''People in Switzerland and Spain think we're one of the big offices in London. They're shocked when they discover we're not," explains Adam Caruso, one of the two partners of the architectural firm Caruso St John. The firm was responsible for the sensuous reconfiguration of Stortorget, the baroque main square in the Swedish town of Kalmar, and has been asked to design a housing scheme near Bordeaux to serve as an exemplar for French architects. But at home, it remains outside the architectural mainstream.
Caruso St John sprang to public attention in Britain in 2000 with the opening of the widely praised New Art Gallery in Walsall. The only architects of their generation - Peter St John was born in 1959, Adam Caruso in 1962 - to pull off a major (£21m) Lottery project, stardom seemed only a step away. But instead of a flood of job offers, the firm found itself with nothing and shrank to just three people. "The Barbican [where it renovated the concert hall] saved us," Caruso recalls.
Since then it has clawed its way back. Last year, it finished five projects: a house, additions to Denys Lasdun's Hallfield School in Paddington, west London, and three art galleries. The firm now has a staff of 20 and has had to take on new office space. But these recent projects are all relatively small, on nothing like the scale of Walsall. Caruso even doubts whether the firm will ever be invited to design another school, despite the vast sums for new buildings sloshing around in the education system. "The only reason we got Hallfield was that there were two architects on the governing body. They weren't impressed by the list of firms the Royal Institute of British Architects provided and asked why we weren't on it."
The difficulty Caruso St John faces in Britain is that its work goes against the grain. Two sorts of architects dominate the public perception of British architecture: the high-tech firms led by Norman Foster and Richard Rogers, whose work needs no introduction, and form-makers such as Zaha Hadid, Foreign Office Architects and Will Alsop, whose work is remarkable for its fluid plasticity. Clients like high-tech because it is perceived to symbolise modern efficiency: embarrassing discussion of aesthetics can be brushed aside by rational responses to pragmatic problems. Meanwhile, with their dramatic gestures and wildly imaginative shapes, the form-makers are seen to be daring, rejecting convention and tradition.
Caruso St John is very different. Three things stand out from its work: an obsession with how buildings are built, a firm grounding in tradition, and an interest in ornament. Not that it has anything in common with the Prince Charles school or such born-again classicists as Quinlan Terry. Its buildings do not look like buildings of the past, but they are infused with their lessons.
Hardwick Hall, Robert Smythson's great Elizabethan house, for instance, was a major influence on the New Art Gallery. Many of its sources are 20th century, such as the great Scandinavian architect Sigurd Lewerentz, whose Church of St Mark, Bjorkhagen, entirely built of brick, is a clear source for the recently completed Brick House, also in Paddington. But Adam Caruso is equally fascinated by the late-Victorian Arts and Crafts architect Philip Webb. "We would like to practise architecture with the erudition and wit of Karl Friedrich Schinkel [the German neoclassicist], Webb and Adolf Loos [the influential early 20th-century Austrian avant-garde architect]. All three of them had strong ethical positions, which allowed their work to transcend the empty aesthetic rhetoric."
Owen Jones, famous for his book The Grammar of Ornament (1856), is another Victorian influence. This is most evident in the new facade planned for the Museum of Childhood at Bethnal Green, in the East End of London, which will be richly patterned in different-coloured stones. For a modern architect, this is a dramatic statement. "Would the museum's facade be more shocking if it used fractal geometries [like Daniel Libeskind]? No - less shocking," says Caruso.
He is undoubtedly right, at least for a profession still bound up in the ideals of modernism, where ornament is crime and novelty is all, where anything built before 1910, or perhaps even yesterday, is irrelevant. "Chaos theory, fractals, the environment - these are all words architects claim to engage with. But why isn't architecture about architecture?"
For those sated on the easy images of high-tech and the form-makers, Caruso St John's intense, serious engagement with what has been the essence of architecture for centuries is truly radical. Its architecture is not easy, but it offers an escape from the dead hand of modernist theory. As such, it potentially represents one of the most important architectural developments of the 21st century.
"Caruso St John: cover versions" is at the AA Gallery, Architectural Association, 36 Bedford Square, London WC1 (020 7887 4000) until 3 November
Giles Worsley is architecture critic of the Daily Telegraph