I think you should ask your architect all of these questions. He may have designers that he has worked with before, and work well together. They will be able to explain to you their design process and who you go to for each element. It is crucial to have a team of professionals that are all on the same page.

I agree with the above! I've worked with several, and the best was the one who had a great relationship with a landscape architect. You do want the architectural and the landscape plans to go into the structural engineer and city at the same time so that that builder can coordinate all of the construction plans, both inside and out. You might also want to ask if the city has any special taxes on additions over 500 sq ft. Ours has a "school tax," for anything over, which for one neighbor, totaled an extra one-time fee of $5000.