most efficient Party Wall detail

Hi folks,

am building a two storey extension on to the side of a 3 bed semi-d. New external side wall will be a Party wall, on the boundary and can be used by neighbour in future.

Thinking of 4" solids on the flat, rendered externally as normal, and then sand cement render on inside to act as a vapour barrier followed by mechanically fixing 80 - 90 mm insulated plasterboard and skim ( no batons on walls ).

Space is a little bit of an issue but getting the correct solution is as important.

How do you have permission to build onto the party wall? You will be overhanging your neighbour's land with your guttering. You should be building inside the party wall so the guttering is on your side.

Parapet !