It’s a question we’ve asked before on the Deep End, but let’s ask it again: if we plan on building a million new homes every five years or so, where do we stick ’em?

Paul Cheshire of the Spatial Economics Research Centre, has a pretty clear idea:

“Almost every reasonable person must now accept the case that we need to build on some parts of currently designated Greenbelt land.” 

However, he doesn’t think we should build just anywhere. Rather he favours the application of objective criteria to systematically identify the best locations for development –  the key considerations being proximity to transport infrastructure, the availability of work and the need to protect land of high environment value:

“These simple principles quickly point to a plentiful – almost inexhaustible – supply of suitable land. Barney Stringer of QUOD generated a beautiful map recently identifying all the land in London’s 514,000 ha Greenbelt which was within 800 metres of a station (a ten minute walk), was not built on and had no marker of environmental quality beyond being in the Greenbelt. Barney calculates these simple criteria give us 19,334 hectares of highly buildable land with good access to the highest paying jobs in Europe and no identifiable environmental cost at all.”

Assuming, fifty dwellings per hectare this would “give us space for 996,700 houses.”

Or would it? You can see Barney Stringer’s fascinating map for yourself on his blog. The first thing you might notice is that London is surrounded by scores of these sites – and that even they were  all developed in full, most of the green belt would remain untouched.

However, on closer inspection, you might might also see that green belt restrictions aren’t only reason why much of this land hasn’t been – and won’t be – developed. Stringer admits as much:

“…[not] all the sites on this map should be developed. Some are well-used local parks with valuable public access, some have high-profile existing uses (Epsom Downs racecourse for instance), some will have other constraints.”

Take the Tunbridge Wells area, for instance, which is quintessential commuter belt territory. In the Royal Borough itself the site identified on the map covers the town’s historic common – and is therefore not about to be sold off to Barratts anytime soon. Moving north to TW’s blue collar brother, Tonbridge, we find another prime site – but that covers the town’s biggest park and recreation ground, plus a major railway junction. Moving west along the railway line to Redhill we find more sites that would make lovely new housing estates – as long as the residents don’t mind being flooded every winter.

Other identified sites in the area cover school playing fields, tourist attractions, race courses and golf courses. Paul Cheshire may well be right in saying that we build far too many golf courses (and effectively subsidise them by allowing them to be used as untaxed burial sites for inert waste), but it doesn’t change the fact that the best green belt building land isn’t just owned by hard-up farmers desperate to sell – it is occupied by profit-making businesses and wealthy home owners who’d like to preserve their rural idylls, thank you very much. Dislodging them implies a massive legal and political battle of fiendish complexity.

As for the argument that the transport links are just ready-and-waiting for millions of new residents, commuter trains are already packed to the gills – so where’s the investment in new capacity going to come from?

Rather than paying for a general upgrade to one of the world’s densest railway networks, plus scores of additional flood defence schemes and a jihad against the golfers of South East England, might it not make sense to concentrate development in a few major sites on the Garden City model?

This is the real choice for policy makers: piecemeal versus focused development of the green belt. Contrary to the lazy assumptions of the piecemeal brigade both approaches require upfront infrastructure investment, the difference is that focusing this investment (for instance, along the Thames Estuary) has a chance of making places better not worse.