I think you are trying to get the cart before the horse but I can provide you with the info .
The manual for HFGH # 93358 says excavate an area for the base of the greenhouse. The area must be 10'2" wide by 12'2" long by 5" deep. The floor of the hole must be level and square. The diagonal dimensions,from Conner to Conner,should each be 15'10"

Assemble the base outside the hole as shown.
With assistance lower the base into the hole. The diagonal dimensions from outside Conner to outside should be 15'71/2" each.
These measurements must be equal for the base to be square.
Fill the hole around the outside with soil.
The base will need to be filled with 40 cubic feet of gravel or another appropriate fill material. The gravel should reach up to about 1" below the lip of the base.

NOW here is the problem as I see it. The base is made from a thin piece of sheet metal not aluminum but thin metal.
You notice the instructions say have someone to help put it in the hole that is because it is so flimsy that if you try to move it you will bend it.
A few years in the ground and it will rust away but they recommend putting it in the ground because it needs something to support it.
You can bolt the base to some timber but it is so thin I think it will flex and maybe bend so I plan on installing several L brackets to help support it.
The rail road ties would be overkill but if you have them or can get them for cheap then use them but they will require a lot of extra digging.
The HFGH is a lot of GH for the money but it is a cheaply constructed structure and needs some modifications to it to make it into a nice GH.
Since it sells for more than a $1000.00 less than a quality built one of that size I feel I can make the necessary modifications to it to make it into a strong piece and save a lot of money but as it ships to you it is not very well built.
I plan on building the base onto 4x4 treated timbers and bolting the base to the timbers every two feet then reinforce the base with the L brackets then put it into the hole with the base above ground where it will not be in wet soil all the time.
After the wall studs are bolted onto the base that will make it a lot stronger but I plan on some extra bracing to make sure it don't collapse in a wind storm.

The metal base needs to be attached to a foundation structure of some sort (concrete, treated wood, etc.). It should not be placed directly in the ground. Don't know your zone, but in areas where there is frost heave or excessive soil moisture, you will need a foundation.

I would also suggest a sill seal under the metal base.
The wood will absorb moisture even though it's treated.
The metal will be subjected to it. The sill seal will protect
it from rust.
Tom

A seal is a good idea, too. Your base (if its like the one on my GH) is galvanized steel (presumably painted). If you place the frame on a treated wood foundation, the wood treatment chemicals will attack the galvanization. Rubber elastomer membrane (used on roofs under tiles to protect against ice dams) would be a relatively inexpensive option.

For foundation materials, creosote treated railroad ties should not be used. Creosote is poisonous and a carcinogen.

When I set up my greenhouse (another China manufactured unit), I made the mistake of not taking the directions for the size of the foundation directly from the base itself. Instead, I used the size as printed in the manual, which turned out to be almost 2 inches too large. What I should have done was set up the base first and measured it.

I agree with Cottage,

At least set up that base before planning the foundation. It'll give you an accurate measurement.

Dave
Milwaukee

At least set up that base before planning the foundation. It'll give you an accurate measurement

Yep that is what I mean by putting the cart before the horse. I'm not going to do any digging until the foundation is done. I have the base assembled but don't have it on the wood yet. Probably will be April before I dig.

Thanks everybody for the input! I agree, I'd better get the goods first, then make the measurements.

I'm definitely leaning toward cinderblocks for the actual foundation.

Check out my website on building my hfgh. There is info there about the foundation. Irecommend putting it on an elevated foundation of some kind justto make it a little taller.

marbs

Here is a link that might be useful: Building my HF greenhouse and more

Your link is broken. You missed the period after www.

http://www.lowtechwebs.com/hf/greenhouse.htm

I realize this is a bit late for y'all, but am just about to buy a hfgh 10x12, and am looking for advice. I plan to place it on an existing concrete patio. Is is really necessary to dig a trench for it in the dirt, or can I just either bolt it directly to the concrete or a 4x4 framework placed on the concrete? Would I have to make special allowance for drainage? Thanks for any help you can offer.

I would put it on treated 4x4 timbers and bolt the base to the timbers. This will allow you to hook on to it and drag it to another location if you wanted to. The base is very thin and will require a lot of bolts if you put it directly to the concrete you will need to drill about 35 holes in the patio concrete.
I posted a list of the modifications needed to make one of these things survive in a wind storm.
Here is a link to my post. Also do a search for 10x12 HFGH you will find a lot of posts that should answer all your questions.

Here is a link that might be useful: Pictures of mods to HFGH

Gardnergal I just tried a different approach to my 10X12 foundation and it may also work for you:

I placed Qty 4 green-treated 4x4 timbers INSIDE the greenhouse floor, criss-crossing them. I cut a slot on the bottom of each timber 1 1/16" from the end about 1/2" deep, such that the timber rests on top of the metal and holds it down. Then I put 4 lag bolts into every timber end (thru the foundation of course). Makes everything nice & straight & square, and the weight of the timbers oughta hold it down good. Then we filled it with pea gravel.

Others will fret that the metal foundation contacting both ground and green-treated lumber is problematic; I don't think the things going to rust-out and/or fall down in my remaining lifetime.

I will post pictures when I get a chance...

Others will fret that the metal foundation contacting both ground and green-treated lumber is problematic; I don't think the things going to rust-out and/or fall down in my remaining lifetime.

MAN sorry to hear that you plan on dieing so soon.

>MAN sorry to hear that you plan on dieing so soon.

Hey, it never rains here so I think it'll last for more than a few years anyway!

Here's a link with pics.

Here is a link that might be useful: My HFGH 10X12 Foundation

Thanks for the ideas. I'm not savvy enough in the construction dept. to solve the problem so creatively!