Something like a Portakabin but a bit more suitable to use as a house
extension and will look OK in that setting (ie will not make back garden
look like a building site).. Must meet building control & regs (fire,
inulation etc) - hopefully modern timber frame might be OK in this regard?

Been looking on 'net, so far found a few kits for full houses, timber
offices, garages etc but these seem intended, at least, for use as detached
structures.

Maybe I need an architect!.