... and why a starship never got off the ground

What if ... Bankside Power Station had been demolished and the Tate had been able to build entirely afresh on the site? What would a gallery designed to represent the dream of the new millennium look like? What would it be made of? How might it work ?

Future Systems, a firm of avant-garde, London-based architects led by Amanda Levete and Jan Kaplicky, began thinking about the opportunities such a scenario could offer 18 months before the announcement of the Tate's competition to find an architect to convert the power station into a museum of modern art.

"At the time," says Amanda Levete, "we had no reason to believe that Bankside would become the focus of the Tate's ambitions. We just happened to think that the site, opposite St Paul's Cathedral, was an excellent one. We did enter the first stage of thecompetition and, naturally, some of the ideas we explored in our `Gallery for the 21st Century' project were incorporated into our proposal for the Tate."

However fascinating - a weatherproof glass membrane enveloping the old building - Future Systems' competition entry did not make it on to the Tate's shortlist of six architects.

"Of course we were disappointed," says Levete. "We aren't rich, and competitions cost us a fortune. But apart from hoping to win, we use them as a form of research and development, working with some of the country's most inventive engineers. We're also competition masochists; we get an intense buzz from entering competitions; if we feel so unhappy when we lose, it's partly because we put so much into them. But what I feel is truly disappointing about the Tate project is that it isn't a new building.

"In fact," she says, "every single one of London's architectural projects up for Millennium Commission funding is a conversion of an existing building or buildings: the South Bank, the British Museum, the Royal Opera House, Albertopolis. This says a lot about the way we think about the future; most of all it says we don't want to go there and, if we do go there, we haven't any new ideas."

The Gallery for the 21st Century is indisputably new and shows what sort of architecture we could be groping towards if only we could stop ourselves from turning every major building opportunity that comes our way into a compromise.

An exquisite model in Future System's studio in London's Fitzrovia reveals a low-lying silver structure in the guise of a doughnut-ring with the hole in the middle, covered in an ingenious glass umbrella that would wash the art galleries and circulation spaces beneath it in passively controlled, yet highly modulated, north-facing daylight. The gallery is like a space station come to land on the banks of the Thames. But if it represents an alien presence, it is more The Day the Earth Stood Still than Alien, a friendly invader and a thing of futuristic dreams. By night, the ceramic-coated walls of this art gallery-as-starship would be bathed in glowing golden light, the glass membrane of its vast suspended roof brought to life by artificial light shiningthrough 8,000 skylights, and an illuminated pedestrian bridge across the Thames would draw visitors from St Paul's and arriving by boat from east and west.

This remarkable design has been devised with Andy Sedgwick, of Ove Arup and Partners, an engineer with great expertise in controlling the use of daylight in contemporary buildings. Sedgwick worked recently on the stupendous Richelieu wing of the Louvre and, perhaps at his most inspired, with Renzo Piano, between 1981 and 1985, on the design of the superb Menil Collection museum in Houston, Texas.

It was at Houston that Sedgwick, Piano and their respective teams showed how it was perfectly possible to display precious artworks in daylight with the minimal use of artificial lighting; the ceiling and roofing system create a controlled, yet varied, play of daylight that subtly changed the way light fell on artworks and so altered the way visitors perceived them through the course of a morning or an afternoon.

Working with Sedgwick, Future Systems is confident that its great membrane roof would work: watertight and free from glare. Suspended by cable-masts attached to a vast steel ring beam, the ceiling-roof would be, in every way, the highlight and defining force of this powerful design.

"What this roof enables us to do," says Levete, "is plan a gallery that offers maximum flexibility for curators and a true fluidity of movement. The gallery is a single, modulated space; its plan is simple - a visitor could take in its logic in moments, the three main floors, all exposed to the eye, leading off from a sequence of ramp-like stairs."

What would it cost? "We've had it reliably costed at £75m," says Levete, "cheaper than the Tate's project at Bankside and not much more than the cost of a pair of F-15 jet fighters."

Beyond questions of practicality (yes, it would work) and price (a good buy), this building would be charged with a beauty matched only by that of spaceships or futuristic cities encountered in dreams. But just as we have lost our faith in the exploration of space, preferring war and shopping, so we have jettisoned the idea of progress: art is best served, so it appears, in converted warehouses, power stations or new buildings that, described as Post-Modern, are antithetical to progress and cynical about the future. That, agree with it or not, is the reality. It would be a joy if someone, somewhere had the courage to build such an innocent, feasible and uncompromised building as Future Systems' Gallery for the 21st Century.

The Gallery for the 21st Century can be seen at the Concord Gallery, 174 High Holborn, London WC1, from 2-24 February.