This option sounds easier than it is in reality. The list goes on and on as to why this is a challenging approach, but primarily, existing garages are problematic because they are often old construction and not structurally sound to support a second floor. So, by the time you reinforce the old garage, you might as well tear it down and start over with something nice and new or just convert the existing garage space to an ADU, park your cars outside and get a storage shed for the "junk" currently in your garage.
For now, Modative is not currently taking on second story additions over garages. We are, however, exploring cost effective options for adding a second story to a garage, but will not have this available until we establish a plan and cost that makes sense. Stay tuned!