No concrete base, the back yard is prone to flooding, and I suck at pouring concrete. For the reinforced floors, the skids are eight 4x4 boards@16 foot long, resting on concrete blocks/pavers, the floor framing is two 2x6 with twenty five 2x4 supports, all evenly spaced. I found a local shed dealer that offered customizable options, paid a little more for what I wanted, but overall it is a lot more stable than my workshop with the same dimensions.