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HMRC internal manual

Capital Allowances Manual

IBA: Buildings and Structures: Meaning of building

There is no definition of building in the IBA legislation. The dictionary defines a building as a substantial structure for giving shelter and so you should treat anything with four walls and a roof as a building provided that it is of reasonably substantial size. Something that is too small or insubstantial to be a building, such as a garden shed, will be a structure.