Hi Libby and Neil,
My husband and I have recently moved to our small holding in west Wales, after spending more than 5 years and thousands of pounds on trying to get planning permission on our land in the midlands. Even with experts backing up our case for builiding a dwelling on our land and other precedents across the country our planning officer was absolutely adamant that he would not issue permission. You may be more fortunate than us, but we have spoken to many many people with similar stories so as one of the other respondants suggested, I would suggest you get a view on your chances from your planning office well in advance of buying any land, and be prepared to have deep pockets!
Sally