New planning policy adopted in North Somerset adopted in 2016 makes development of gardens in towns for additional houses extremely difficult. The policy says that the new house must be of an equivalent size to its near neighbours on the same street and the resulting plot sizes of the existing and additional must be no smaller than the neighbours. So unless you start out with a garden twice the size of your neighbours you are unlikely to get permission.
Having discussed this policy with an eminent planning consultant it seems that it may not stand up to the scrutiny of an appeal. We have a test pre-application in the system to see what the reaction is.