wrote:

Not quite. If you are excavating within 3m and lower that your
neighbours foundations or within 6m if you are excavating lower than a
line drawn at 45 degrees downwards from the bottom of neighbours
foundations.

True, but foundations for a new extension will almost always be
deeper than for the house it is attached to, because of changes in
building control. For the second requirement to apply (outside 3m),
his foundations would need to be 3m deeper than the neighbour's
foundations. This also seems pretty unlikely, if the houses are on
the same level.

Al