structural engineer needed for residential ext?

Hi —
I’m extending my semi-detached house and have hired an architect for all stages of the process. The project includes replacing a single-storey extension with two storeys plus attic bedroom. My hunch is that I should hire a structural engineer to advise on the specs for the job, particularly what materials/foundations to use in the two-storey extension and what/if any of the existing structure can be saved. The project budget is <200k. My architect can recommend an engineer, and I trust the architect, but I am inclined to go to a third party.

Anyone got good opinions or recommendations on this? What’s the usual approach?

thanks,
T