Boris Johnson's housing adviser says deal is 'real achievement', but admits not all of 50,000 target will be new build

London mayor Boris Johnson has got agreement from 21 out of 33 London boroughs on housing targets designed to ensure 50,000 affordable homes are built in the capital by 2011.

Richard Blakeway, Johnson's housing adviser, said the work meant the mayor now had commitments from boroughs to build 40,000 out of the 50,000 homes he pledged in his mayoral manifesto.

The news came as Johnson launched a public consultation on his housing strategy, with a commitment to halve overcrowding in social housing by 2016, primarily through building new large affordable homes.

Blakeway said the agreement with the boroughs over targets for each of the boroughs was a “real achievement” of the new mayor. He said: “If this gets delivered them we're on course to build the most amount of affordable housing since 1994. For those boroughs that haven't agreed a target they're all around the table.”

However Blakeway admitted that not all of the 50,000 target would be met by new build, with the GLA reserving the right to buy up existing homes to meet the figure.

When elected Johnson scrapped the target of former mayor Ken Livingstone that 50% of all new homes had to be affordable. Instead he said he would ensure 50,000 affordable homes were built by 2011, through reaching agreements on targets with the boroughs.

The policy was immediately criticised by Mike Tuffrey, the Liberal Democrat London Assembly housing spokesman. He said: ““It is bitterly disappointing that after seven months of negotiation we discover that the mayor has fallen so far short of his key pledge he made to Londoners.

“We need a mayor who's prepared to be the Teddy Roosevelt of housing - speaking softly to the boroughs but carrying a big stick too. Boris Johnson needs to tell boroughs he'll take over planning their large sites and use the £5 billion he has at his disposal to get house building moving.”