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it IS confusing! For what you’re talking about you’ll need the following; an architect, a structural engineer, planning permission and building control, plus possible surveys regarding drainage, asbestos, possibly other specialist stuff if needed. Probably not a party wall agreement. Oh, and the trades of course, as if it’s that easy! You’ve done right to start early, it takes time to read and absorb all the info, make your plan and put it into practice. You can find all of the above yourself, or an architect can do some/all of it for you, depending on what they offer. Best of luck!