General contractor vs architect or civil engineer to submit plans

[Marcos Urvina]

I went to the dept of building & safety to ask about the permit process to build a 400 sq ft studio in my back yard so i can rent it out. They told me to get a civil engineer or architect to submit plans. I was under the impression that general contractors can submit plans also.My question is why would i go through a civil engineer or architect and not a general contractor?