With a hip to gable loft conversion or a stonking great rear dormer which has got political and where the planners deem that for instance the materials are not similar enough then you have probably got virtually no chance of getting planning approval so you are stuffed.
That's not the case. You have the oportunity of submitting an app just the same for a loft as for an extension.
Presumably, the competent designer will specify a design and materials in accordance with whatever PD guidance applies.