Gallery shows off its lighter side

The new extension to the National Portrait Gallery will transform its current modest image

When the National Portrait Gallery opened in 1896 its declared task was to promote "an appreciation and understanding of the men and women who have made and are making British history and culture". Unfortunately, Ewan Christian, its architect had responded to the challenge with all the pomp and gloom of a Victorian portrait. Now new light is going to be shed into the gallery through a wonderful extension funded by the Heritage Lottery and involving an imaginative property deal between the National Portrait Gallery and the neighbouring National Gallery.

The directors, Charles Saumarez Smith and Neil MacGregor, shook hands on it five years ago. The Portrait Gallery surrendered 400 square meters of gallery space in the East wing to the National in exchange for the right to build on their shared courtyard, a site as long and narrow as a lift shaft turned on its side.

Out of respect for the neighbouring National Gallery designed 60 years earlier by William Wilkes, Ewan Christian in 1896 copied it for the façade of the National Portrait Gallery on St Martin's Lane. This was so successful in persuading onlookers that was the National Gallery that it now seems appropriate that it really is. At its far end, is the isolated entrance to the National Portrait Gallery, "almost wilfully reticent" as Charles Saumarez Smith describes it. Behind it now, on the same axis is the £15.9m new wing, skilfully inserted by the architects of the Royal Opera House, Sir Jeremy Dixon and Edward Jones. Once inside the new wing, an 11.3 metre-long escalator, delineated with a blade of blue light, travels to the mezzanine gallery.

Elevators are normally associated with the London Underground, airports and shopping malls, but narrowing it deliberately, this one becomes an architectural element in a dramatic white box.

Jeremy Dixon likes that diagonal movement in space - there is a slightly shorter version in the Royal Opera House. High above daylight from clerestory windows creates a constantly changing play of light upon the broad oak-beamed floor and white plastered walls. A shadow gap of a fine recessed line where the skirting boards would be in the Victorian galleries is a detail worthy of Charles Saumarez Smith description of the architects as "classical modernists".

There are two new galleries. The Balcony has 165 square meters for modern portraits, all white and bathed in diffused natural light. Here hangs the ample new Julie Birchill, the petite Joan Collins, the formidable Margaret Thatcher, the heroes and heroines, victims and villains of our age.

Above it on the next floor, the Tudor Gallery is in dramatic contrast, denying daylight, walls clad in charcoal army greatcoat felt, exquisite paintings lit from angled overhead spots of fibre-optic light. The room has the almost palpable anticipation you find in the theatre, darkening before the performance with just enough light to make the portraits come alive. Fibre optics give the painting sparkle. When the Queen opens the Gallery on 4 May, her namesake, Elizabeth 1 painted by Ditchley, will have a sheen on her red hair, a starchy crispness to her pie frill ruff, each gemstone jewel bright, the chalky whiteness of her porcelain complexion enhanced.

At the extension's heart is the new lecture theatre and an IT study centre which allows visitors access to all of its 10,000 portraits as well as some of the 250,000 in its archives At its head, a rooftop restaurant has spectacular views, past Nelson's Column, from St Martins in the Field to New Zealand House with the Houses of Parliament on the horizon, framed by Whitehall. In a snapshot, it is, the architects claim, a portrait of a nation.

Everyone famous in Britain passes through the National Portrait Gallery. Columnists and cartoonists, supermodels and sportsmen and women, the fashionable, the film stars, heroes, villains and victims, the popular and the political are pinned to its walls. But as the stature of the National Portrait Gallery grew, its building was unable to match it. It needed improved circulation. It needed a significant new building in a site burdened by its Victorian gloominess - Jeremy Dixon calls Ewan Christian "the Eeyore of architecture" - to draw people into the upper galleries housing the permanent collection. It needed a coherent plan so that visitors could find their way around.

As important as the increased space bathed in light, is the sense of plan and purpose this modern building brings. Visitors subliminally absorb the layout of the building rather than abandon their quest in a building that used to be as confusing as Harrods.

"Behind the scheme is the extent to which arts institutions have changed their role over the last two decades," says Charles Saumarez Smith. As a result of pressures to generate funding, in addition to money provided by the Treasury or by local authorities, museums and galleries have to welcome corporate entertaining, sponsorship, as well as more places for the public to linger, browse, eat or shop. "Museums and galleries, if they are publicly funded, should participate as broadly as possible in the public sphere."

When Charles Saumarez Smith came to the NPG from the Victoria and Albert he brought with him the knowledge of working to an architectural master plan. "Museums inevitably, I think, have strong centripetal tendencies. There is a danger of an overall institutional identity being broken into a multiplicity of different competing galleries in the ultimate experience of post-modern fragmentation," he wrote in Museums and the Metropolis, which explains how he brokered the deal with the National.

It is rumoured that he might be the next head of the Victoria and Albert after Dr Alan Borg's retirement. If he is, then maybe he will be able to find a role for the Daniel Libeskind "Spiral" which began as an orientation building for all those confused within the vast V&A, then became a design and craft display centre, and now has nothing intended for it, despite receiving planning permission. All style, and no content. It would be a shame if it didn't make it off the drawingboard. Perhaps Charles Saumarez Smith could make it all happen.