Has Anyone Got Experience with Putting an Addition over a Garage?

thinking in another direction: the previous owners of our home converted the oversized 2car garage into a living room....20x22.....removed the interior walls of the garage, opening the new living room into the dining room, kitchen, & old family room. This created an open floor plan/great room, totaling about 1000 sq feet! Front door to far wall (which abuts to the carport & now has a full wall of brick & a big fireplace)....35'. Front wall of living room to rear wall of dining room (a wall of french doors/glass) ... another 30-35'. The old living room (on the other side of the front door), then became our master bedroom....very spacious!

What I like about this renovation is that it does not change the footprint of our home, we still have a carport.....& I LOVE our 1000 sq feet of daily living space!

& recently, we flipped the dining room into an extension of our tiny kitchen: we added a 4x6' island where the dining table used to be. The island is multi-purpose with a breakfast bar (2 stools), large item storage (mixer, blender, lots of prep tools - all stored in pullout bins behind closed doors), a bar sink, & a full-size dishwasher (which was missing in our little kitchen).
The dining table is now offset from the fireplace, with an antique buffet next to the fireplace. It works well & we couldn't be happier with our updated format!

Oh, & addressing your comment about the foundation: get an engineer's opinon on whether or not the existing foundation will support that addition!

Not sure what your building codes are in MA, but here in CA, chances are your garage would not have the proper foundation or framing to support a second story without some modifications. (Bottom story has to have 2x6 studs framing out the space to support the exterior walls, and the foundation has to be big enough to support 2x6's.) But it's not that big of a deal - add framing, beef up the foundation, rip off the roof, and up you go. Without plumbing, it's not a scary project. Probably $30K if you hire out all the work.

Starting around $25,000 it all depends on how fancy you want to do things and such, hubby does this for a living here in MN.

We got two quotes when we considered adding a bedroom and bath above ours. One was for $30,000 and the other was for $50,000 (they explained that 15,000 of that quote was for the plumbing). We live in Texas. We decided against it for our house, but we were surprised, we expected to pay more.