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you mentioned something in the difference between needing to seek planning permission and permitted development. Does it sound like this might apply here?
I'm not Brommers but the planning portals on the local authority website will have a list of stuff that's been done / is trying to get done and you can deduce what is permitted development from there as it usually says on the Approval notices under what circumstances the approval was granted.
Most (at least the surveyors I looked at) recommended you let them know what you have in mind so they can factor that in while surveying. How much use they'll be, I don't know.
Our, hopeful, purchase has half of the ground floor extended for a kitchen, so I let the surveyor know that we might want to extend the rest of the ground floor for a bigger kitchen/diner and maybe a shower room/wc/utility room. Speaking of which, I think @Brommers, you mentioned something in the difference between needing to seek planning permission and permitted development. Does it sound like this might apply here?