I don't know if you need an Architect since the home is so stylized. Just copy exactly what is there, unless you want the addition to look different. If so, just do a section with a parapet to it contrast the tile roof or even add a contrasting material (metal or tile siding). While I agree it is a traditional AZ stucco/tile home, it does have a modern feel to it with the clean lines and gray colors.
I'm not against Architects as they definitely have their place. But as a landscape designer (not a licensed landscape architect), I always find architects seem to design the most expensive way possible. You want a designer/builder that will also quote building it. That way they will be a honesty in the design as they have to bid it as well. I've had projects from a landscape architect that was 20-40% more in cost because of the way they designed it or the material the picked to go into the project. I typically change the design just a little or specify another material for a huge savings. Architect seem to get fascinated with a particular design idea and try to force it at all cost. A designer/builder, knows what materials cost and how they work, so they can maximize what you get for the money.