@Danny Have a look at the NPPF (National Planning Policy Framework) and it's reference to "sustainable buildings".
Basically, any greenfield site can be built on under the NPPF as long as they meet quite strict sustainability criteria. Locally sourced materials, solar/wind power, ground source heat, minimal or no use of cementinous products etc etc.
The planning laws now mean that they cannot refuse planning for buildings that meet these criteria. Neighbours and local councils can get fcuked as long as trees are hugged and no polar bears cry basically.
It's not viable for big estates, but for single houses it can mean that the land is dirt cheap because you wouldn't get planning for a 'normal' build.
Have a read up on it, then find an architect who specialises in it. Then when you're in your eco castle you can send me a mug.