My family and I moved to a new house last spring and I no longer had my south-facing windows for starting seeds and transplants. I built a very useful greenhouse for well under $100--although I did have some scrap lumber on hand and some old hinges and screws. The greenhouse measures 12'x 14' (168 sq. ft.).
For starting seeds and transplants, you will want to place the structure in full sun. Select a level, well-drained site for the greenhouse. If your site is not level you can level it by digging a trench on the high side instead of raising the low side, as the base boards should contact the ground at all points to ensure a good seal. Cedar or green treated lumber should ...