Each planning authority must keep and maintain a register of brownfield land in it's area. Brownfield land is land which has been previously developed and which might be suitable for housing development. Registers may be kept in two parts. The first part (which is mandatory) is for sites categorised as previously developed land which are suitable, available and achievable for residential development. The second part of the register (which is optional) allows the planning authority to select sites from part one and grant permission in principle (PiP) for housing-led development. Further guidance can be found at http://schemas.opendata.esd.org.uk/brownfield.