Janice757

It is as simple as that: the architects admit they decided to do exactly the opposite of what they did before. Or as Jacques Herzog says, "To make it too hermetic would be the wrong answer." Since what they did before was extraordinarily restrained compared to some of the competing designs for the museum, it follows that what comes now must be extremely exuberant. And thus it proves.

It is not only remarkable for an art museum to be extended so soon after its completion - the optimistic target date to finish Phase 2 is 2012, when the Olympics come to London - but it is astonishingly big. It will cost as much as the original gallery in real terms - much more with inflation, bringing the estimated final cost to ?15 million. It will tower over the original building (though not its chimney) from the rear, re-orienting it away from the riverside and incorporating large chunks of the old structure.

It has all been made possible by happenstance: the rear, southern section of the old power station has remained in use as a transformer and switchgear hub for the City of London, but now all that kit is to be upgraded. Today's equipment is much more compact, and so can be shoehorned into the eastern end of the building. That means that Herzog and de Meuron can get to grips with the whole south-western end, using not only the old transformer hall but also an existing below-ground cluster of three enormous former oil tanks, which will become performance spaces.

However, this time round new building will dominate the old rather than vice-versa. And how. Given a list of space and function requirements by the Tate which attempts to engage with contemporary art and art-education practice but is bound to mutate, Jacques Herzog and his team has opted to pile all the spaces up into a kind of eroded ziggurat, rising to 70 metres. This is to be clad in cast glass - the rough, bobbly sort.

It looks highly complex and it is, but not as much as it might be. These are not pods slung from some structural armature, rather a relatively straightforward concrete post-and-beam arrangement that happens to have highly irregular edges. Herzog and de Meuron's Harry Gugger has scribbled a rough sketch to explain this.

You have to wonder about the wisdom of a glass-clad building facing south - especially given that most of the gallery spaces inside will have to have solid walls. You wonder equally about the energy efficiency of all that broken-up surface area. Then again, the architects intend to use ground-water energy storage, along with surplus heat from the transformer station and extensive insulation, to make this an exemplary building in energy terms. Though they are a bit vague over how they will clean it. Never mind. To quote Tadao Ando's interpreter on being presented with similar technical questions: "Mr. Ando, he solve these problems."

There is also the matter of whether the hip new annexe, being so big, will rather put the original building into the shade and take over. No, says Herzog. "We don't want to create ghettos. There won't be an old Tate Modern and a young Tate Modern."

It will surely comprehensively eclipse the "final" element of Tate Modern's plan: to allow a new Design Museum, much larger than the old, to snuggle up against its south-eastern end. Visually, this may be so but in cultural terms I suspect the arrangement will work pretty well. I wonder if the Design Museum really wants its independence, or if it wouldn't mind becoming another Tate franchise? Either way, it will be in the right place. In planning terms the aim of all this - and an ambitious landscape plan which will send tendrils into the Southwark hinterland - is to create a new cultural quarter in this part of London. We'll have the Architecture Foundation's new building, designed by Zaha Hadid, nearby. There are many theatres and other galleries round about. It is already regenerating nicely, if with too much in the way of bland corporate office blocks (and one by Will Alsop).

For some of us who saw the unveiling of the designs, however, the most striking thing was the way the scheme uncannily recalls sculptor Rachel Whiteread's huge polystyrene-box installation "Embankment" in the Tate Modern's turbine hall last year. Jacques Herzog saw it and liked it. But no, he says. That's just more happenstance.

How come you got billion of ideas?