Great question! This goes directly to the "Organize" part of this entire plan.
When we bought this house it came with a dedicated inside pantry across from the laundry room. It had some very well built shelves to the ceiling on one wall, and I added a cabinet with a counter and a set of open cabinets above that, on the far end. I also added lights. The room is 5 x 10 (50 square feet) but the shelves and cabinetry provide at least double that.
The double car garage in our house was sheet-rocked and taped, but never painted. It looked like hell and was dark, and what little shelving there was was made from 2x4's and 1/2" plywood. 2 years ago I cleaned everything out and sprayed the ceiling with texture, and then painted the whole interior. Then, I cleaned the entire floor and painted it with epoxy floor paint that included chips to sprinkle in the wet paint to make it non-slip. I also installed several new flourescent lights.
When that was done, we found some excellent utility cabinets on sale at Lowes. They were closing out that model and were marked down almost half. We bought enough cabinets to install on both sides of the garage, and I put in a raised platform to mount the refrigerator and freezer on, that keeps us from having to bend over to see what's in there. The storage underneath is very useful.
I also installed a 16" wide wire mesh shelf with hangars every 16" above the top of my garage door. It is big enough for a set of plastic containers to fit on it and we use it for off the floor storage of things we don't want buried in the attic.
My daughter was impressed enough to get me a sign last Christmas that reads "Garage Mahal"...here is a pic from a year ago when I was still setting up the opposite side of the garage. These tall cabinets are now my outside pantry.
The white blocks under the legs are for leveling. Here is the other side...
I cannot say for certain how much square footage is used, but I have plenty of room left in the cabinets for the rest of the things we need, and we can still park 2 cars inside the garage. I also use some otherwise unused space in my shop for storage, maybe 50 square feet total, so far.
The bottom line is that we built the capability to store quite a bit of stuff, and we are not impacting our floor space in the house at all. It wasn't hard to do, and we managed to accomplish the improvements out of hide and over a relatively short time.