Thanks. I assume the planning process is entirely separate to the party wall decision?
Would I flag issues about building 'on' the boundary now or is it fruitless?
The notice I have received is planning.
Party wall is separate and once planning permission is granted all you can really do is delay the build by not agreeing to a party wall agreement
What you should agree to do is hire a separate surveyer and have them go over the issues raised.
Also take lots of photos from your side before works start so if any cracks or damage appear your side you have evidence
Also is this just a single level extension?
I'm taking a rough guess here but a single level extension would add about 15sq if that. Seems insane as I'm guessing the build cost is going to be 30-40k