A report commissioned by the Greater London Authority says government must pump an extra £250m a year into social housing to reach Ken Livingstone's goal of making half of all new development affordable.
Affordable Housing in London says the subsidy must be raised to £600m – a total of £7.2bn under the 12-year plan – to hit the mayor's target.

Livingstone said the report, written for the GLA by research institute Three Dragons, showed that his targets were achievable in most boroughs if a flexible approach was taken.

But the research also concluded that the 50% target was not realistic everywhere. One in three councils in poorer areas should be asked to achieve a 35% target, because imposing a higher percentage would deter developers by reducing their profit margins.

The House Builders Federation said the report underlined the folly of the 50% target in the capital. It claimed the government would not offer such subsidies.

A spokesperson said: "Ultimately all housing could be affordable if there was enough public subsidy to make it so. But that simply will not happen, and it is vital for all parties in this debate to remember this."

50% is achievable in most boroughs if a flexible approach is taken

Ken Livingstone, mayor of London

Tony Arbour, chairman of the GLA planning advisory committee, said that the interim report on the mayor's spatial development strategy showed that only one new-build development had been constructed with 50% affordable housing – at Imperial Wharf in the borough of Hammersmith and Fulham. He said this indicated that the target would deter development.

Arbour is calling for a review of the role of the committee, after claims that it was ineffective because it had no statutory power.

Under his proposals, GLA staff would investigate planning applications and broader planning submissions to evaluate the consistency and transparency of the mayor's decisions.

Livingstone and his deputy, Nicky Gavron, would be called as witnesses to hearings of the planning advisory committee and the mayor could be questioned.