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There seems to be a first mover advantage here - at our old place, the neighbours went into the loft and their dormer goes to the center line.
At one stage we wanted to go into the loft too, and would have needed to either build up the whatever-the-upstand-bit-is-called, or have some awful bodge with flashing over the top to stop crap getting into the gap.
My wife, who was the owner of the property, naively signed the party wall agreement. What she should absolutely have done was to have insisted on her own surveyor. Nothing but aggro because of not doing so.
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There seems to be a first mover advantage here
Well, it means that if you do the same, and have the horrible flashing bodge, then both you and the neighbour put themselves in to a world of hurt later down the line.
I wonder if you could actually prevent them from doing this, even with a surveyor acting in your interests. The best you could hope for might have been to go halves on extending the party wall up.
Friends of ours down the street agreed with their neighbour to split the costs of having it re-done properly when they converted theirs, so that now they have a brick firewall between them.
I think PD only refers to the requirement for a set back from the eaves of 200mm. So you can build up to the centre line of party wall. Ironically you can't typically get planning permission for this kind of 'full width' dormer - because it looks shit and is much better architecturally to be set in from either side a decent amount. But PD is PD.
If your neighbour's have built up to centre line or close to it they have forced your hand to either - connect your dormer (with adequate waterproofing detail + fire resistance for the new 'party wall' - could be brick but not necessarily) or. set your dormer cheek in far enough for it to be constructed properly and maintained. - so at least 50cm between the tiled dormer cheeks - or thereabouts. This one obviously seriously limits your internal space.
You do see dormers built right up to each other but the construction, waterproofing and maintenance issues are best avoided.
On the other party wall where there is no neighbour's dormer you need to consider if you will a. raise a party wall in brick - allowing them to build a dormer up to it in future, or b. have the cheek finish on the centre line, which will lead to the same negotiation later when someone wants to do a dormer that side.