The best place to start is with the building inspector for your township. The building inspector can inform you of the specific parameters you have to work within. Once you know your limitations you can have drawings developed. Prior to this design stage we can assist with idea generation for the project.
Once the drawings are complete (and approved by the building inspector), you can begin to compile a list of all the work you will need the contractor to quote on.
If you intend to shop around for comparison quotes this is an essential step so that you can accurately compare submissions.