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Paul_C
does it need to be transportable or could you build in situ on a raised base?
It doesn't matter where it's built, so long as it remains capable of being moved whole once assembled. So as long as it falls within the set limits and it is possible to move it, it's a caravan for legal purposes. So as long as you can somehow remove it whole from the base and tow it somewhere else, fine.
I'd actually toyed with the idea (if I could ever find a suitable piece of land on which it would be possible) of getting just a basic shell with all exterior doors, windows, roof etc. built and set up on a base, then taking my time to fit out the interior with furnishings, partitions, doorways, wiring, plumbing etc. once sited.