You know, as the summer's gone on, the Labour party has begun to depress me a bit.

It isn't just that the party looks all but certain to select as its leader yet another white man from North London, whose views on foreign affairs are, well, no, perhaps this isn't the place for that. No, it's that, in the parallel race to become the party's candidate for the London mayoralty, all the housing policies on offer are all so bloody wet.

This assessment may surprise some because, as in almost any conversation about London, housing has popped up in the mayoral race rather a lot. On the surface, at least, the candidates for the Labour nomination have talked about housing loads. Some have backed rent controls; others have talked landlord licensing.

In all, no fewer than four of the six (yes, there are six) candidates have listed housing as the main theme of their campaign. Whoever the party picks, it seems probable that Labour's mayoral candidate will promise to toughen up the requirement on developers to build affordable housing.

So why am I still so bloody miserable about the whole thing? Because the housing problem is, at root, a land problem. And, with only a single exception, Labour's field hasn’t even gone near the land question.

The reason London has a housing crisis is simple. The city's population is shooting up, and its housing stock isn't shooting up to meet it. A pretty big reason for that is that the city is, to the first approximation, full: there just isn't that much land left to build on.

Faced with such a situation, a city is faced with two choices. It can expand outwards; or it can redevelop previously built-up areas ("brownfield") at higher densities, to shove stuff more in. London, thanks to its green belt, has for decades focused entirely on the latter. Five of Labour's six potential mayors have said, very clearly, that this would not change under their leadership.

Image: Neal Hudson/Savills, with labels by CityMetric.

But there’s a problem with this “brownfield first” strategy: it is, by its very nature, achingly slow. There's no enthusiasm whatsoever for knocking down chunks of the city and doing them again. So we've had to make do with individual sites, which are often titchy; or larger areas that aren't already populated, which are normally contaminated or a long way from anywhere or both.

The Thames Gateway is a case in point. A vast stretch of former industrial land by the river east of Docklands, it’s been talked off as London's salvation for decades. But it’d cost so much to redevelop that most builders don’t fancy the job, it's got no public transport to speak of, and nobody wants to live there anyway because it's horrible. It's an answer to the question, "Shit, where can we build something?" but it's not going to address the sheer number of people trying to squeeze into zone 2.

London's population grows by that much about once every three years. It's not even an entire mayoral term

Labour's runners and riders would no doubt respond that they aren't simply pursuing the same strategy as their predecessors. Several have talked about asking the Treasury for new borrowing powers, to get councils building again (good luck with that). The most impressive plan for increasing building rates is probably Tessa Jowell's proposed Homes for Londoners agency, which would arrange funding and assemble land to get the market moving.

These are good ideas. But they don't solve the land problem, because however badly used publically-owned land is at the moment it remains in horribly short supply. Jowell has said that TfL owns the land the size of the London Borough of Camden, which is home to around 235,000 people. That sound like a lot – but at current rates, London's population grows by that much about once every three years. It's not even an entire mayoral term.

If we're to have a hope of putting a brake on the insane speed with which London property prices are rising, the next mayor has to go further than redeveloping a few railway yards.

There are, as far as I can see, two options. One is knocking down large areas and rebuilding them at higher densities. But that means either widespread use of compulsory purchase, evicting social tenants, or both – so, despite support from figures like Lord Adonis it’s hard to see this one coming off.

The other is to build outwards: look at the bits of green belt that aren't working in London's interests, and where the only reason we protect them is because everyone’s terrified of the thin-end-of-the-wedge lobby.

Only one mayoral candidate has had the guts to talk about this. Step forward David Lammy, the Tottenham MP, who has warned that the current regulations "protect wasteland and car parks in Outer London but not parks/playing fields in Inner London". He's right. So why have none of the others agreed with him?

Politics, is the obvious answer: to win back the mayoralty, Labour needs to win in outer London again. "Let's build on the green belt" is not a message calculated to make that happen.

But if London’s next mayor really wants to solve the housing crisis, they’ll either have to move the city towards large scale rebuilding works, or renegotiate the green belt. Those are the options. There are no others.

I understand why Tessa Jowell or Sadiq Khan aren’t talking about tearing up the green belt. What frightens me is the chance they might mean it.

Jonn Elledge is the editor of CityMetric. He is on Twitter, far too much, as @jonnelledge.