This is not about consent to build an extention. Its about avoiding
hassle with an immediate nightnare neighbour.

Yes, so it is up to them to determine what they will object to. If they're
going to object on the boundary line, will they be any different at 30cm,
60cm? The law doesn't state what you can build without offending tetchy
neighbours.

Once you've got planning permission, there's nothing they can do. As far as
planning permission is concerned, they can object, but are unlikely to be
successful unless they have a genuine concern or your application fails on
its merits anyway. They can't object on the basis that they don't want you
to do it. They have to claim that it will mean they are excessively
overlooked or something, which is not possible if the extension has no side
windows.

If there's going to be trouble, I'd just build on the boundary line anyway.
Your neighbours are required by law to allow you, provided PP has been
obtained. They are even required to let you access their land to maintain
the building (i.e. repoint).

Christian.