Does your local code require a footing?
Usually you only need this when attached to the house
My greenhouse is 7x10 & only on a 12" pre-existing foundation
I was only allowed to build it on the pre-exisiting foundation
This due to the fact it was less then 15' from the property line & 15' is required as a set back
The Inspector looked at it & gave me the OK
I dug down inside about 12-18" in order to be able to walk around
I also added a 55g drum to collect rainwater from the roof
If I had been able to do this from scratch I would have gone down maybe 3' & poured a 12" "footer" Then built a 2' cement block wall on top of that.
I am planning on digging out around the edges at some point & put foam board in. Since this is against the house the basement helps keep it warm. I do not have the roof fully completed & need to put the door on. But its very warm in thereon a sunny day even when its only 32 out