IN THIS week’s print edition, I have a piece looking at that most-overlooked part of London: the 40-mile stretch of industrial riverside from Canary Wharf out into the estuary. One of the places I visited was a new housing estate in construction on the banks of the Thames near Barking. I thought I’d quickly return to it, because it illustrated very neatly some of the problems with that NIMBY favourite: why don’t we build more on brownfield land. Here’s why:

IN THIS week’s print edition, I have a piece about that most-overlooked part of London: the 40-mile stretch of industrial riverside from Canary Wharf out into the estuary. One of the places I visited was a new housing estate in construction on the banks of the Thames near Barking. I thought I’d quickly return to it, because it illustrated very neatly some of the problems with that NIMBY favourite: why don’t we build more on brownfield land? Here’s why:

First, no one really wants to live on the typical brownfield site. The Barking Riverside site was formerly occupied by a collection of power stations which shut down in the late 1970s and 1980s. It is still bleak. Enormous electricity pylons stretch as far as the eye can see, running from a nearby substation. On the opposite side of the riverbank sits a sewage treatment plant. Further up the river is an oil depot. As the chap showing me around admitted, “these are all things that a city needs, but not necessarily things that you want to live next to.” The developers have done an excellent job of trying to make the surroundings appealing—but there is only so much you can do.

Second, most unexploited brownfield land is outrageously expensive to build on. The Barking site is on a floodplain, so huge amounts of land must be moved to raise the houses up higher. The attractive ponds and “eco-roofs” are designed to help excessive water drain away instead of flooding the roads and playgrounds. Some of the smart Scandinavian-style houses are being built on landfill 15 metres deep—that has to be decontaminated. Meanwhile, thousands of furry animals—water voles and the like—must be temporarily relocated during the construction.

A policy of putting brownfield land first means that building only starts when prices reach a high enough level to make pretty grubby land viable. That’s why, even though house prices grew at double-digit rates for much of the noughties, private-sector building in the UK never surpassed 200,000 units per year. The planning system in effect creates a ratchet effect, because overtime, as the easier, more popular sites are built on, it takes ever higher prices to achieve the same levels of building. And if prices fall, because (for example) a financial crisis reduces the availability of mortgages, then building dries up extremely quickly.

No one wants to bulldoze England’s prettiest countryside if it is unnecessary. But when campaigners such as the Campaign to Protect Rural England suggest that there is an easy solution in brownfield land, they are deluding themselves. It's not just that there isn't enough of it. Without heavy subsidies developers simply won’t build on such unviable land, for the good reason that it's expensive and people don’t want to live on it. Either we allow more building on green fields, or we succumb to an endless succession of house-price booms, and never enough building.

Update: some good further reading on why a brownfield first policy is a bad idea from Paul Cheshire, of the London School of Economics. He points out that much brownfield land is rich with wildlife, whereas much "greenfield" intensive farmland is pretty devoid of life. If you want to protect nightingales and water voles, it might be better to allow building on more greenfield land.