Boris Johnson’s official biography says he “oversaw the building of a record number of new affordable homes” in his first term as Mayor of London.

And he says there will be a “major future pipeline of additional homes” in the future.

But critics claim lower earners are increasingly being priced out of buying or even renting a home in many parts of the capital.

What has Boris actually achieved, and will it be enough to ease London’s housing crisis?

The mayor has repeatedly set out a target of building 55,000 more affordable homes between April 2011 and March 2015.

This doesn’t mean he has approved or funded all them personally. Affordable homes funded by the Greater London Authority (GLA), and those built by private developers as part of planning agreements with councils, all count towards the target.

In a London Assembly meeting last month, Boris appeared to admit that not all the homes would be ready in time to hit the target.

And his office confirmed that to FactCheck today, saying 18,590 affordable homes would be completed by the end of this financial year.

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Added to the total built since April 2011, that means 53,730 will be built during the four-year period, not 55,000. And that’s assuming that all these planned homes are finished on time.

Before we get accused of Boris-bashing, we ought to point out that he is on track to deliver more than 100,000 affordable homes over two terms in office, and that is more than were built under the previous eight years.

It may well also be technically true that 2014/15 will see the highest number of affordable homes built since 1980, although this simply means that massive amounts of building work is being crammed into this year rather than spaced out equally over the last four.

Two big questions remain.

What does “affordable” really mean?

Governments have changed the definition over the years, so the goalposts keep moving.

Within the overall umbrella term of “affordable housing” there are big differences in affordability. Homes designated for “social rent” means tenants will get charged around 30 to 40 per cent of the average rent charged in the local private rented market.

“Affordable rent” means the rent could be as high as 80 per cent of local rental prices, although the average will be 65 per cent for GLA-funded schemes.

That is obviously a very different proposition. In many parts of London, a so-called “affordable” home could be well beyond the reach of a low-earner if a developer wanted to charge the full 80 per cent.

Over the last four years the number of more expensive “affordable rent homes” being built each year has risen sharply, while the number of cheaper “social rent” homes has more than halved.

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Will supply keep up with demand?

Boris has set out plans to build 42,000 new homes a year over 10 years. Some 15,000 a year of these will be “affordable” although the definition is vague.

This looks like a low target, compared to several independent estimates of the housing that will actually be needed to keep up with London’s growing population. Savills say London needs 50,000 new homes a year.

And indeed, the mayor’s own Strategic Housing Market Assessment says London needs 48,800 homes a year over 20 years, with about 25,000 a year in the “affordable” bracket.

If you want to clear the backlog of under-supply in just 10 years,  you would need to build 62,000 homes a year, the researchers said.

The verdict

Boris Johnson delivered more affordable homes than his predecessor. Lack of cheaper housing is a long-term problem that has built up over many years, while London’s population and house prices have soared.

But he looks set to miss his four-year target for affordable homes.

His long-term housing plan is just an aspiration. Actually hitting the targets will depend on all kinds of variables in the planning system.

One trend that has emerged since the crash might hinder things: developers have been agreeing to build a minimum number of affordable homes when applying for planning permission for new housing, then trying to wriggle out of the deal.

Builders often try to claim they won’t be able to make a profit if councils force them to stick to affordable housing targets.

It’s difficult to say how big a problem this is, but last year the Bureau of Investigative Journalism found that 60 per cent of the biggest housing developments in the planning system were not meeting affordable housing targets.

Boris Johnson has made increasing use of his powers to intervene in the planning process, approving developments despite councils raising objections over issues including affordable housing targets.

He defended his recent decision to let Royal Mail turn its Mount Pleasant sorting office into flats by saying that building new homes was “absolutely crucial” and the plan had been in danger of falling through after Islington and Camden councils rejected it.

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