Around 87% of land in the UK is �greenfield�. That is land which has never previously been developed, or fully-restored former waste disposal landfill sites or minerals extraction land. This could be both within and outside built-up areas.
The remaining 13% is either developed or is deemed as �brownfield�. Brownfield is often referred to as previously-developed land, is that which is or was occupied by a permanent structure, including land within boundary of the developed land and any associated fixed surface infrastructure. This could include vacant and derelict sites and buildings, or land that used for alternative uses, such as employment.
The current Government drive is to build houses upon brownfield sites whilst retaining as much Greenfield, or green belt as feasibly possible. Unfortuately as I have noted above there is more greenfield land available and this becomes the target of many a developer.