IMO a bedroom over a garage is my least favorite place for a bedroom but if you must then you need to also have a full bath to serve the bedroom or it is just IMO not going to work. I do need to see the floor plan and how it connects to the main house . You need to find out if this is doable where you live and remember most garages are not meant to be living space so there could be all kinds of issues you will need to deal with. First get permission then hire an architect.

Thank you for your comments. The “bonus” room is already living space and part of the home. It just happens to be constructed over the garage. The former owners used it as their sons bedroom. I’m not sure from whom I would need permission? I was really asking which would be optimal for resale mostly.

You would find out from the city, you live in, whether this can be done. There are codes and permits needed for all. What you need to find out, is if it can be permitted. Do that, before you make any plans to change anything from what it is.

I need a permit to build a wall and a closet? Really? Seems sort of extreme but I’ll check it out. Thanks.

Without seeing a floor plan, I can't picture dividing the space into 2 bedrooms without walking through one to get to the other. A single bedroom and sitting room may be doable though.

https://tandemcontracting.com/make-bonus-room-work/