Are you pretty sure you have the budget to take care of this project? If so, then I would bring a design/build remodeler and a designer on board from the start and let the two work as a team. I don't think an architect is needed for such a small project. You do need to be clear and upfront from the start about what your budget is or you will have lots of great ideas that are not affordable.

If you are unclear about what the cost of such a project would be and need to do some exploring first, then I would have a design/build remodeler come in and give you some initial estimates to see if you are in the right ball park before moving forward.

Here is an article that defines each and when their services are best used. http://www.todayshomeowner.com/do-you-need-an-architect-or-a-designer/