In France “Tiny houses” (tinneeows) are popping up in lots of gardens. The planning regs allow it here.
Someone mentioned uninsulated, the one I poked around when picking up some free wood was made of a wood and 100mm polyurethane sandwich. That’s better than R4.5 whereas a typical UK cavity wall with polystyrene beads in the cavity is R1.5. Tiny houses are cosy.
Some are used for Booking/AirB&B which is far better than a buy to let being used which takes a dwelling off the long-term rental market.
Some are on wheels so class as a caravan/mobile home and can be parked anywhere a caravan can with no building regs; just electrical, water and sewage hook ups needed.
I’m all in favour, affordable eco-responsible housing.