I have spent many hours trying to answer this question this year. One case that is key to the discussion is Cantrell. But there is a complication in that the journey through the appeals system was not easy!
The question started with construction works to a nursing home in Woodbridge, Suffolk. The contractor disagreed with HMRC’s decision that the works should be standard rated. The Tribunal decision is called Wright & Fuller Ltd (VTD 15952). The decision was appealed by the recipient of the supply, Mr & Mrs Cantrell T/A Foxearth Lodge Nursing Home. The High Court (QBD) heard this appeal. Oddly, Simon’s Tax Cases seems to have reported it twice! At [1999] STC 100 and [2000] STC 100. The Court remitted the decision back to the same Tribunal to re-hear it.
The Tribunal re-convened at the Nursing Home, and the decision is called Cantrell (VTD 17804). (The Tribunal member had died in the interim, so the same Chairman sat with a different member.) This decision was again appealed.
The second High Court decision is called Cantrell (No.2) [2003] STC 486. This time the Court decided not to remit the decision, but simply allowed the appeal in favour of Mr & Mrs Cantrell. This decision included this key comment:
An annexe is an adjunct or accessory to something else, such as a document. When used in relation to a building it is referring to a supplementary structure, be it a room, a wing or a separate building.
Ultimately the Cantrell decisions are referred to in HMRC guidance on annexes. And I am hoping to use this same material to win at Tribunal! https://www.gov.uk/hmrc-internal-manuals/vat-construction/vconst02330