From your OP image it looks like the roof truss does not extend to the garage end which I question. However, I assume this will probably require permit and code compliance but again it is not clear if this will be condition space ( code requirements different) which I think it will not be since it is an add on. I would first find your the local requirements , if any , in regards to code. Even a visit to the local codes and inspection office ( city engineers) may get you some helpfull input before going to the expense to hire a PE. Some of those offices will even have hand outs showing you what the design requirements are for their local area. These may have the regional climate conditions factored in. My guess is a beef up in some of the beams will be required