It won’t surprise many to know that this higgledy-piggledy heap of rectangles is the planned extension to Tate Modern, for it is the modern art museum that, over the past few decades, has come to establish the phenomenon known as destination architecture. It was probably the Pompidou Centre in Paris that started it all off in the Seventies. With its brightly painted inside-out plumbing it worked as a sort of visitor pumping machine. Ever since, other cities have aspired to compete. In recent years museums in New York, Barcelona, Berlin, Lisbon, Madrid and Manchester have all announced new homes, extensions and overhauls.

But, it is Frank Gehry’s Guggenheim in Bilbao that appears to be the model for this new Herzog & de Meuron Tate building in that both look at the structure as a sculpture in its own right. The new Tate extension harks back to the cube: the fundamental abstract art form and the cornerstone of our modern aesthetic. It plays with that problem that tantalised the turn-of-the-century Europeans: how could a canvas become an architectural construction? It echoes the jumbled picture planes of Cubism; of such great founding modernists as Picasso or Braque or Gris.

The plans are striking. But then so was the overhauled Gilbert Scott power station that first brought Tate Modern success. Now this extension threatens to overshadow it; and worse still, to overshadow St Paul’s by throwing the scale of the London skyline out of kilter.

My first question is: why not put it in a run-down area that needs the sort of regeneration that Tate brought to Bankside?

My second is: should the extension be a government priority when we are about to lose works by Poussin and Titian from the National Gallery for lack of funds?

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