Where I grew up, on the Essex fringe of London, there’s a street that was never finished. The numbering of Goodwood Avenue, Hornchurch, begins at around 350 but before it reaches 200 the road just stops. Beyond, there’s nothing but marshy fields stretching out to the Thames, four miles away.

This particular estate was begun in the 1930s when the capital was still expanding. But it was left unfinished after the second world war, when preventing sprawl was the order of the day. The creation of the metropolitan green belt fixed the boundaries of London as wherever the suburbs happened to stop. So Goodwood Avenue stops midway at an unconvincing simulacrum of countryside that’s hemmed in by homes on three sides. There are two tube stations within a mile, yet despite the housing shortage, there’s no building on this land. Not because it’s unsuitable; simply because nobody’s ever built there.

'The simple, depressing truth is that most people who own homes in the suburbs don’t want those suburbs to expand'

There are places like this not only all over London, but around a dozen other British cities, too. Green belts separate Coventry from Birmingham, Nottingham from Derby, Sunderland from Newcastle. Around some smaller cities – Oxford, Cambridge, York – the green belt is many times larger than the city it surrounds. Sometimes this land is picturesque, often it is not: wander the green belt and before long you find yourself in a field of nettles, or a wooded clearing full of discarded, stinking cans. Yet suggesting there might be better uses for these places is generally the quickest way to radicalise local people.

The case for the green belt is ostensibly compelling. In the first half of the 20th century, cities ballooned as trains and then cars made it easier to work in the city while living in spacious new suburbs. In 1890, the built-up area of London stretched perhaps five miles from Charing Cross; by 1939, it was closer to 15. Middlesex had disappeared under a sea of bricks.

The metropolitan green belt was meant to prevent the city from swallowing up yet more countryside. Today, dense, walkable cities are generally more efficient and more interesting than sprawling, car-friendly ones. And constraints on outward growth encourage developers to regenerate derelict sites rather than cheaply concreting over fields.

‘While the green belt may have contained London’s sprawl, it has simply displaced this growth to the outer ring of commuter towns.’ Photograph: Matt Cardy/Getty Images

But a land use policy designed for the 1950s is an imperfect fit for the 2010s. The modern green belt is often not green at all, but grey or beige, full of quarries, dumps and breakers’ yards. Even where it is green it often takes the form of private golf or pony clubs rather than anything as useful as a park. Huge chunks are intensively farmed agricultural land with limited access, chemical pesticides and a total lack of biodiversity. And while the green belt may have contained London’s sprawl, it has simply displaced this growth to the outer ring of commuter towns. Longer commutes means bigger carbon footprints. Try getting by in Milton Keynes without a car.

The green belt is also clearly implicated in the nation’s housing crisis. There are many reasons why we don’t build enough homes where demand is greatest, including the collapse of council building and the vagaries of the private developers’ business models. But a big one is that we simply don’t have the space. For example, Oxford is so tightly constrained by its green belt that there is no room to build significant numbers of homes. It’s not a coincidence that, once incomes are factored in, it is the least affordable place to live in Britain.

So far, politicians have been reticent to explain the problem. Every major candidate in last year’s London mayoral election – including Sadiq Khan – pledged to protect the green belt. Despite the city’s housing crisis, the Liberal Democrat MP for Oxford West and Abingdon, Layla Moran, is a firm opponent of green belt development, to the frustration of some in her local party.

This is far from politically irrational: people love the green belt. The label is a masterstroke of branding, its words easily conflated with “greenfield” (undeveloped land) when it fact they mean something subtly different: some green belt is brownfield. They conjure up images of rolling fields, hedgerows and areas of outstanding natural beauty, and news reports tend to be illustrated accordingly.

There are powerful forces in place devoted to making sure people keep loving it. Almost any attempt to expand Britain’s housing supply is treated as the sort of thing that will turn Tunbridge Wells into Megacity One. (“New threat to Britain’s green belt” the Daily Mail thundered last week about a policy that didn’t explicitly mention the green belt at all.) The Campaign for the Protection of Rural England has a long and ignoble history of pumping out hysterical press releases about the tide of concrete that it alone can hold back. Last month it promoted a fundraising campaign to prevent the creation of a “London-on-sea”, which was apparently going to engulf much of East Sussex, Essex and Kent. Sadly for those hoping to own a home there, it turned out that they had made the whole thing up.

The simple, depressing truth is that most people who own homes in the suburbs don’t want those suburbs to expand – because it will lower their house price, or block their view, or ruin their favourite walking route with the dog. There are a lot of these people, and they are settled, they are vocal, and they vote. No wonder that politicians listen to them.

Every time they do, though, they are making it harder for Britain to build the homes it needs. They are selling younger people down the river. And gradually, the group that benefits from the green belt is shrinking, and the group it blights is growing. Younger people vote now, too.

The good news is that we really don’t need much green belt land to fix this

In cities like London and Oxford, Cambridge and York, it’s almost impossible to see a solution to the housing crisis that won’t require green belt reform. There isn’t enough brownfield land outside of the green belt. And “densification”, while necessary, by definition means demolishing existing homes to rebuild sites to contain more of them. I’ve noticed that people who propose redeveloping council estates rarely to live on them.

The good news is that we really don’t need much green belt land to fix this. In a 2015 report, the Adam Smith Institute found that just 3.7% of the capital’s green belt would be enough to provide a million new homes, all within walking distance of a station. Almost nobody believes we should scrap the green belt altogether, but by planning properly – working out which bits have decent transport, and are sufficiently ugly they would be better used for housing – we could have a transformative effect on the housing crisis. Looking down from the air, you would barely even notice.

And if we don’t do that? Prices will rise, overcrowding will increase, commutes will get longer, and the economy will suffer as our most productive cities fail to grow. People love the green belt. Perhaps you love it too. But that, to me, seems like a very high price to pay to protect a few nettle-strewn fields.

• Jonn Elledge is editor of the New Statesman’s cities site, CityMetric