Looking at the space I wonder why you wouldn't use this as your ensuite and reconvert the current lower living space for your family and playroom with more conducive access to the outside and kitchen.

You are pretty far along in scheduling to not have an actual game plan here. Why have you scheduled a contractor without having a final layout from your architect? How could a contractor even give you a bid based on anything? This is not going to be cheap! And you want to pop the roof or add big dormers to try to get more floor space? Not gonna happen without more planning and at least a 100K+ budget. Your ideas do not fit the existing architecture. The whole structure would need to have the second floor removed and rebuilt.

I do not see any windows ecept in the office Post some pics of the actual space but right now It does not seem to me to be worth the effort and have you actually got permission to do this , and IMO the only way the space will get any bigger really is to remove the roof and build an actual whole floor above the garage . It is hard to really comment without seeing the space in real life.