On Sunday, March 25, 2018 at 4:35:48 PM UTC+1, Peter Johnson wrote:

The builder extended existing circuits, lighting and power, on my
extension a few years ago. The building inspector was told this and
there was no problem getting the completion certificate.

That's reassuring; thanks Peter.

To be honest it was always my plan to live with the new extension for a while before deciding on the final layout of the new room. This means that the majority of the electrics would likely be delayed anyway and so if I can get the works signed off in that interim period I should be okay anyway.

That said, at what point is an extension considered 'complete' for sign-off purposes? Perhaps I'd need to get as far as plasterboarding (and possibly plastering?) given that such components form part of the thermal and fire envelope?