaluable for scholars is that they highlight key changes in decorative styles and social customs over the course of the century, providing clues to the directions in which interior styles might develop in the new millennium. Living in 19th-century buildings designed to house now-obsolete machinery is just one of them. Space is going to be the biggest luxury of all, a fact the Geffrye Museum trustees recognised in choosing such a bold team as Branson Coates to take them into the 21st century.