Been there, done that, everywhere I've lived that I could. Back in colledge days, 1976, it was a few 2x4's leaned against the south wall of a rental, covered with plastic. The produce was nice, but the the greenhouse was built over a window, and heated the house too.
Later, in the trailer house, we soon added a 14x20 room facing southwest that had five 3x5 windows. By afternoon we'd open the door and let the heat roll up into the trailer from the room, 3 feet lower. The widows would be covered with shadecloth in the summer, and the windows left open.
Now I have a lot of southeast wall. two homemade passive collectors 3x7 and a window 4x8 heat the living room. The smaller entry porch connects by a window to the kitchen. We had some ugly glass over that last winter, and I have just bought some plexiglass to make a much prettier, and permanent way to make the porch heat the kitchen. Those gather heat all morning, the perfect time for it, while a large bedroom window heats in the afternoon. Our first winter in this house, before adding passive solar, insulating, and trimming a tree that shaded the front window, we were running about $200 a month to heat the house, which is all electric. Today , after a substantial rate hike, we pay $50 a month for heat.
Passive solar rocks! but it needs to be done to a house that's been tightened up, and up insulated for the real savings. On a leaky house, it still saves money, but the effect does not last all night like it does for us now.
Polytunnels were instant greenhouses made of giant tubes of polyethylene weren't they? A really good cheap instant greenhouse can be made with the shade covers for cars, usually a 10x20 tent frame of steel pipe. Simply cover the thing with 3 mil plastic, with the end on the wall of the house, where a window or door is. bingo 200 square feet of solar heater.
If you have some south wall, It's hard to beat passive collectors. All you need is an old sliding glass door panel, a few strips of wood, a piece of sheet metal, and a couple air vent covers. Wood strips frame a square on the wall, inside goes the black painted metal. Cover with the glass. Behind the metal, one hole in the wall lets air in the bottom, another up top blows hot air all day. No fan needed, but you need to cover the vents at night, or it becomes a cooler.
