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You could also try to get quotes from a specialist extension company or two who operates in your area, as they would do it all in-house (unlikely to employ a proper Architect, more likely ‘architectural technician’ or similar descriptor) with Structural Engineer and Building expertise and possibly some form of “Project Management” (I put in “” as this is more of a minefield). As ever, ask to talk /visit their recent jobs, get full quotation and check T&C if they don’t offer an actual contract.