Inspired by this thread from @mrfish, Can we have a thought experiment as to just what it costs to build an insulated garden office with reasonable facilites? (Also because I still need something like this). I wamt to think through cost-effective options and see if I have not though about any aspects.   My £3k number is because more-or-less this is what it costs to buy a secondhand insulated, wired, site office of this size and spend £400 shipping it in.   My specification is