Andrew wrote:
You should be able to get the planning application
plans from the councils website, but the detailed
Building Control plans are not available, presumably
because of copyright.

You could probably find a structural engineer or draftsperson to draw up
plans based on the planning application from next door - not photocopies,
but the same layout/etc. That would be cheaper than employing an architect,
although whether it's cheaper than asking the original architect 'same again
please' is another question.

Theo