1. Get an ordinance survey map (cheap, buy from ordinance survey).
2. Look up your local authority planning division and read the regulations regards building on side gardens. See the requirements for back garden size and access/entrance.
3. Roughly measure the free space allowing for separation from your own house.
4. It has to be detached house, you cant add a new house on to a terrace or semi-d.
5. If it looks ok, go to the planning office of the local authority- public counter.
6. Tell them what you have in mind.
7. They will tell you if it is NOT feasible, otherwise they will say it could suceed.
8. Get an architect to draw up a planning application only and get it submitted.
9. Review similar applications in your area as this is generally driven by precedent.
10. Get planning and sell.