Was weighing up this choice recently leaning towards 200mm cavity. I've built last 2 houses at 150mm. I plumbed the numbers into Google AI just to see what it thought on a 8x11m box 2 storey house. If the info it gave me back is right then I'd probably stick at 150mm. I thought it wouldn't be too much dearer but I think things will add up. I do remember someone @Mr Punter? Saying it was best to internally insulate the additional 50mm to keep everything more standard, footing width, ties, closers, lintels etc. The energy savings just don't seem worth it 50+ year payback and if you are constrained on overall size of dwelling your losing 2 or 3% in net floorspace. Anyone else make this decision and why?