Previously Developed Land
There are various definitions of previously developed land in use. For the purposes of this guidance (PPG 3: Housing - 2000), such land is defined as follows:
Previously developed land is that which is or was occupied by a permanent structure (excluding agricultural or forestry buildings), and associated fixed surface infrastructure . The definition covers the curtilage of the development . Previously developed land may occur in both built-up and rural settings. The definition includes defence buildings and land used for mineral extraction and waste disposal  where provision for restoration has not been made through development control procedures .
The definition excludes land and buildings that are currently in use for agricultural or forestry purposes, and land in built-up areas which has not been developed previously (e.g. parks, recreation grounds, and allotments - even though these areas may contain certain urban features such as paths, pavilions and other buildings).
Also excluded is land that was previously developed but where the remains of any structure or activity have blended into the landscape in the process of time (to the extent that it can reasonably be considered as part of the natural surroundings), and where there is a clear reason that could outweigh the re-use of the site - such as its contribution to nature conservation - or it has subsequently been put to an amenity use and cannot be regarded as requiring redevelopment .
1. In other words, the urban land uses as defined by the DETR's Land-Use Change Statistics (excluding 'urban land not previously developed'). See Annex B of Land Use Change in England No 14, which is available at http://www.planning.dtlr.gov.uk/luc14/index.htm.
However, this does not mean that the whole area of the curtilage should therefore be redeveloped. For example, where the footprint of a building only occupies a proportion of a site of which the remainder is open land (such as at an airfield or a hospital) the whole site should not normally be developed to the boundary of the curtilage. The local planning authority should make a judgement about site layout in this context, bearing in mind other planning considerations, such as policies for the protection of open space and playing fields or development in the countryside, how the site relates to the surrounding area, and requirements for on-site open space, buffer strips, landscaped areas, etc.
This relates to minerals and waste sites, which are to remain un-restored after
use because the planning permission allowing them did not include a restoration
condition. All other such sites will be restored to '
5. The definition does not supersede or in any way change the policy in respect of the redevelopment of major developed sites in the Green Belt set out in Annex C to Planning Policy Guidance note 2: Green Belts.
This Definition is and extract from Planning Policy Guidance Note 3: Housing (2000). For links go to PPG's.