Do you have any construction experience? YOU will have to make it level. You will need some sort of firm footing for the greenhouse. Not having seen the land, I guess you will need to dig down from the higher side, cut a chunk out of the hillside to make a level space. I would then trench it all around, build forms and pour a level concrete footing. Then you will be ready to set up your greenhouse.

I faced a somewhat similar situation. I ended up renting the smallest backhoe I could find and flattened the area and then dug a trench down to the frostline for the footing. You're going to want a solid floor that will drain well; you could flatten things out by dumping a lot of 5/8 minus gravel on the low side, but you still will need something to retain it. You might be able to get by with a retaining wall of landscape timbers pinned together and into the ground with rebar, but a concrete retaining wall or perimeter foundation would be better.

You don't say how large your greenhouse will be, but perhaps the attached link will help; it is from an older thread on constructing a 6'x8' HFGH on a slope - excellent pictures and descriptions. Basically, he built a concrete block foundation on the lower side to create a level surface for his wood foundation. There are a some other good ideas there, too.

The link was posted by GW member 'marbs' on 2/27/05. If you want to view the entire thread and his original post, go to http://forums2.gardenweb.com/forums/load/strucs/msg0220164716982.html

Here is a link that might be useful: Marbs' Harbor Freight Greenhouse

I had exactly the same problem when I put my greenhouse up.
Levelling the ground was not an option due to the # of large rocks in the soil. We built a foundation of landscape timbers, secured by reebar. We then filled in the whole greenhouse floor with stones to level it. On one side they are a good 2ft deep if not more, on the other just a few inches. It has held up well for me. The floor drains well and holds some heat to help out on cold winter nights. A masonary foundation would probably be longer lived but between costs and labor involved I went with the landscape timbers option.
PQ

I put down a row of concrete blocks and then put the foundation timbers on it so that I coud raise the greenhouse. Raising it worked very well. I have a higher doorway entrance, but getting the increased height was worth it.

Check the construction out at my link below.

marbs

Here is a link that might be useful: Building my HFGH greenhouse

Yep,had the same problem when I build my greenhouse.I just dug down about 4 inches,and framed it up level,and had concrete delivered for the footing.It's about 2 inches high on one side,and over a foot tall on the other end.Depending on your frostline depth,you may have to dig down much deeper than I had to.

I did the same thing as Ponderosaq. It was the quickest and easiest. We dug holes and made little pier footings and put 2x6 down for the base. One side has one 2x6 the other side has 2. We did dig down but it was just a tiny bit. We put ours up in March when the ground was still frozen, se we wanted to do as little digging as possible. We dug it by hand. Then we filled it with stone, more on the lower side to level it out. It works great! Here is a pic.

Here is a link that might be useful: {{gwi:291386}}

Thanks for all the good ideas.
My greenhouse will be 21x36 and mostly PC with PE vents.
I'm so thrilled, and can't wait for it to get here, hope it won't be warm by the time we get it up.
Next task is to figure out propane tank size.
susan
will post photos of construction (if I can figure out how to do so)