Serota has to tone down Tate Modern extension

When architects designed an extension for Tate Modern made up of a futuristic 11-storey glass structure and giant blocks protruding from a ziggurat, the gallery's director, Sir Nicholas Serota, had his work cut out convincing a divided council planning committee to give permission to his "spectacular" landmark.

So there was confusion when, after securing a hard-fought victory, Sir Nicholas this week stood before Southwark Council again, two years after his last appearance, to ask for planning permission for a different design after scrapping the wacky ziggurat idea.

The six-strong council committee, chaired by Councillor James Gurling, did little to disguise their surprise over why Sir Nicholas should be back so soon with radically toned down plans for Tate Modern 2 when he had been so effusive over the original version by architects, Herzog & de Meuron.

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After a lengthy discussion and yet another silver-tongued hard-sell, Sir Nicholas yesterday won over the committee – with a unanimous vote this time unlike the divided decision in 2007.

The committee also granted Tate an extended five years, rather than the usual three, to finance the £215m project, after taking into account the current financial climate. Its completion is planned for 2012 but a five-year extension would mean completion in 2013 if Tate was to make full use of it. Yesterday, Tate said it had so far raised £74m – a third of the overall costs for creating the extra gallery space.

Prior to approval, committee members raised the fact that Sir Nicholas had "sold us" the original idea and said they would need to be "persuaded" to approve this comparatively subdued design. A source said: "The original plan was to have a building that really stood out. The committee said they had to be persuaded that this different design was more appropriate."

Sir Nicholas said that after the original discussion with the planning committee – which consisted of different members who had encouraged reflection on the design – he had had time to rethink. In the end, Southwark Council commended the revised plans, saying: "The proposed new building will be an extraordinary and unique addition to London's townscape."

Sir Nicholas, in turn, said: "We are delighted that Herzog & de Meuron's revised plans have been granted approval... We look forward to creating one of the most exciting cultural buildings in Europe..."

The revision was made in consultation with artists and curators. Disused oil tanks in the former power station lie at the heart of the extension and will be retained as raw spaces for art.

Tate Modern was originally designed to cater for up to two million visitors but current visitor numbers reach up to 5m.