I love the idea of reusing materials that would otherwise be thrown out to build a greenhouse – like this one made from old windows. This is something I have been seeing quite a bit lately when searching for DIY greenhouse ideas. Here is a great example of one from cheft on Instructables.
The windows came from houses that were being torn down in his neighborhood, and he purchased the lumber to build the frame and the other materials – screws, caulk, etc – for about $300.
He began by laying the windows out on the ground to plan out two pair of equal sides for the walls, then used that layout to build the frame. He constructed it shed-height and stabilized it with cinder blocks buried in the ground, rather than pouring a permanent foundation, to avoid having to get a building permit. This greenhouse is 7 ft x 10 ft x 6 ft. This method could easily be used to create a larger or smaller greenhouse, depending on needs and available materials.
See the full process here.