On Thursday, 27 February 2020 11:54:40 UTC, George Miles wrote:
But would Building Control let me use the existing walls
with whatever foundation they do or dont have?
Would I have to get a surveyor to do some sums?
Or must I demolish everything and rebuild?

Structural engineer and a trial pit to determine soil type and existing foundations (if any)

If it's a non-habitable outbuilding and a covered passage between that and the house it won't have to meet full BRegs anyway.

And Planning are concerned with roofs as well as walls.

Owain