I'm about to stick build our 2 storey wrap-around extension and we're building bang up to the boundary on both sides. We're the end terrace of a small row of victorian cottages, so one side we're around 30cm from a conservatory, and the other side we're around 3m from the flank wall of the next house. We're tight on room, so the wall make-up as per plans (I may change the insulation if I can get better acoustic properties within a similar depth and still meet the required U value) is:
skim
37.5mm insulated pb with taped joints
120mm Kingspan K112 between studs
140x38 studs (can't remember if spec'd as C16 or C24, but will use C24 as that's what my local sawmill carries)
Non-combustible sheathing (just about to start looking into options for this)
Breathable membrane
Cross battened
Standing seam steel
We'll have normal (unprotected, not fire spec) alu non-opening high-level windows on one flank of the ground floor only. There's a max area that you can have for unprotected openings, so I'll put in a fire door to compartmentalise the the ground floor (where new rear kitchen meets new side hallway) as fire resistant windows are sooo much more expensive.