A simple solution is to pallet rack to 12'. It comes in a variety of lengths and depths, is strong enough to support whatever you want to store, you can use a hoist to lift heavy stuff up between the beams. It is easy to build a workbench surface in it. You can use it to build a mezzanine with enough room to park a car or other stuff under. You can back to back install racking so you use a 4 post to lift things like lawnmowers, motorcycles and other heavy stuff up so it can be stored in the racking. It is strong and stable enough to climb on or to mount a ladder on it that slides along the length to provide access.
For example, 12' racking set in a 13.5' height space:
This is a great garage and a really good Idea. I never thought of this. ThankX