Government planning policy guidance used to set out detailed criteria for dwellings required for agricultural workers, and established that where a business was in its formative stages, a temporary dwelling should be approved first, before something more permanent. Most temporary dwellings deployed were mobile homes.
New Government Guidance in the National Planning Policy Framework (NPPF) doesn’t spell out the circumstances where such dwellings might be approved, other than to say that new dwellings in the countryside might be permitted where they’re needed to serve rural businesses. There’s nothing in policy to say a temporary dwelling must be a mobile home, or that, if it is, it should conform to the size limitations that would bring it within the statutory definition of a caravan. That said, the size limits to remain technically a ‘caravan’ are 30 metres long, by 6.8 metres wide and 3.03 metres high.
At the planning stage attention is more likely to focus on the need for you to live on site, rather than the precise design of a temporary dwelling, so ensure you have a watertight argument showing you have to live on site and that your rural business is viable or likely to become so soon.