Either one will do, but depending on the state that you live in you may want to check with your local permits and development office. A lot of times you can get the same answers out of a good contractor, and save yourself a lot of money at the same time. Good luck!
It depends on which state and which city you’re in. Usually for residential, one will suffice, depending on how much information they will provide on their plans. In Los Angeles, to pull permits for residential additions, the city will require an architectural set of plans and a structural set. For residential, architects and engineers are usually allowed to draw up both set of plans but you can also hire both individuals to draw up their respective plans.
It is always recommendable to hire a structural engineer as they will have to calculate the gravity members and seismic resistance elements of your addition, making the construction more efficient, whereas an architect will just call out more than required so that the plans are approved by the city.
In most projects, an architect is hired to draw up the architectural portion of the plans and an engineer is hired to draw up the structural portion. This way each individual can focus in what they specialize in. But at the end of the day, you can do it anyway you feel will most benefit you and your project.
Depends on local authority (town, township, county). In certain towns in NY, we need to have both approve/design it.