A gallery for the 21st century

Usually , when you think about paintings and glass casings, you expect to see the glass on the outside and the art nestling safely within. But it doesn't work like that in the new Early 20th Century room at London's National Portrait Gallery. The glass is laid out in a series of slender, slabby boxes, like giant see-through suitcases sharply squashed. And then the art is hung on the outside of the boxes. The conventional art-glass relationship has been turned completely inside out.

Only, the paintings aren't so much hung upon the boxes as screwed into their surfaces. The plain glass sheets are studded all over with little plastic carbuncles into which you screw your chosen painting by its frame. And the carbuncles that don't happen to be needed for the moment just sit there twinkling like magic under the conservation-level lights, making you think about all the other ways a curator might have arranged an exhibition. It's a plug-in, clip-on heritage show. It's gimmicky and yet also modest, wittily attention-seeking and yet a clever, technically sensible design.

The man responsible is Piers Gough, who has, as he says himself, a reputation for a "rather raunchy" sort of architecture. Fifty years old this year, he was a student at London's Architectural Association in the late 1960s, which was when he first started thinking of his sensibility as Pop. He and his firm, CZWG, are famous for using a lot of colour in their buildings, and unexpected shapes and allusions - shiny blue and a statue of a shire horse at the curvy Circle flats in Bermondsey, shiny greeny-blue and a striking wedge shape at the popular and prize-winning Westbourne Grove public toilets. He's never done a gallery before, although he was shortlisted for the National Gallery's Sainsbury Wing 11 years ago. (The Sainsbury brothers were apparently "freaked out" by his plan to paper an entire gallery in gold leaf, and that was that.)

Though his work is clearly witty and mischievous, Gough is a serious avant-garde designer and urban campaigner, utterly opposed to the blandness of Prince Charles. He's Pop without being populist, if to be populist means giving people what they think they want without encouraging them to think their preconceptions through. And he's a gifted verbal communicator who likes to maintain a strong public profile. All these things make him a perfect choice for rejigging the previously dull and worthy Duveen Wing of the NPG.

There are important pieces in the NPG's Victorian and Early 20th Century galleries. But fundamentally, the NPG is about famous and familiar likenesses before it is about fine art. That, as Gough says, gives a designer an unusual freedom with his imagination. It's a freedom he has used to the full. The idea of the glass boxes is, he says, that they should "people the gallery with pictures". "It's like being at a cocktail party, seeing people through people and so on." This, of course, is exactly how we tend to go round the NPG anyway, making a beeline for the chaps we know already, strik-ing up the odd new acquaintance as we go on our way. It's nice to have this friendly philis-tinism so acknowledged. And it's nice to be made to think through a gallery's social and aesthetic function.

And there's another function that Gough's installations make you think about a little: the gallery as mausoleum, peopled by floating early 20th- century ghosts. A chevron-slotted ceiling holds spotlights focused to hit exhibits and nothing but the exhibits. In blunt contradiction to the modernist associations of all the glass and hardware, the walls are papered in a dull-grey silk. It's more disconcerting than you'd expect to glimpse a painting's reverse side when you weren't meaning to. The shiver you get from seeing something usually hidden reminded me a little of the shock I felt when I first saw the Pompidou Centre, with all those digestive innards crawling openly across its front.

The glass boxes, together with a new hanging system and some curiously kitchen-unit-like wooden shutters for the Victorian gallery, have cost the NPG pounds 1.2m. That's peanuts in gallery-renovation terms, but then again, the job hasn't involved any structural work. Will Gough's system still be there in a decade? Or will its emphatic conceptual-artiness, in the long term, prove a bit much? You know you're entering the Goughland part of the gallery, incidentally, when you come across an upside-down triangle of Victorian busts arranged in 6-5-4 formation. And you can decide for yourself which you find the odder: the busts themselves, or the curious installation of which they have been made a part.

NPG, WC2 (0171 306 0055). Admission free. Piers Gough will be discussing his designs with NPG director Charles Saumarez Smith, 12 Nov, 7.15pm; tickets pounds 10 in advance.