Here, I'll show you a few videos of greenhouse construction, with my commentary. The are good points to be made from each, and even though two of these are rather large, those principles still apply in smaller versions. The first video is the smallest, simplest, and closest to what is needed for personal consumption. (You may want to mute the sound on the first one. All the info is typed and the muzak was killing me.) Despite the fact that I sound critical of all three, each has put a good deal of thought into it, and built good structures. In other words, I looking at the hybrid of the three.
Overall, I like the simplicity of this one, and particularly how it maximizes the exposure to the Southern sun. It really demonstrates how easy it can be to build, and that you can do it in the city, unless you're in an authoritarian HOA or municipality which demands both green measures and prohibits their construction.
You can use the same principles from "Framing the Little Home-2" to build it, so don't fear the complications of framing. The difference is that you don't need 16" centers for this. Four foot centers will probably be the most you want to stretch it, but the only weight your dealing with is any snow that accumulates, and in all honesty, we want to get rid of that by internal heat anyway, which leaves only the weight of the materials, and that's pretty light. In fact, this would be easily built on the South wall of the Little Home.
The first thing I would change on this one is the foundation. Wood does not last long when it is in contact with dirt. It seems to pull the moisture out of the dirt and rapidly rot. I would recommend that concrete block be used to an area above the dirt line. Conversely, the foundation blocks are a great way of avoiding having to dig deep holes and pour concrete footings.