A report by the Adam Smith Institute found that by using 4% of London's green belt, one million homes could be built within a 10-minute walk of a railway station. The Economist has criticised green belt policy, saying that unless more houses are built through reforming planning laws and releasing green belt land, then housing space will need to be rationed out. It noted that if general inflation had risen as fast as housing prices had since 1971, a chicken would cost £51; and that Britain is "building less homes today than at any point since the 1920s". According to the independent Institute of Economic Affairs, there is "overwhelming empirical evidence that planning restrictions have a substantial impact on housing costs" and are the main reason why housing was two and a half times more expensive in 2011 than it was in 1975. The Campaign to Protect Rural England argued that "Green Belt land is important for our wider environment, providing us with the trees and the undeveloped land which reduce the effect of the heat generated by big cities. Instead of reducing this green space, we should be using it to its best effect. We know from our research that three-quarters (79%) of the population would like to see more trees planted and more food grown in the areas around towns and cities." Valuation of land and impact on house prices Property companies state that land values follow house prices and that a developer assesses what new build house price is achievable in any particular location with reference to prices and sales rates in the second-hand market and on nearby comparable new build sites. At a basic level (assuming no affordable housing, S106 or CIL), they then multiply that new build house price by the number of homes to be built on the land and to arrive at the gross development value (GDV) of the site. The underlying value of the land is then the GDV less the cost of development and less an allowance for profit. According to a study by Knoll, Schularick and Steger, up to 80% of the rise in houses prices between 1950 and 2012 can be attributed to the increasing price of land. This is for two reasons. Firstly, before 1950 improving transport meant that more and more land was economically usable, but this effect subsided after 1950. Secondly, zoning restrictions did not allow