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A few questions here, would you be able to share the plans as you are remodelling internally or just to understand you are re-working a downstairs layout including moving, altering or removing structural walls?
Personally I would assume the architect would have provided you with a quote that allowed for some iterations and I would have seen the designer as an intermediary second opinion who influenced the final plans. I would get the architect (they provide you with more assurance and security if they draw up the plans) to provide the detailed drawings next so you have a clear and accurate final set of plans for the structural engineers to work from.
A final set is paramount and even though planning permission may not have been required for your project you are likely to still have to involve building control who will also want a final set of detailed drawings with all the structural calculations on.
Personally I would want the architect to act as the principal designer even if they didn’t present the final solution. By providing and completing the final drawing for you they will also check everything is feasible that you have finalised and then adhere to those drawings. The detailed drawings will let you plan plumbing, electrics and any insulation or underfloor heating e.t.c. that you are planning to encompass too.
As for employing an architect if its a considerable internal layout change the I don’t see why one wouldn’t employ them and more so if you have structural wall changes and for example are moving the kitchen from one end of the property to another for example they will test to the feasibility of that better than anyone.
Hope that makes sense as a start.