When Boris Johnson pledged to take on "inappropriate" tall buildings in suburbs, he may not have realised what he was taking on. Encouraged by Ken Livingstone's enthusiasm for height, proposals for towers have started popping up all over London, such as the mini-Manhattan sprouting in Stratford almost unnoticed.

The sheer scale of deciding which of these towers is "inappropriate" is huge.

It would be bold but difficult to confine all new towers to the "clusters" of the City, Canary Wharf and Croydon and would mean stopping dozens of proposals in their tracks. It also raises problems of definition: how many existing towers does it take to make a "cluster"?

The easier decisions facing the Mayor concern the more highprofile skyscrapers such as the 1,000ft funnel proposed for next to Battersea Power Station.

Middle-sized towers of 20 to 30 storeys in less glamorous locations raise murkier questions, compounded by the differences between boroughs. In planning terms it is harder to argue against a tower in Newham, where there are few conservation areas and listed buildings, than in Westminster.

We can expect many more controversies like the one over Queens Market. But Boris has made his promise and now needs to find a way of making it work.

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