I can't tell you how much I dislike garages that are in my face in front of an entry to make the front of the house all garage as the first thing you see. But that said I do not like bedrooms above garages either. A garage is not living space so you will need to dempolish the garage and then build an addition just like any other addition. Can you do what you want where you live there are often rules about sq footage of living space in proportion o the lot. As for cost that can only be decided after getting all the info you need and then quotes from contractors in your area. IMO you also need an architect since IMO additions are best when they look like they have always been there

The costs will be similar, with the master suite given the edge as being more economical because you won’t need to redo your driveway and landscaping too. 100-250K, depending on location and expectations. But its more about the results that each project would bring as far as curb appeal and interior flow. A master off of a family area needs transition type area and sound buffering between the two, for instance.

Plus there is the difficulty of tying into the main house and the plumbing rework are two big cost unknowns until your local contractor and architect team weigh in after a site inspection.