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  STC 100 and  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)  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.