CA90250 | Structures and buildings allowance (SBA): outline: meaning of building
From HM Revenue & Customs · Capital Allowances Manual
There is no definition of building in the SBA legislation, so the word takes its everyday meaning.
The dictionary definition of a building is a substantial structure for giving shelter and so you should treat any item constructed with walls and a roof as a building provided that it is of reasonably substantial size. An item that is too small or insubstantial to be a building, such as a tool shed, may be a structure.
Works within the curtilage of a building do not necessarily qualify simply because they are within the curtilage of a qualifying building, and need to be examined in their own right against the conditions for relief.