First thing to do is check with your city’s building department and see what you need to do to get a permit to make a bedroom in a garage. Some cities won’t allow this but many will. You may need to map out your most basic floor plan so that you can roughly determine the electrical system, heating, framing details and insulation. Then get with your city’s building department to see if there’s anything else they need to approve the project. You will need a window somewhere in there for it to be approved for a bedroom.
If you do go ahead without contacting your city’s building department for approval, you may down the road if you try to sell your house that the extra bedroom won’t be added to the value of your home, and may even likely take away from the value due to the cost of taking it down or bringing it up to code.
To start the project, first thing would be to remove your garage door and the opener and frame it in. Be sure and install insulation not only in that new wall, but add to the other exterior walls (can be blown in) as well as over the ceiling in the attic. Next, you will want to frame in where the bedroom closet will me. One simple way is to use the mirrored closet doors than run on tracks you tack into the ceiling and the floor.
You will want to remove any big fluorescent lighting and put in nice interior lighting that matches what you have in your home. Then drywall the walls of the bedroom. Be sure and frame and drywall the area along the ends of the closet. Make any drywall repairs as needed to the whole room. Now, it’s time to repaint and replace any outlet or lights switch covers as needed. Last of all is your flooring. Carpet with padding is great if the concrete flooring is pretty level. You can also choose tile or other flooring. And then you’re done!