If you are considering a large or involved project, you will need design services. If the contractor does not have design-build capabilities, you should consider hiring an architect. Has he worked with an architect before? How closely are you willing to work with my ideas? Depending on the size and scope of the project, you may need a structural engineer, which the contractor should be familiar with. Discovering how willing the contractor is to working with outside suggestions. Disagreements on design, materials etc. may lead to project stagnation.