Below is not my own work:
"A caravan, be it a touring or static caravan or a large twin-size mobile home, is regarded as an article of movable personal property known as a ‘chattel’ and there is no public law preventing one being kept in someone’s garden, but there are Laws that regulate the ‘Use’ of land.
The siting of a caravan within the garden of a property does not require express consent provided a ‘material change of use’ has not occurred. Gardens are used for the enjoyment of the main dwelling house. If a caravan is parked in a drive or sited in a garden and used by members of the household in connection to the enjoyment of the house or as extra accommodation for visiting guests, provided the occupants continue to use the facilities of the house, then the siting of the caravan has not changed the ‘use’ of the land. However, if for example a caravan is sited in a garden and used as business premises, separately rented or used as a primary independent dwelling, with no relation to the main house, the local planning Authority could decide that an unauthorized ‘material change of use’ has occurred, for which planning permission will be required.
Mobile Homes and Caravans can be sited and used in a garden without the need for express planning consent. If the use is not considered part of, or incidental to, the house, then a ‘material change of use’ may have occurred. If the caravan is not considered to conform to the definition of a caravan then ‘building operations’ may have been carried out. In either case, planning permission will be required."
So the important points are:
- It must be transportable (and this would apply equally to a caravan and a bus)
- The persons using it must be connected to the household as either family or guests
- They must continue to enjoy the "facilities of the house"
- Renting to strangers as a business would need planning permission (which, IMHO, would be unlikely to be granted).