The OECD found that the responsiveness of housing supply to demand in the UK was the weakest among developed countries, due in large part to green belt policies. The green belt concept was first introduced for London in 1938 before the 1947 Town and Country Planning Act enabled local authorities to designate the status themselves. The policy was introduced to contain urban sprawl following huge post-war housing developments, and expanded greatly between 1951 and 1964. Since 1979, the green belt has doubled to cover 13 per cent of land in England (while only 2.3 per cent is covered by buildings).