CA90250 - Structures and buildings allowance (SBA): outline: meaning of building
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.
Anything that qualifies for plant and machinery allowances (taking into account the provisions in CAA01/S21-23) will not qualify for SBA CA94300. It follows that expenditure excluded from being on the provision of plant or machinery by CAA01/S21 (and not within CAA01/S23) may qualify for SBA.