The secret of a facelift: can you see the join?

Sir Denys Lasdun was true to his word when he said that his Royal College of Physicians could be altered, adapted and extended over 100 years of occupation. So why not involve him in remodelling the National Theatre? By Jonathan Glancey

Sir Denys Lasdun telephones to ask if I would meet him at the Royal College of Physicians in London's Regent's Park. He wants me to see the extension he has designed for this most handsome and assured Sixties building. Yet, when I arrive, it is hard to know whether or not the architect has been pulling my leg. What extension?

There is one, of course. You can see Lasdun sitting inside its serene upper-storey in the photograph. The fact that from the outside, the chaste white college appears to have remained unchanged is because Lasdun always intended it that way. Here was a striking modern building, owing much to Cubism and Le Corbusier that was, nevertheless, designed to cope with change, and over a very long period.

Architects like to claim that they design with change in mind, but although their sentiment is genuine, harsh reality finds buildings that age gracelessly or need to be hacked around, often unsympathetically, to accommodate new rooms and new roles. In a speech he gave at the Royal Institute of British Architects in 1965, one year after the completion of the Royal College of Physicians, Lasdun said the building "can be altered, adapted and extended through a century of occupation". A third of a century on, his words have been borne out. Very recently, the college asked its architect back to extend the building and, unless you look very closely, you can hardly see the join.

There is a join, and Lasdun enjoys seeing if visitors can spot it. And here it is: a new chapter house-like extension emerging from the building's north wall. The only way to be sure that it is brand new is to look at its curved surfaces of dark-blue industrial bricks, and from these back to the dark-blue industrial bricks of the original. The new bricks are metric, the old bricks imperial. "This is an extension, not an exercise in conservation," says Lasdun as we go back inside the college, "although, as you can see, it is possible to blend in new work without slavishly copying the old. The college is designed in two layers, or what I call strata. The formal rooms - entrance hall, library, dining room and so on - are housed in the grand white building you see as if standing on piloti, or columns, from Regent's Park. The offices, workaday rooms and lecture theatres, are contained in the dark brick structure that threads in and out of the raised section at ground level. Where the top, or white, half of the building is formal in plan, the undercroft, or brick, half of the building is informal; it meanders. And so you can alter, extend and adapt the building at ground level without ever upsetting the symmetry of what Palladio would have called the piano nobile."

This makes sense both aesthetically and pragmatically. Over the past 30 years, and in the coming 70, the college has been and is unlikely to change the layout of the formal reception rooms on the upper floors of this beautifully tended building. This is because the function of dining halls, for example, never really changes. Those of Oxbridge colleges are up to 700 years old and are still serving the purpose for which they were built. Change, however, is needed periodically in the extent and layout of administrative offices and meeting rooms, and this is where the Royal College of Physicians is such an intelligent building: it allows for natural change without this threatening the perceived formal nature of the building's composition.

Inside, the extension comprises a lecture theatre on the ground floor and a beautifully lit council chamber above, its roof a concrete vault that brings to mind that of a medieval chapter house. "It is designed to feel very calm," says Lasdun, "because college debates tend to get rather heated."

Although Lasdun's extension seems a natural part of the original building, it has its own special character inside. It clearly belongs, but is a gentle move forwards from what already existed. In short, it is a model of how to extend a building without the architect being either precious or contrary.

Strange, then, to discover that, despite a lot of fuss in both the national press and behind the scenes over the past two or three years, he is still excluded from plans to remodel parts of his masterpiece, the Royal National Theatre, on the south side of London's Waterloo Bridge. If anyone should be asked to carry out new work, should it be necessary, it is Lasdun.He might be in his early eighties but he remains younger in mind and heart than many an architect 40 years his junior. He would hate to bow out knowing that his greatest building has been butchered and without him being given the chance to see if and how the building can be enhanced.

The theatre wants to demolish one of the building's concrete walkways, to fill in the porte-cochere that leads to the lower-level entrance and ticket office with a new book shop and to push out the great, cathedral- like window of the main lobby. This triple-pronged attack on the fabric of the 20-year-old theatre will, undoubtedly, undermine its aesthetic integrity. The removal of the walkway, for example, would be like extracting a column from the portico of the Parthenon. At the moment it is blocked up, meaning it is neither serving a useful purpose norattractive to look at. The new book shop, meanwhile, will block carefully framed views of St Paul's Cathedral and the bend on the Thames known as King's Reach. This is a particularly insensitive move on the part of the theatre trustees, as the whole point of the layout of the great terraced building is to relate the experience of the auditor to the foyers and then to the terraces and thus to the city beyond. If the experience of watching a play in the Olivier Theatre is the closest thing one can imagine to being a part of an audience in ancient Greece (albeit without the azure skies), this is because it is what Lasdun was trying to achieve in a contemporary way and with modern materials and technology.

The book shop could, in any case, be found a home outside the theatre, across Waterloo Bridge in Covent Garden perhaps, where it would enjoy more space than at the theatre itself, which is a place to meet and watch plays rather than to shop. Moving the great lobby window will be nothing less than an act of barbarism; it would be as if John Piper's stained- glass window at Coventry Cathedral were pushed back several yards to make room for a few more tourists in Sir Basil Spence's famous building. No, the National Theatre is all of a piece, and if the theatre trustees want change, then they should turn to Lasdun. "It's not as if I have to do the work myself," he says. "But I would like some say in the practice chosen, and it would be rather nice if it was one I could speak to freely."

Lasdun is right to feel aggrieved. If changes to the National Theatre were clearly going to improve it, then he would be the first to welcome change. It is not, however, an easy building to tinker with. It was built, whether you find it easy on the eye or not, with great intelligence and passion. It was meant to be a building that playgoers and others walking by could occupy both inside and outside as if its massive tiered terraces were a part of some geological formation. It has a powerful logic of its own and one that as younger generations discover it becomes progressively more accessible and appreciated.

Lasdun's experience of the way we experience new buildings goes back longer than that of almost any architect still working. Born in London in 1914, and trained at the Architectural Association, Bedford Square, his first building was a house in Newton Road, Paddington, much influenced by Le Corbusier (1887-1965). It was constructed in 1937 when Le Corbusier was very much in his prime. He worked for Wells Coates and Berthold Lubetkin, took part in the D-Day landings building emergency forward landing strips for RAF fighter squadrons, and set up in his own right in 1960 when he won the commission for the Royal College of Physicians. "They wanted something like old Herbert Baker's South Africa House at first," says Lasdun. "I said no. I wouldn't compromise and assumed my stubbornness meant I had lost the commission. Perhaps clients were braver in those days, I don't know, but I was wrong and got the job."

Aside from being hugely experienced and much admired by critics and historians, Lasdun has lived long enough to begin to see the tide swing back in his favour. There is no need for him to go post- Modern or Deconstructivist, or whatever, in 1996 to court favour with changing tastes. The number of young people who like the National Theatre is legion, and any number of twenty- and thirty-somethings setting up home say they would "die" (better if they lived) for one of the superb duplex flats he designed in a block facing Green Park from London's St James's Place ("You would never get permission to build something as radical as those flats today," says Lasdun). Equally, many young people understand how Lasdun can be a passionate fan of Nicholas Hawksmoor while having been Britain's most convincing advocate of Cubism in his glory years.

"Lasdun's character," wrote the architectural historian Alastair Service in 1979, "is outwardly one of kindness and quiet charm, but beneath this lies obsessive qualities, passion and concentrated energy." Seventeen years on, he has not changed. That passion and energy, allied to a formidable intelligence and understanding of the nature of change - positive, brave, forward-looking change - is why he is a friend to so many younger talents and why he is the first architect the National Theatre should look to if it really feels it must press ahead with alterations. And, if any of the theatre's trustees should still harbour lingering doubts, the Royal College of Physicians would be only too happy to show them Sir Denys Lasdun's very latest work.