How the Swiss will transform Bankside...

Jonathan Glancey looks at what will be - and what might have been - the home of the new Tate Gallery at the former power station

Jacques Herzog and Pierre de Meuron, models of cool, Calvinist restraint and Swiss precision, have won the Tate Gallery's competition to transform the monumental Bankside Power Station, on the Thames opposite St Paul's Cathedral, into a millennial gallery of modern art.

They might have been tailor made to suit the temperament and aesthetic sensibilities of Nicholas Serota, director of the Tate Gallery, for whom the Bankside project is a major investment, both personally and professionally. Public funding for the conversion - somewhere between £80m and £100m - is not guaranteed, although should Millennium Fund money fail to come the Tate's way (it is hoping for £50m), Mr Serota has been nothing short of meticulous in weaving the ambitions and largess of the rich and arty into his and the Swiss architects' plans. In carrying patrons and collectors with them - even taking them on a tour of Herzog & de Meuron's exquisitely crafted Swiss buildings before Christmas - Mr Serota has forged what should prove to be a watertightlink between the avowed architectural taste of the avant-garde art establishment (neat, minimal, ordered, everything unpleasant swept under the carpet) and his own delight in architecture at once pared down and poetic.

In choosing the Swiss, Mr Serota and his fellow competition judges have rejected the undisputed talents of five rival designers, among them David Chipperfield, the one British architect considered (he would have knocked the chimney off Bankside) and Tadao Ando, long said to be a particular favourite of Mr Serota's. The others were Renzo Piano (Italy), Rafael Moneo (Spain) and the Office of Metropolitan Architecture (Netherlands), fronted by Rem Koolhaas.

Mr Serota was introduced to the work of Herzog & de Meuron by Ricky Burdett, director of the Architecture Foundation and long-time champion of the Swiss duo. As one of the competition judges, Mr Burdett helped to steer them through waves of well-known international architects into the harbour of Artistic Favour. Of course, this strategy could have backfired if the judges had actually disliked Herzog & de Meuron's work, but as the reality is very beautiful (and difficult to capture in newspaper photographs), the risk was nicely calculated. Before the competition, the two were virtual unknowns in Britain: last week they were the unanimous choice of the Tate judges.

This might sound as if the competition for the conversion of Bankside Power Station was all but redundant; if the client - the Trustees of the Tate Gallery de jure, Nicholas Serota de facto - wanted architects of Herzog & de Meuron's puritanical grace, why not commission them direct? Why have teams of enthusiastic, often hard-up architects toiling long, unprofitable nights over complex drawings and models, when they were not really wanted in the first place?

Two generous answers spring to mind. The first is that competitions encourage both the best known and most obscure architects into spurts of creativity (who knows what surprises lie in store for them and their judges?). The second, and circumstantial, isthat the Millennium Commission is unlikely to fund architectural schemes not chosen through competition.

Even so, the Bankside venture suggests that those commissioning buildings with a strong idea of what they want in mind would be better off employing the architect of their choice direct, rather than going through the professional and emotional roller-coaster of a contest.

Let that rest. The Tate has got a firm of architects for Bankside that should suit it handsomely. Herzog & de Meuron (both born in Basle in 1950, qualifying from Zurich Polytechnic together in 1975 and setting up practice in Basle in 1978) will leave thestructure of the power station virtually unscathed. This is a wise move, given that Bankside is not to be demolished, thus saving the Tate from the anguish and wrath of conservation bodies and from spending vast sums of money on the outside.

Naturally, there will be some changes. The north front of the power station, approached, possibly, by a footbridge over the Thames from St Paul's, will feature layers of glass walling - clear, fretted, milky, acid-etched - bringing daylight to three floors of galleries stacked east and west of Bankside's 325ft chimney. At the top of the building, a glazed steel box the length of the structure will support the weight of the galleries below, bring extra daylight into the depths of the museum and cause it to glow prettily at night. The museum's main cafe and restaurant will be sited on the power station's flat roof, now a field of asphalt raised into the London smog.

The main entrance will be from the west. Visitors - up to 20,000 a day are expected - will descend a large ramp into the vastness of the 500ft-long turbine hall, which will be left much as it is today but with the generating equipment stripped out, the brick and steel walls insulated and the whole vastness cleaned and polished. This giant atrium or galleria will act as a great junction box distributing visitors to points of orientation, cloakrooms, shops, lavatories, lifts and escalators. It w ill not be air-conditioned. It will also be big enough to contain artworks as big as houses - literally; in one of their drawings, Herzog & de Meuron show Rachel Whiteread's ill-fated plaster-cast House standing in the former turbine house, and quite dwa rfed by it.

Cutting across this east-west axis at first floor level, a 65ft-wide bridge (reached by lifts and escalators) will make a north-south cross-axis. It will also lead to secondary entrances to the north (riverside) and south (Southwark).

The air-conditioned galleries, stacked east and west of the chimney, will be extremely simple spaces that can be made any length curators need or want. Facing the Thames, they will benefit from the great bands of glass the architects propose for the front of the building.

Does all this sound simple almost to the point of banality? Simple, undoubtedly, banal no. If you took a look at the plans and other drawings of a typical buiding by Herzog & de Meuron, you might be forgiven for assuming that the real thing is as plain and as yawn-inducing as a Sixties-style concrete box. This is not the case. Although simple in the way they are planned and organised, Herzog & de Meuron's buildings - nearly all in their native Switzerland - are as thoroughly detailed and as well engineered as a ... Swiss watch; not a flamboyant Swatch or ostentatious Rolex, but more the sort of Omega that a pilot or mountain climber might choose. None of their drawings prepare you for the rich and variegated, almost savagely Ruskinian, exteriors

of some of their best buildings - a warehouse clad in an assortment of wooden planks and tin in Laufen, a timber house shoehorned into a Basle courtyard - nor the handsome materials employed inside them, nor the lovely play of daylight that lifts them from the abstract lines of the drawing board into likeable and memorable architecture. They have designed a house for an art collector near Basle, an art gallery in Munich, and - to prove how high their standing is at home - the Swiss Pavilion for the Sao Paulo Biennale.

What you will not get from Herzog & de Meuron is a National Gallery extension or any other kind of crudely camp Post-Modern architecture. These boys are toughies, intellectually and aesthetically, but not cool in the sense that the fashionable British school of minimalist architecture, witnessed in any number of art galleries and architecty-restaurants, are frightfully Cool. Argumentative to the point of what some people see as arrogance, Jacques Herzog, spokesman for the Swiss practice, speak s with passion.

"I support the idea of the architect as artist, but I think that to apply the image of art to architecture is the worst thing you can do. Contemporary architecture tends to behave like an advertising copywriter; it exploits the field of art, taking advantage of art in order to renew its own image without reflecting its conceptual foundations - and everybody gets tired of applied images. To escape being trapped in the world of Post-Modern graphics, the architect can also over-react by finding himself con verted into a pure pragmatist."

These are fine words and will, no doubt, baste Bankside in gently glowing architectural finesse. Yet fine words, handsome deeds, the support of fashionable, rich patrons and all the cleverness of Nicholas Serota will not defuse the highly charged cries of those who feel, deep down, that the opportunity to build a brand new museum of modern art to coincide with the millennium has been lost in an oily canvas of compromise, competition, convention and conservation. All one can say is that the projects' detractors will have to come back to Bankside in six years' time to see whether Herzog & de Meuron have managed to breathe new energy into the redundant temple of power that broods accusingly across the Thames from Wren's famously compromised masterpiece.