Architecture: One good functionalist deserves another: Should Norman Foster remodel this Modern masterpiece? The debate is fierce, says Jonathan Glancey

A PRIVATE house in Chelsea is at the centre of a row between Sir Norman Foster, one of the foremost architects of our time, and the Twentieth Century Society, which is dedicated to preserving the finest buildings of this century. Sir Norman has been asked by the publisher Paul Hamlyn and his wife, Helen, to remodel their listed home at 64 Old Church Street, opposite the Chelsea Arts Club in London.

The severe, long white villa was designed by Erich Mendelsohn, a contemporary of Le Corbusier and one of the most fascinating architects of this century. Mendelsohn has a particular appeal to historians and conservationists because of his exotic personal history and because so few of his designs survive - just three in Britain.

Mendelsohn was a Prussian Jew, born in Allenstein (now in Poland) in 1887. He trained in Berlin and Munich before qualifying as an architect in 1912. Inspired by the work of the German Expressionists, he worked as a set designer and painter before serving as an engineer on the western and Russian fronts in the First World War.

Only after the war did he begin work on what were at first bizarre buildings (the Einstein tower at Potsdam, 1921, for example, an Expressionist masterpiece) and then a succession of superb streamlined Modern Movement designs: the Schocken department stores in Stuttgart (1928) and Chemnitz (1929) - upon which the design of Peter Jones, the department store in Sloane Square, London, was based a decade later - and the Metal Workers' Union Building in Berlin (1929).

Mendelsohn left Germany with his wife, Louise, a celebrated cellist, as soon as Hitler took power in 1933. Before settling in Palestine and later in the United States (where he died in 1953), he spent several years working in London - where he was much feted. The Modern Movement in Britain was still a struggling infant; Mendelsohn was a giant of established German Modernism.

When Mendelsohn announced that he wanted to stay in Britain, Charles Reilly, the great professor of architecture at Liverpool University, wrote: 'The great man was to become an Englishman . . . it was like adding a Continent to the Empire, and one which would not cost us anything.' In the event, between 1933 and 1936 Mendelsohn realised just three small islands in Britain: the Nimmo House, Chalfont St Giles; the De La Warr Pavilion, Bexhill; and the Chelsea house, known as the Cohen house after its first owner.

All three were designed in collaboration with Serge Chermayeff, a Russian Jew who was still working as a professional ballroom dancer (at 7s 6d a time) and an interior decorator for Waring and Gillow when his hero, Mendelsohn, arrived in Britain. Chermayeff was later to teach Norman Foster at graduate school at Yale; Foster became a favourite pupil and was encouraged to stay on to work with Chermayeff after taking his master's diploma.

Denis Cohen, the Chelsea house's original owner, died in the early Sixties and the house was bought by the Hamlyns. Now they want to alter it - ever so slightly. They want to turn Cohen's squash court (an integral and windowless part of the house) into a library, replace the unsympathetic conservatory - added years after Mendelsohn completed the house - with a new, low-key design, add a lift and change the servants' accommodation.

The Hamlyns have dropped their original plan to alter the factory-like street front (except to paint it white again; it is currently a wholemeal beige), but they want to add windows to their proposed library, which would mean altering the Cubist garden front.

If the house were an 18th-century design or a Victorian villa, it would be no surprise to find conservationists huffing, puffing and going red in the face with indignation. Naturally they would expect the job of restoring such a building to be placed on the drawing board of an architect with a solid track record in conservation. But this house raises a number of issues new to conservation.

It might seem appropriate that Foster - one of the late 20th-century's finest architects - should be asked to restore and remodel the work of one of the early 20th-century masters. This is not, however, the view of the Twentieth Century Society, nor of members of the international conservation body Docomomo (Documentation of the Modern Movement).

They believe that Sir Norman's progressivist approach to design makes him fundamentally unsuited to tamper with what Alan Powers, the secretary of the Twentieth Century Society, and James Dunnett, an architect and spokesman for Docomomo, call a 'finite work of art that cannot be changed in any way without destroying its integrity'. Mr Dunnett has written to Sir Norman comparing the Cohen house with the white villas Le Corbusier built on the edge of Paris in the Twenties.

A conflict between Sir Norman and the 20th century seems a bizarre idea; for if any architect can be said to personify many of this century's key elements of progress, technology, change, and to combine them with an undeviatingly optimistic view of the future, it is Sir Norman - architect of such overtly Modern buildings as the new terminal at Stansted airport and the Sainsbury Centre for the Visual Arts at the University of East Anglia.

Sir Norman is pure 20th-century man in other ways, too: he is a skilled pilot and a collector of abstract modern art, his education has been international and his work is spread across the world - there are sophisticated Foster buildings in Tokyo and Hong Kong, in Barcelona and Nmes. Yet all this means that for conservationists, he typifies the kind of modern architect for whom progress and the future is everything and history, if not bunk, is simply a blind spot over a silk-suited shoulder.

Mr Powers says: 'The Twentieth Century Society is not against Sir Norman, but we believe that no one should be allowed to tamper with a listed masterpiece of the early Modern Movement.' This is, as he is the first to admit, a curious case. In fact, the story of the renovation of the Cohen house is destined to set several precedents in conservation policy, if not in law.

Until now, conservationists in Britain have been concerned with the battle to save the nation's best historic buildings. Even in 1993, they must keep an eye on unscrupulous local authorities, private owners and developers who will demolish or radically alter any Georgian building if they can make a fast buck by doing so. The 20th century seemed to have passed conservationists by until recently.

Finally, however, they have been forced to admit not only that the 20th century exists (they are more comfortable with it now that it is almost over and is the stuff of lists, catalogues, auction rooms, exhibitions and museums), but that it has produced its share of architectural masterpieces.

Sir Norman is well aware of Mendelsohn's quality as an architect and the rarity of his surviving buildings (most of his German work was destroyed in the Second World War). He believes, however, like a good functionalist - Mendelsohn was one too - that the Cohen house can be improved without undermining either its authenticity or its artistic value.

To conservationists such as Messrs Powers and Dunnett, this is heresy. In their view, if the Hamlyns find their Mendelsohn house inadequate, then they might consider moving elsewhere. 'Adding windows to the Cohen house,' says Mr Dunnett, 'is like adding a circle to a Ben Nicholson abstract.'

'I sympathise with the Hamlyns,' says Mr Powers, 'but Foster's design is not sufficiently respectful. Why can't Hamlyn write in the dining-room and get Foster to create an artificially lit library within the old squash court? That way there would be no need to add extra windows and Foster would be in his element.'

Some members of the Twentieth Century Society and Docomomo have a more favourable opinion of Foster as conservationist. Christopher Dean, architect and co-ordinator of Docomomo, for instance, has written to Sir Norman pledging his support.

For Sir Norman, the assignment is a labour of love. 'We hardly need such a small, fiddly job on our books,' he says. 'Indeed, some of my colleagues tried to dissuade me from taking it on. It's the sort of job that loses us money, but I see it as a creative dialogue between what Mendelsohn set out to do, what the Hamlyns want and what we can add without in any way undermining the flavour of a very special building.'

The Twentieth Century Society still wants to halt work on the house (due to begin in May), while the Hamlyns are keen to get a move on. Sir Norman's designs already have approval from English Heritage, and unless Mr Powers can marshal his forces to rapid effect, the work is likely to go ahead on schedule.

Whatever happens - and it seems improbable that Sir Norman and his team will spoil 64 Old Church Street - architect and conservationist can at least agree on one thing: 'This is part of a learning curve for us,' says Sir Norman; 'We are learning to live with the 20th century,' says Mr Powers.

Whereas Sir Norman - ever confident that the future can be better than the past - believes that the Cohen house will be better for its rebuilding, Powers, Dunnett and company believe that history will not absolve interference. 'Mendelsohn and Foster,' says Mr Powers, 'will not add up to a doubly good building. One might detract from the other. The early Modern Movement canon is small enough in England as it is. We cannot afford to undermine what we have.'

(Photographs omitted)