Site Preparation for Storage Units - Before installing storage units, lay down cinder blocks, concrete piers, a concrete slab, or at least crushed stone. But don't stop there. Make sure the supports underlying the floor are made of pressure-treated wood.
Buying an outdoor storage building with a loft greatly increases storage capability, allowing you to store rarely used supplies overhead. For the exterior, build your own lean-to and attach it to the outdoor storage building to form a sheltered area for firewood.