Contact designers in your area and choose one to go with you to see the house they should be able to tell you if it is worth doing and a very rough estimate of cost I do this quite often in my design business.

Since this would be a major renovation (remove roof, add on to the entire back of the house, major renovation of kitchen and two bathrooms, building an outdoor pool, etc. etc.), I guess I was thinking a design/build contractor. Am I wrong?

I may be partial, but you should contact a local architect first. They are trained to plan spaces and evaluate structural requirements and make it look good. They can create drawings to communicate to you different design solutions and drawing to communicate what changes you want to a contractor. Call a few and discuss with them your situation and concerns until you find an architect you feel comfortable working with.

Thank you for your suggestions. Would either a designer or an architect be able to give us a ballpark price estimate of renovations?

I agree with Mark Bischak that an architect is best. Architects can see the whole scope of the project and are able to visualize difficult design solutions. We also understand the structural engineering that is required for a project. (An interior designer does not have that training). Most architects should also be able to do a "ballpark" estimate of the costs. If you find a design build firm that has an architect as part of the team, then ask them. You should be able to pay for a couple of hours of their time......