@Cyclehire: "So how do I know what standard rules apply to all conservation areas in England. I can only find general advice rather than rules."
In Planning jargon, Conservation Areas are one of a number of land classifications that fall under the broader title of 'Designated Land'. The others are National Parks, the Norfolk Broads, AONB's and World Heritage Sites.
The Planning Portal lists COMMON PROJECTS.
If you click on whichever of these is relevant to you, it will tell you what you can and can't do, generally, and any general restrictions that apply to conservation areas will be described under the bits that refer to 'designated land'.
So, for example, THIS LINK tells you about extensions, and says that (bullet point 4) you're not allowed the increased size allowance for extensions under the Neighbour Consultaion scheme and (last three bullet points on main list) you're not allowed side extensions, 2 storey rear extensions, or cladding of the exterior.
Otherwise, unless there is an Article 4 direction in place, you have the same PD rights as normal.
If you want to find the information all in one place, and with more detail on its interpretation, refer to the HOUSEHOLDER TECHNICAL GUIDE