This is the situation:

neighbour will not let me go anywhere near their land. I want to build
an extension onto the back of my house. One of the walls will need to
be adjacent to and with zero clearance to their already built
extension.

Normally walls sit centrally on the foundations upon which they sit.

So...how do I dig/pour the foundations for the wall for my extension
so that the wall of my extension adjacent to theirs has zero clearance
and yet sits in the middle of the foundations when it is built and
also does not involve going onto their land/property?

I think when I start digging at some point I 'might' hit their
foundations for their extension on my side of the property.

I hope that I have made the situation clear but also feel that I have
not!

So in general, if there is a wall sitting on foundations and you want
a new wall adjacent to it with zero clearance, but it's a neighbours
and you can't interfere with the current neighbours wall or
foundations....how do you build your wall?