Why is it important to establish the planning unit? Pursuant to section 57 of the Town and Country Planning Act 1990 (TCPA 1990), planning permission is required for ‘the carrying out of any development of land’. ‘Development’ is defined in TCPA 1990, s 55(1) as: • the carrying out of building, engineering, mining or other operations in, on, over or under land, or • the making of any material change in the use of any buildings or other land Consequently, TCPA 1990 controls two distinct types of development by mandating that planning permission is required for operational development and material changes of use. See Practice Notes: Operational development and Material change of use. In determining whether a proposal constitutes a material change of use, the courts use the concept of the planning unit to determine the area of land to be considered when identifying the primary use of land (and its ancillary uses), to then assess whether any material change of use within or of that unit has occurred. Tests to help define planning unit The seminal