Zero-rating the construction of buildings: is a building being constructed: interpreting Note 16 in relation to extension, enlargement and annexe
Deciding whether the works constitute an enlargement, extension or annexe to an existing building is a two stage process. It requires an examination and comparison of the building as it was or (if more than one) the buildings as they were before the works were carried out and the building or buildings as they will be after the works are completed.
The question then to be asked is whether the completed works amount to the enlargement of or the construction of an extension or annexe to the original building. That question is to be asked at the date of supply. The answer must be given after an objective examination of the physical characteristics of the building or buildings at the two points in time, having regard to similarities and differences in appearance, the layout and how the building or buildings are equipped to function (Cantrell and another  STC 100).
It is not necessary to give those factors equal weight, and a development may well amount to an extension even though markedly different in design and appearance from the existing building (Mr & Mrs Elliott (QB  STC 369)
An annexe is an adjunct or accessory and, when used in this context in relation to a building, it means a supplementary structure (Cantrell and another (No 2)  EWHC 404(ch)).
Also of relevance is Bryan Thomas Macnamara (VTD 16039), which says, with our emphasis in italics:
Note 16 deals with [the series of building works excluded from the expression ‘construction of a building’] in descending order of their degree of integration with the existing building. Conversions, reconstruction and the alterations of existing buildings, the most closely integrated are excluded. Enlargements of existing buildings are then excluded, the word ‘enlargement’ connoting structural work producing an overall increase in the size or capacity. The word ‘extension’ in relation to an existing building refers, we think, to building work which provides an additional section or wing to that existing building; the degree of integration is one stage less than with enlargements. Then come ‘annexes’ which, as a matter of principle, are also excluded. The term annexe connotes something that is adjoined but either not integrated with the existing building or of tenuous integration.
The decision in Colchester Sixth Form College (VTD 16252) explains how to apply the above test:
The correct approach, we think, is to consider the existing building as it was and then to consider the end result of the construction works and to ask whether, viewed objectively and in the light of all the relevant information, the work done amounts to the enlargement of, or an extension to, the existing building, or to the construction of an annexe to it.
Notice 708 Buildings and construction, at Section 3 gives more guidance on annexes and how an annexe can be distinguished from an extension and an enlargement.