We poured a concrete floor for our structure. After leveling the space by digging down a bit, we put up 8" forms. For the next step we put down recycled 3" closed cell foam and covered it with poly. We left the center open for drainage and brick so we could access the power and water if repairs are needed. The pipe next to the shovel is the hydrant.The floor tilts to center 1"/3' to keep water for freezing the walls loose from the base.
After the concrete was poured, finished, and decorated with leaf impressions it was etched, and stained a dark color to help absorb any direct sunlight. Crushed stone and quarry waste were tamped in the center and the brick set.
We used 1 x 6 ripped composite deck boards, Tek screwed over sill foam to the concrete to attach the base of the GH, rather than the stone channel that came with it and started assembly. The clips that bolted to the uprights were modified to be screwed to the composite, this resulted in a sturdy frame to work from.
We back filled against the concrete and job done.
Last winter I mulched around the GH with straw to help protect the edges from the cold and held it in place with the strips of carpet I use on the garden walks. I didn't quite get that done this year, I ran out of time before I had back surgery.