How big is the run and many birds are you looking to fit in? Remember not to overcrowd them.
A dog run is a great start point for a run, they are solid and easy to add on to if chicken math kicks in. If it doesn't have a top, you can buy premade chainlink panels at Lowes or HD to bolt in. You don't say where your at but keep in mind the angle the garage roof dumps water and snow (don't want a avalanche to wipe you out or soggy chickens from dripping a roof).
Raising a small coop off the ground allows you to count that square footage in the run and wrapping flashing around the legs will help stop mice from crawling into the coop if you don't turn around a give the birds a ramp. Also a raised coop makes cleaning easier, you don't have to work on your knees.
Don't forget....skirt the run if it's on dirt, line the inside of the run with hardware cloth up to a little over chicken height to prevent reaching thru, throw a shade panel or tarp over the run to help protect the birds from heat and hang a roost pole in the run, birds love to hopp around and will roost outside on warm days.
Look thru the coop pics and design ideas on this site while designing your coop. There a some very unique ideas floating around this forum
Most of all, have fun doing it. Hope to see photos when you start