Development and Planning (Use Classes) Order 1975.
These Regulations stipulate that where a building or other land is used for a purpose of any class specified in the Schedule, the use of such building or other land for any other purpose of the same class shall not be deemed for the purposes of the Development and Planning Act 1974 to involve development of the land, provided that remains the necessity of obtaining planning permission for a change in the use of any building or land if such change involves an intensification of user amounting to a material change in the use.