For an attached garage, you will likely need a proper foundation. Factor that into your cost.
Personally, I wouldn't bother. Build a shed if you need more storage space. Build a car port if you need somewhere to put another car.
If you plan on selling the house, make it someone else's problem.
It will cost you $10K+ to do it. think about it. A garage package will cost you $5-7K from totem. That doesn't include labour. The concrete will likely run you $2K for a pad, and way more if you build a foundation. If you have to do a foundation, it will cost you money to dig and also tear down the existing structure. You have to repave the driveway.
It is going to take a couple of weeks to get a permit from the city. etc.
I see a $15K project that will add very little in terms of equity in your home (compared to the rest of the house).
Develop you basement if it isn't already or redo an old kitchen.