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Amazing – thank you!
The wall will ('should') be right on the party line. That's what started off the problems – the designs showed it over the line on our property.
The council didn't care about that, and that's where things started to unravel. If the council don't care about plans that are wrong, where do we stand once something is being built?
Thankfully, that issue seems to have been resolved with the neighbours, but we're still concerned about the apparent lack of attention to detail for everything else we've seen so far – hence the need for independent expert opinion.
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Bit hard without understanding the drawings / property etc but...
From both a planning and party wall perspective the absolute maximum acceptable is for the centre line of the new wall to align with the centre line of the property boundary -essentially an extension of the existing party wall that your properties already share - so this is
approx 150mm on your land.Beyond that and they are building on land no sole owned by them (which is part of a planning declaration).
They cannot build anything without a party wall award which is the process to tie this all down. If they were to build it taller than the drawings/PD rights or significantly beyond the boundary line onto your property then I guess it's a candidate for planning enforcement.
Planning / permitted development is separate to party wall rights / legislation so council not relevant here as long as they build to the approved drawings.
On Party wall - they build directly on the line of junction (so half of the wall on your side), up to it, or just inside. Building directly on the line actually has the benefit that in future you could enclose upon that wall with your own extension in an easier manner than otherwise.
Whatever they want to do tho - is all covered by the party wall act. They must inform you offically in writing (serve you a notice) and give you time to respond (often done by a party wall surveyor) . Then on the basis that you don't tick the "sure go ahead" box - you either accept to share their proposed surveyor (cheaper for them) or refuse that, find your own and your neighbour is obliged to pay your surveyor's costs in addition to theirs. The surveyors agree the "party wall award/s" and it become legal . This will include what wall where on what foundations and with what build methodology. It will(should) also include condition surveys of your house and land before construction to cover you should big cracks appear after or during etc.
I assume they would propose to cantilever a wall foundation so it is only on their side of the boundary unless you are agreeing to share the wall.
Mechanism for agreeing their access onto your land to complete the work is also part of the award.
Other might have more "west" london recommend's but i've used these guys for projects (more outer north west based) but they apparently work all over. They were nice to work with.
https://www.woodwardsurveyors.co.uk/about-us/