SOUTH BANK SHOW.

Fitted into the shell of Giles Gilbert Scott's heroic Bankside Power Station, the Tate Gallery has an impressive new home for part of its collection that will act as a wider stimulus for artistic creativity and urban regeneration.

The conversion of Bankside Power Station in Southwark to house part of the Tate Gallery's collection of modern art represents the reinvention of a national institution as well as the regeneration of an industrial monument and its surroundings. The Tate Gallery holds one of the three or four most significant collections of twentieth-century art in the world, on a par with MoMA in New York. Each day, 20 000 people throng through its existing galleries housed in a nineteenth-century palazzo on the river Thames at Millbank. The phenomenal growth in art audiences coupled with a serious shortage of space eventually convinced the Tate that it should divide its collection into two. A new institution, Tate Modern, will contain an expanding accumulation of international twentieth-century and contemporary art, together with fluid, flexible spaces for multidisciplinary exhibitions and performances, while the existing Millbank building will be adapted to become the national museum of British art from 1750 to the present d ay.

The Tate's record of architecture patronage is commendable. In recent years it has built new galleries to house its collections in Liverpool (AR July 1988) and St Ives (AR July 1993), as well as Stirling's Clore Gallery for the Turner bequest at Millbank (AR June 1987). Its decision to locate the new Tate Modern in Southwark, an impoverished London borough, might seem unorthodox, but Giles Gilbert Scott's cavernous former power station had the dual advantages of space and an outstanding riverside setting. Moreover, the immediate area is undergoing a stealthy regeneration. During the sixteenth and seventeenth centuries, Southwark was London's bawdy pleasure district; more recently, its cheap warehouse spaces attracted colonies of artists and craftspeople. Now there is talk of a new arts quarter and Richard MacCormac's Jubilee Line station (AR May 1999) reinforces the borough's links with the rest of London.

An international competition for the new Tate was won by Herzog & de Meuron. Compared with other shortlisted designs, the Swiss practice's proposals adhered most closely to the building's original organization. There were no overtly wilful gestures, no cutting into the building or applied decoration. Instead, through a series of minimal interventions, the heroic, brooding volumes of Scott's industrial monolith are simply and rationally converted to make a variety of spaces for art. Nicholas Serota, the Tate's director sees these 'as a set of instruments which will be played in different ways by artists and curators in the years ahead'.

The most visible intervention is a two-storey 'light beam' that runs longitudinally along the building, its emphatic horizontality counterbalancing the strong vertical mass of the central chimney. This new structure brings light into galleries and is illuminated at night; its huge mass pulsating with light above the Southwark skyline evocatively signals the building's reincarnation.

Designed in two phases between 1948 and 1963, Bankside reflects Scott's interest in the early Dutch Modernism, manifest in a taut brick skin incised with long vertical openings. Herzog & de Meuron leave this largely intact, but gently diffuse the building's necessarily hermetic character with a big ramp on the west side that funnels visitors into the soaring, cathedral-like volume of the former turbine hall. Now carved open to its full height, this forms an imposing public street for performances and displays of art, particularly large sculptures and installations. Public facilities, such as education rooms, shops and information spaces are arranged around the turbine hall, which acts as a constant reference point for visitors as they circulate through the new museum.

Despite variations in size, configuration and lighting conditions, the gallery spaces constitute a recognizable overall family. This partly stems from an early decision to maintain a minimum room height of around 5m, rather than creating a hierarchy of floors, which lends itself to stratification by medium or school. On the north side of the turbine hall, three floors of galleries are organized around a central concourse. Multiple entrances to groups of rooms enable visitors to construct their own routes through the building. Gallery walls are generally plain white, with floors of either timber or concrete. Some spaces are lit from the side through Scott's existing slot-like windows, others from above through clerestoreys. Some rooms are totally enclosed, others have bracing views out over the Thames or back into the turbine hall.

In the longer term, the strip on the south side of the hall (in use as a switch house, since Bankside still functions as electricity sub-station) will be converted to create additional gallery spaces. These will be of a rougher, more mutable character than the galleries on the north side, for experimental exhibitions, performances and collaborations between different artistic disciplines. Reflecting the changing nature of art museums, Herzog & de Meuron's finely considered architecture serves as a stimulus to creativity rather than a static constraint. Clearly the new Tate is an important, complex organism and once the art is installed in May, we shall return to it in greater detail.