If you know exactly what you want, then in my opinion you don't need an architect. The question will be whether the wall you want to knock down is load bearing, if it is you will need a structural steel put into place, so will need a structural engineer to guide on size etc.
I personally would try and get a couple of builders out, good ones will tell you what you need and and will also have structural engineers they have worked with before.
If you know exactly what you want, then in my opinion you don't need an architect. The question will be whether the wall you want to knock down is load bearing, if it is you will need a structural steel put into place, so will need a structural engineer to guide on size etc.
I personally would try and get a couple of builders out, good ones will tell you what you need and and will also have structural engineers they have worked with before.