Tall doors prevent Great Goofs
If you decide to install an 8-ft. door, you'll have to build the walls at least 9 ft. tall to accommodate it. But tall walls are better anyway. They allow you more room to maneuver 4 x 8-ft. sheets of plywood and 8-ft.-long boards without hitting the ceiling or breaking light bulbs.
Weekend mechanics love warm floors
PEX tubing carries warm water through the slab, where it releases heat, warming the floor and garage. Since the floor is warm, you can keep the heat set at a lower level and still feel comfortable. Materials for a DIY in-floor heat system cost about $2 to $3 per square foot. A professionally installed system costs about twice this much. And you don't need a boiler. You can use a conventional water heater or an on-demand water heater as a heat source.
To insulate the tubing and prevent heat loss through the slab, you install sheets of rigid insulation board under the tubing and around the edges of the slab. And of course you'll want to insulate the garage walls, ceiling and overhead door and pay close attention to sealing air leaks around all the doors and windows too. For information on installing and purchasing in-floor heat supplies, go to pexsupply.com.
Shed dormers add second-floor headroom
Consult an architect or structural engineer to help work out the framing details. You'll probably end up hand-framing the roof, but don't worry. It's not that hard, and you'll gain a real sense of satisfaction from building it yourself.