As cities grow outwards, they begin to take over the surrounding countryside. Authorities have created Green belts on the undeveloped land around a city to prevent urban sprawl and to prevent the towns and cities merging into one. They also encourage development within the town, and protect the countryside. The need for new houses, particularly large detached and semi-detached houses in pleasant surroundings, means that some areas of green belt land have been built on and others are threatened. That's because it is cheaper to build on empty land than to redevelop brownfield sites in older parts of the town or city. 