My brother in law is in construction and has built and project managed dozens of houses, small apartment blocks etc. He said that if a building job doesn't require planning permission (small extension, temporary structure, certain farm buildings, large sheds, pergolas, verandas, sunrooms etc) then the customer might ask if he can save money by skipping the architect and engineer. A competent builder can generally work that stuff out, but it does still require rudimentary drawings (elevation, plans, end view) and some spacial figuring out. And like a lot of construction guys he has almost zero computer skills, so this shit is literally pencil, paper, squares and rulers.

I always wondered if there's a VR app (or market for one) that can map out an area and let you virtually model your project within it using standard building components (9" solid blocks, cavity blocks, lengths of timber in standard sizes, various sheets materials etc). It should be able to do basic calculations (rafter spans, roof pitch etc). Once you're done, it spits out CAD files, bill of materials etc. Bonus points for looking up local suppliers and offering to order everything for you, or even modifying the design to accommodate what's available.

It's a completely different project and target audience to OP's app, so sorry for the tangent.