These documents provide non-statutory advice and guidance on particular issues to expand on subjects referred to in legislation.
Circulars are used to explain policy and regulation more fully. Many circulars are quasi-legislative and include a direction or requirement to take specific action. Letters are used to provide key communication between central and local Government.
Many circulars and letters include guidance on implementation of aspects of planning policy.
Though circulars can apply to England and Wales, you should not assume that is the case.
Also, in some circumstances the same circular will be issued under different numbers by different government bodies in each country and will contain specific information for each country in their respective version.
For example the document 'Planning Controls over Demolition' was issued as 'Circular 10/95' by the then Department of Environment and as 'Circular 31/95' by the then Welsh Office.
Welsh Office Circulars were issued (in many cases jointly with other Government Departments) between 1965 and 1999. Some of these, bearing on land use planning, remain in force (partly in cases) in Wales.