I think by your own admission you’ve described it as a “dodgy dormer” and if it needs to extend above the ridge tiles to provide adequate head height it is always going to look pants. To sort it properly is a big cost – raise roof height etc.
From a planning perspective if it is just maintenance i.e. the original structure remains and you are just renewing the cladding then you can just proceed without an application.
If it requires taking down and reconstruction then it is considered development and would need a planning permission – which is unlikely if it continues to be above the ridge line.
If it is at the rear and your not in an AONB or some restrictive conservation area and you were to build it below the ridge line it could be permitted development and you’d not need permission.
You can check what permissions relate to the property with your local planning authority either on the phone, through their website or on Planning Portal. They won’t have much detail beyond headline info for really old stuff but it should all be on the system.
Is the house really really cheap?