Chris,

Thanks for that.

Like others, I am finding it a bit hard to get my
head round the situation.

I understand that: there have been times when I've been in much the same
situation.

I'll try to post a more detailed explanation a little later, with some
photos if I can manage it, but the basic dispute is this:

For whatever reason, the end wall of the new rear extension and dormer was
not built in exactly the position shown on the council-approved plans; the
brickwork (of the extension) and the timber construction (of the dormer) are
both within my side of what the drawings describe as the "alleged centre
line" of the party wall, but a uPVC end fascia, some cladding and some
tiling all overhang it. If the centre line is accurate then the maximum
overhang is 40mm (the tiles) and the minimum is 20mm (everything else).

If the overhang exists then I am in breach of my planning permission and
will have to reduce it. But whether or not it does exist, and if so by how
much. depends on the *exact* location of the boundary line, so that is what
has to be determined.

Bert