Magic roundabout.

An elegant and genial contribution to London's public realm has been made on an unpromising site and with an unusually reductive brief. The city has a new landmark.

As you go south across Waterloo Bridge in London, you are rapidly made aware of an extraordinary phenomenon: a new public building that adds to the urban quality of the British capital. Looking up and down the Thames, you are assaulted by huge arching monsters designed by people like Terry Farrell, who are doubtless intending to make their buildings seem friendly and approachable, but either the size of the programmes or the architects' lack of understanding of human scale has been too overwhelming, and they squat looking over the river like giant amphibians, hungrily waiting to devour you and the whole of the rest of the city - which still, after all the bombing and (much worse) the developments of the last half century, retains a degree of delicacy that is rarely paralleled in the other great economic centres of the world.

Avery Associates' new glass rotunda at the south end of the Bridge reflects the essential gentleness of London without being either in the least reticent or mincing like the work of the Prince Charlesists. It is a memorable and elegant new landmark.

Its grace is the more remarkable because both brief and site were not at all promising. The building contains the biggest British IMAX cinema, created for the nearby British Film Institute. Cinemas have rarely made cheering contributions to the public texture of cities. They are too introverted: somehow much more so than theatres; they are black boxes, with little of the social and spatial apparatus of porches, foyers, and bars of their predecessors. Imax cinemas are much the biggest around, and the architects had to deal with an enormous tall volume and make it part of urban conversation.

The site was terrible. A huge hole had been made by earnest planners in the 1960s to provide green and leafy space in the middle of one of London's biggest and nastiest traffic roundabouts where none of the surrounding buildings has any presence or relationship to its neighbours. Waterloo Station is a chaotic mess (Nicholas Grimshaw's noble termination to the Channel Tunnel railway, AR September 1993, is hidden from the road behind a dim 1920s Classical front). What was hoped to be a soothing urban space in the middle of all this was reached by tunnels under the surrounding roads. The strategy went catastrophically wrong. Tramps and vagabonds soon occupied the place and its tunnels; the plants died; it became a frightening, violent and most unpleasant place to visit. For a long time there were many discussions about how to deal with the disastrously disorientated people who lived down there in filth and squalor. In the end, Lambeth, the local council, heroically arranged for every single person to get a home to live in which has proper sanitation, heating and protection from the weather. (A few of the distressed seem to have crept back now, but their presence is manageable.)

The roundabout more or less dictated a round building, and so did the brief, the main requirement of which (apart from the huge screen which is over 20m high, and 26m wide) was to be able to get a full audience of 482 people into and out of the auditorium every hour. This necessitated a very simple and generous circulation system. You go in at the lowest level, where the underpass used to be. Here behind glass walls are the entrance hall and ticket office and a cafe (which was surprisingly sunny when I went there, considering that it is at the bottom of a hole). Up the cranked stairs from here is a large foyer, which will be furnished so that people can wait in comfort. On up again through the light and soundproofed lobbies to emerge right up against the screen. Then the audience splits and climbs up the very steep rake to radially arranged seats. (If you are handicapped, you ascend to the topmost level by lift.) The radial plan arrangement, with its two aisles, seems to make the scale less daunting than it might have been, and it appears to make the screen more immediate. The audience leaves the chamber at the back under the projection box, then descends to the lowest level again by two separate stairs.

Clearly, if a black-box cinema is to be totally surrounded with busy traffic, it has to be very efficiently isolated acoustically. The smooth outer glass wall (hung from the steel frame) is separated by a continuous circular gallery from an inner one made of two skins of multilayered plasterboard. The arrangement provides enough acoustic mass and separation, and is complemented by 150mm of acoustic absorbents over all interior surfaces. A further acoustic problem was generated by two underground railway lines that run at no great depth under the site. Long piles were sunk on either side of the railway tubes and these are topped with oil damped springs to prevent vibration being carried up to the concrete slab on which the steel rotunda was built. Staircases all have a structural break between ground and first floor to prevent the system being short-circuited. Incidentally construction was very difficult in the middle of all the traffic, and one of the determinants of using relatively light circular acoustic walls was the need to use wet building methods as little as possible to minimize delivery times.

It is this wall which gives the cinema its external elegance and scale, making it layered and slightly mysterious. To my mind, its delight has been rather reduced in daytime by (at the client's request) covering the whole inner drum with a giant mural by Howard Hodgkin. From certain aspects, particularly when the picture is at its most red, it makes the building seem unnecessarily heavy, almost as if the inner cylinder is made of brick. Still, the mural is made up of gigantic photographic enlargements of a quite small image by the artist, and the installation is not intended to last more than four years, so it should be possible to arrange for something more sympathetic in future.

In earlier versions of the project (Avery has been turning it over in his mind for a long time), the cylinder was to be covered by vegetation, a little reminiscent of reconstructions of Hadrian's tomb in Rome. Planting has now been reduced to growing up the walls of the surrounding hole, and spanning its rim to the circular slab from which the drum springs. So an awning of wisteria, jasmine, clematis and ivy is gradually growing over the open space, ameliorating the noise and fumes of the traffic, and making the rotunda seem as if it is growing from a green hill.

Beside the plants, the underground spaces will be enlivened by specially commissioned public art installations. Avery Associates are working with Diane Radford on a glass and light work for the tunnel from Waterloo Station, and the one from the National Film Theatre and Avery's Museum of the Moving Image has already been given a touch of magic with an installation by Ron Hasleden in which 8000 deep sapphire diodes gently glow beguilingly.

Other improvements in the routes and form of the tunnels are planned, and shops and cafes are to be added. If it can be properly maintained and controlled, this most unlikely place could become a significant and liked part of the public realm, signalled by the elegant round beacon on top. P.D.

Architect

Avery Associates (London):

Bryan Avery (lead designer)

John Dawson (director in charge)

Jon Neville-Jones (project architect)

Simon Ewings (lead package architect)

Structural engineer

Anthony Hunt Associates Ltd

Acoustics consultant

Bickerdike Allen Partnership

Lighting consultant

DHA Design Services Ltd

Glass

Saint-Gobain Eckelt

External cladding

Bug-AluTechnic AG

Photographs

All photographs by Richard Holttum apart from nos 4 and 10 by Peter Cook