Party wall agreements

[Scraggyspanner]

Hello,

We're planning an extension at the back of our property. Our neighbour already has a similar extension that was built by previous owners before they moved in.
It seems that their extension is actually built right on, or even slightly over the boundary line, so surveyors have suggested we use it as the party wall and build onto it.
We're on friendly terms so I mentioned it to her and she initially said it sounded sensible, then spoke to her brother who said sharing a roof would be unwise.
Grrr!
From what I understood, we can force the point as they are on/over the boundary anyway, but I'd rather not affect neighbourly relations.
Has anyone got any similar experience?

Not sure why you'd be sharing a roof. There'll be a parapet between the properties running along the party line and your roofs will be separated by this. You will use their wall and foundations, for which they're entitled to ask you for a £ contribution towards, determined by a surveyor. They may not ask for this but be prepared if they do.