Finally had a chance to catch-up with Centre for Cities report on where to build homes for Britain's most successful cities.
In the ten least affordable (British) cities building out every brownfield site delivers a total of 425,000 extra houses. If you go outside of their existing built-up area and use land within 25 minute walk of an existing train station you could add up to 1.4 million new homes (at reasonable densities). If neighbouring authorities could be made to cooperate that total rises to 3.4 million homes within 2 km of existing train stations.
The trouble, of course, is that those 3.4 million homes (within walking distance of existing infrastructure) would need to be built on green-belt land. In total, around 12.5% of the green belt land around those cities would be needed for development.
Achieving agreement on this scale of development on green belt land will clearly be difficult. Although, as the report notes, developing brownfield land can be a complex process and may require cities to take additional actions (e.g. land assembly) and investment (e.g. new infrastructure). And that brownfield land is only capable of delivering a fraction of the homes that could be built around existing infrastructure in the greenbelt.
In short, as the report makes clear, both options have their challenges, but making housing more affordable in our most successful cities will require a more sensible debate to designate land on its merits rather than according to whether it is currently designated as brown or green field.