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From legal perspective, use the architect and ask them to move the original plans toward where you want to “freeze” them (as correctly put). “Concept planner” hopeless as not a recognised profession / insurance. Structural engineer v important and will be used to working with another professional ie the arch. Arch will be PD under CDM (quite normal for SE not to be). The builder will be the PC. I can help you set up /check you’ve set up contract network properly.