The only exception to National Planning Policy is to allow the development of a single dwelling in the open countryside even in green belt.
There are no others.
The basis of the exemption is to allow individuals to build homes of their dreams within their available finances which genuinely enhance the local landscape and preserve it.
This landscape, by definition, must be special and important in any analysis.
There is no set design frame for such new dwellings. They are often very large but must include energy saving, water recycling aspects and genuine ergonomic proposals.
The more interesting, aesthetic and ground breaking the design and concept, usually the greater the chances of gaining planning consent.