In my opinion, it makes no sense at all to add on both width and depth. It makes more sense to tear down what you have and build a new garage. Here is why I say that.
To add on width and depth to the existing garage, all four walls need to be moved and rebuilt, and the trusses/rafters need to be replaced. For example, let's say you leave the wall standing on the left side of the garage. It is 21 feet long so you need to add on another 9 feet to it, but first you need to pour more concrete for the floor since it will be bigger when the garage is completed. Once the floor addition has been poured and cured, you need to add on the extra 9 feet to the wall that is still standing. The wall with the overhead door will need to be revamped totally because it will be too short and you will most likely want the overhead door in the center. That means the wall will need to be pretty much totally rebuilt. The left wall and the rear wall will need several feet added on and depending on whether you want windows or a side door or whatever else you may want, both walls will likely need some revamping as well. Then you need all new trusses/rafters to fit the larger size garage.
Long story short, in my opinion, it is much simpler, and most likely less expensive, to start from scratch and build the garage the way you want it to be built instead of trying to screw around using the materials from the old garage.
Personally, I would take a very close look at the old concrete. If the old concrete has any problems with it, tear it out and pour new concrete. No matter how you do it, you will be spending a large chunk of change on this garage project. Do yourself a favor and spend a few thousand more and get something that will last for many years and will be exactly what you want it to be.
