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 seem to remember that you might not be able to leave "zero tolerance"
(I assume you mean butting up your wall against theirs) from discussions
on uk.legal

Maybe a builder can advise on anything salient in planning regs etc.
about access to adjacent premises for maintenance purposes etc.

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