Mike Taylor wrote:
Your BCO was wrong
Your neighbour can only build on his side of the boundary and for technical
purposes the boundary goes vertically down and up form the ground. The
foundations have to be on his property so does any overhang, gutters, tiles,
window catches etc. You need to go backto your BCO and check the plans of
your neighbour's extension then talk to the BCO.

I agree entirely; but surely this is a Planning issue, nothing to do
with Building Control?

David