We've done this for a client, bumping a bedroom out into the garage.
As long as you're not altering the load-bearing walls, but simply adjusting partition walls... it's usually not a big deal.
Watch the ceiling and floor where the "extension" happens, it's often hard to blend it together.
Also, you'll want to make sure the doorway & vapor barrier between the garage and house isn't compromised.
Lastly, I' m not an architect, or an engineer, and I'm not there looking at the project.
So please exercise your own judgement, and bring in a licensed professional to assist with the design if needed.