Foundation for a greenhouse?

I bought a harbor freight greenhouse that is 10x12. I am however splitting it with my dad. he is probably going to place his half against the south side of his garage.

I would like to replace the back half of my greenhouse with other materials, wood, stone or concrete.

Anyway, to increase the height I wanted to build the greenhouse on top of a foundation that is about 1' to 2' over the level of the ground. I also wanted to have the floor lower then grade to further increase the height of the greenhouse. Im not sure yet how much lower then grade i would want to go, between 6" and 2'.

I imagined a concrete footing and wall that would extend a few inches above grade, from which I could use logs or stone to extend the wall up another foot or two.

It just seems like a footing that is 3' deep (30" frost line) is really overkill for a greenhouse, it doesin't need to support hardly anyway weight. The kit weighed about 200lbs total, so about 100lbs of weight would be distributed on about 22' of foundation which is nothing.

So now I am thinking instead of a footing all the way around, I should just build it simliar to a deck and do 6 seperate holes with 4x4 or 4x6 posts extending up to the top of the wall, and bolt beams and basically build a deck that is below grade.

I have also been thinking about storing heat during the winter which would require a large amount of space below the floor so I can store many 55 gallons of water, or brick and stone, or whatever to gather and store heat during the day and emit it at night.

Any recommend a foundation that is not too overkill and is cost effective? I want it to be insulated and last nearly forever.

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

My wife bought the 6' x 8' greenhouse from Harbor Freight. It worked for what she wanted but, we had high wind gusts one day and I happened to get to see it fly up and over the house. When it landed, the upright supports were pretty badly bent. Now I am trying to build wooden supports for a new frame. Don't forget good drainage for the greenhouse since you will be watering plants in it.