The basis of planning law is the Town & County Planning Act. This has been re-invented many times over the years. It doesn't really help you much.
Central Government determines high level policy, but local authorities determine specific policies which are published in a Local Plan. Every planning department has one of these and it determines the specific policies by which planning decisions are made. This is the one for Oxford City (which is nowhere near where I live).
http://www.oxford.gov...d_Local_Plan_occw.htm
Section 7 deals with housing policy - look at it as an example and you will hopefully see what I mean. You can find the one applicable to your area on the web - they are all there.
The general gist under which things work is that central Government sets targets for new housing requirements by county, and it is up to each planning area to identify suitable sites within that area for future housing needs. This is of no help to you - I am talking big developers here - who try and second guess where these areas are and buy up options on huge swathes of land well in advance. General preference is for redevelopment of previously developed land - so-called brownfield sites.
Every planning authority determines existing 'settlement areas' in their plan - these are clearly bounded and cover every significant city/town and some smaller settlements - big villages perhaps. The general policy inside these is that new development is permitted subject to other policies being met. Outside these areas is 'countryside'.
Continued