He's gone out of his way to insist that those in most desperate need haven't been elbowed aside in favour of those on middle incomes. But as his post-election flip-flop over White City showed, the mayor has form for failing to insist on social rented being part of the newbuild mix.
How strong is his defence? Part of the argument is that, given help, around 63,000 council and housing association tenants could secure mortgages. Each one that did would free up a space for one of the 330,000 households on London councils' waiting lists. But how many existing tenants will apply? How many would be wise to? Will his shared ownership schemes really make much difference?
There's a caveat to London Councils' generally warm response:
The organisation has also warned that the strategy may not be robust enough to fully rise to the challenges posed by the credit crunch in London.
Then there's that other problem, affecting the full "affordable" range. BBC London asked the ten councils, most of them in east London, being prevailed on to deliver the bulk of Johnson's 50,000 target if they thought they'd manage it. Four declined to comment. Two said maybe, if given more help. Four said a flat "no." The crunch is hurting and his policy has lots to prove.
