I love this idea! I recently bought a house that a massive garage at the back with a loft, man door, and an electrical panel. You could tell the previous owner had a plan for it like you do. We'd liked to have finished it and used it as another unit, but unfortunately Toronto isn't quite there yet. I talked to my architect who's really on top of new Toronto developments. He let me know that Toronto is pretty close to normalizing laneway housing, so that you don't need to go to such lengths to have these types of renovations approved. From my understanding one of the stipulations is that it must actually be on a laneway though, so separate access I suppose.
More important than a good contractor, I think you should find a really good architect to give you some deeper insight into the project. Most general contractors should be able to handle the construction side of the project sufficiently.
I'd love to see this and more creative housing in Toronto take off. Someone managed to get a shipping container house built recently near College & Lansdowne, I don't see why you couldn't make this work!