Our neighbour has built their extension along the boundary line (not over it) on their side. However, the footings are on our side, but we did agree to this. We are now looking to build a simple ground floor extension on our property and are wondering if we can simply come off their wall. I think we'd have to have small gap between the properties filled with some form of insulation. We'd then simply have a stud wall built next to their wall.

Does this sound ok? Would we still need a party wall agreement?

I'm thinking that we may also have to connect any new foundations we would need for the new extension, but I think these can be connected via steel rods drilled into the existing foundations?