There are infinitely many ways of making a greenhouse.
In round figures, if you do it yourself, you will save a good half on the price of a ready made kit.
As one data point, I built a 12 1/2' x 28' hoop house (6 1/2' center height) for under a thousand bucks.
Mine gets used primarily to store and dry our
firewood, which it does superbly. For that application, the ground must be sealed off somehow. I used black 6 mil vapor barrier polyethylene, covered with tar paper to protect the thin delicate plastic from the sun and foot traffic.
I built mine almost exactly twice as heavy as the commercial kits, so no worries about snow load caving in my greenhouse.
There are a ton of youtube resources on how to do this. Here's the short version:
If you use a polymer glazing, it absolutely has to be treated to stand up to the UF, aka greenhouse plastic. The regular untreated kind will die in a year. Mine is rated for 4 years, and often lasts 5 or 6.
Make a curved jig to bend your EMT (electrical conduit, or more correctly Electrical Metallic Tubing). Keeping in mind after you bend it, it springs open a bit, so you have to make the jig a slightly smaller radius than you want to end up with.
Pound some 18-24) pipe (or bigger emt) into the ground and attach to pressure treated 2x6's. Your hoops slip into the pipes in the ground, and I put a self drilling self tapping screw through both to hold them.
In my case, (2) 10' pieces of emt make a perfect semicircle for a 12 1/2' wide green house. Make it as long as you want with a hoop every 2' if you get significant wind or snow.
I put a diagonal corner brace at each corner, and two purlins, or horizontal pieces that run along the roof. Every place they cross, the emt is screwed together with those self tapping, self drilling screws.
I made a little post over on the
wood heating forum if you want pictures and so on.
http://www.hearth.com/talk/threads/i-built-a-12-5-x-28-greenhouse-to-store-and-dry-wood-working-awesome.129149/page-2
It suits my needs perfectly, because I don't care if it freezes in the winter. Basically, it's a giant
solar dehydrator. It does provide some spring/fall protection for some tomato plants and fig
trees.
Various ground coupling techniques are a lot more work and/or a lot more money and probably the cheapest way to heat one in the winter, together with double glazing and more insulation.
hth
