Many times the owners will just be grateful that you can collect it and put it to good use. It may not be in the best condition, and will often having many broken panes of glass or none at all. If you worry about fitting glass into a greenhouse then polycarbonate panels are a safer and lighter alternative to greenhouse glass. Like glass each piece can be cut to your specific size, it still allows light and heat through.
Cold Frame
If you do not have room for a greenhouse or it is too much of a project, then how about creating a cold frame. These are great if you want to sow your seeds early or protect tender plants over winter, a cold frame can work just as well as an unheated greenhouse. Cold frames are much smaller and providing you have a hammer and saw it can be made using recycled pieces of wood. Check out YouTube for some great diy cold frame instructions.
Have you had any gardening upcycled successes?
Disclosure: This is a collaborated post.