Timber chalet - exterior stud + joists spacing

Rennovating, more like rebuilding, a timber chalet and I am increasing the thickness of walls using 4x2" to meet building regs insulation (was 75mm). My question is what spacing should the studs be? Existing is 600mm centres - I was considering 400 or is this overkill. Also planning on floor joist over slab @ 600mm centres. Any suggestions to meet regs - thanks