Thanks for the input all. I’m a mechanical engineer, so producing the drawings should be easy enough for me to do in theory. I’d want a structural engineer to design the steelwork primarily for their PI. Also I have no idea how buildings are actually put together, in terms of membranes, structural connections, materials etc, so it might be better getting the detailing done by others in any case.
The main input I’d be looking for from an architect would be layout and making sure the end result doesn’t look too cobbled together. As an engineer, a lot of the various DIY jobs I’ve done so far on the house are functional, strong, cost effective, and fairly ugly. I’d like to address the last one there!