Do I need an architect?
This is really a regional legal question more than anything. You'll have to ask your local building code enforcement office. Typically, though, no, no one needs an architect...just someone that can approve the engineering (be it 3rd party, or the jurisdiction, itself).

Now, there are arguments for and against hiring an architect in general (outside of sign-offs) but that's a different question. 

FWIW, we just bought land and are designing it ourselves. Alas, I am not an engineer, so will be having to go 3rd party for that.