If u don’t use the same centre wall, each extension will be about 0.6m narrower as you will both not practically be able to build right up to the boundary. He can build close but if the neighbour goes ahead and is built from one side (he cant come onto your land for new building works, only maintenance, without consent), then built from one side, the side you will be looking at will usually be a mess as builders cant finish it off
It is an impossibility for both of you to use different builders to do same centre wall.
The way around this is for you to contribute 50% to cost of centre wall only on grounds that you can use it and build onto it. On the same basis you agree that he can come onto yrs to do his building provided he reinstates any mess. That way you are also looking ata finished wall, not one built from one side and badly finished until your extension is finished. It does need a formal agreement which can be a letter. Better from solicitors to cover all angles, but no legal reason
That way he can build now, right up to centre line, you get one wall completed now at half price. You can build when you like. If you both don’t agree that, you both lose out.
Make sure he doesn’t try to rip you off on cost of the proportion of just the centre wall
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