I'm pulling together plans for a quick propagation greenhouse that I can throw up for the winter/spring propagation cycle. We have highly variable temperatures and predictably devastating spring winds, but rarely more than a few inches of snow, so those are the requirements for the structure to handle.
The leading candidate at the moment is a 12'x60' scratch-built galvanized steel hoop house, because it's quick, sturdy, and (relatively) inexpensive. This would provide 720 square feet of grow space, which comes in just under my target minimum of 1000 sq ft, but a wider house would require heavier duty steel, and a longer house would be harder to heat (1 to 5 seems to be the point of diminishing returns for heating. Any longer than that and you might as well build a second house). I would be using double layer inflated poly, knee walls for optional conversion to roll up sides in the summer, diagonal bracing at the corners, and quick, inexpensive endwalls. Also, the most efficient heating that I can cheaply, easily, and safely install.
The location is still undecided, so I have a bit longer to play around with different options, but I'll post plans and pictures as the project comes together.
There's always the chance that I'll find a second-hand frame I can reuse, but so far I've come up empty. In the back of my head is a little voice telling me to just get one of these:
http://www.blackrockdomes.com/...
