This article originally appeared in Planning Magazine on 10th February 2017

Richard Garlick's point (see related articles) about correcting a system that favours the landowner by basing viability tests on existing use values is fine in isolation. But we have created the system by steadfastly refusing to consider greenfield development - which in the South East means green belt - and a continued lack of development plans.

If I own an office building in zone 3 London, why would I sell it for its value as an office? The government effectively changed the value of such sites by pushing forward with permitted development (PD), so that site is immediately valued by the market as a residential development exempt of affordable housing. The site next door, functioning as a leisure use, has exactly the same characteristics, and the landowner knows what the PD site has sold for. He isn't beholden to sell it for leisure values, and because all land uses are constrained by green belt and lack of development planning, every other use class is fighting for the same site. So basic supply and demand economics pushes the price up.

The only way viability can be contained is by placing a compulsory affordable provision on all housing sites. That won't happen, because it will damage brownfield supply.

In fact, the government should probably simply slap a 20 per cent land tax on the sale of all sites, and put that in a ring-fenced pot to directly deliver affordable housing. All they would then need to do is find the sites to buy.

Ian Anderson, executive director, Iceni Projects