Pyramid on the Thames: Tate Modern's 'unique' extension gets the go-ahead

The Herzog-designed extension to the South Bank gallery is described as "extraordinary and unique" by planners in Southwark. Councillors vote on Tuesday and are expected to approve the revised plans.

The gallery must raise £145 million to build the 70-metre tall building, for which some funds are already in place. Director Sir Nicholas Serota expects the building to be completed by 2014.

Designs for the extension, known as Tate Modern 2, were drawn up five years ago. They have been revised by Swiss architects Herzog & de Meuron - the firm which created the gallery from a disused power station - because of soaring visitor numbers.

The original design, a pyramid made of glass cubes, was redrawn to look as if it is "growing out" of the original gallery. The 11-storey building will provide nearly 25,000sq m of floor space, including a 1,500sq m gallery.

It has become vital to the future of the gallery because of rising visitor numbers. Tate Modern was designed for 1.8million visitors annually, but has reached an average of 4.6 million over recent years and is one of Britain's most popular attractions.

A Tate spokesman said: "There is huge pressure on public facilities. More space is needed to maintain and develop our programme."

The only hold-up in the scheme is a minor objection from Boris Johnson over sloping floors.

His planning officials have told Southwark Council they have concerns that some of the floors and spiral staircases are not accessible to wheelchairs.

But Southwark Council said it posed no "serious barrier to mobility". The gallery is understood to be confident it will not hold up the scheme.

A spokeswoman for the Mayor said he was supportive of an extension to the Tate but added: "He did raise valid issues in respect of accessibility which officers are seeking to resolve."

The council report states: "The architectural qualities are of the highest standard. Its scale and confidence of execution give it an enduring quality that will find its place in the public consciousness over time."

'Brick walls are folded like paper'

Rowan Moore, the Evening Standard's architecture critic, wrote about the new version of the Tate's proposed extension last year: "What was a glassy pile of boxes has become brick and smooth-skinned. The new version is still emphatic, a hefty, hard-to-miss brick tower, but it is no longer a battle of design ideas. The big move is to use the same material as that of the old power station that houses Tate Modern, while using it to make a brick building such as has never been seen before. The bricks hover. Light shines through them. Brick walls are folded like paper... futuristic. Quite simply, the design has got better."