Here's the key wording from the North Dorset Local Plan to 2011 that you would have to overcome: -
Policy 1.6 - Development in the Countryside
In the countryside areas beyond the defined settlement boundaries, most forms of residential and commercial development for general needs will not be permitted. The following uses however, may be granted permission, subject to the relevant policy and assessment criteria;
(i) Development required for Agriculture and Forestry
(ii) Rural Buildings; Re-use & Adaptation
(iii) Rural "Exception" Sites for Affordable Housing for Local Needs
(iv) Housing for Agricultural & Forestry Needs
(v) Housing; Dwelling extensions & replacements
(vi) Employment; Development for Local Needs
(vii) Countryside Tourism
(viii) Countryside Recreation
(ix) Infrastructure (e.g. roads, sewers, energy, telecoms etc.)
That means it's against policy to put new dwellings up, unless for agricultural workers.