Not a greenhouse as such, but I built a leanto polytunnel affair.
originally it was composed of 4 8ft canes and a beam which I'd bought to replace one from the shed which I thought to be rotten but which on closer inspection turned out to be solid with holes drilled to accept the thin ends of the canes. 4 pieces of old pipe were hammered into the ground for the thick ends, a sheet of PVC film purchased and some scrap wood scrounged on the site, used as batons to secure the sheet to the front of the shed.
It was not without problems, entering and leaving was a very undignified procedure due to the sheet being attached to the shed with just a small corner flap to crawl in through, there was much condensation that ended up destroying my tomatoes and which also made everything in the shed damp. There was too much sideways movement in the end canes, and the final straw was that the sheet was not UV resistant and disintegrated once the winds got up in winter.
So, pictured below is MKII, the same basic design but with an extra bracing cane added on the curved section, held in place with brackets made from scrap steel at the ends and some thick aluminium wire I dug up on the middle canes. Anti hotspot tape is provided in the form of foam draught excluder strips. In view of last years issues I bought some NEW WOOD and built a proper door and frame (Which is just too low for my head) and some triangular vents above the door and at the other end, I was planning to use some twinwall polycarb from a lidl coldframe which got blown apart as windows, but never quite got there...
I crammed 9 tomato plants in there last year, but that was too much for comfortable movement, so I'll limit it to 6 this year, and maybe train some physalis up the front of the shed.
At some point I will build a bigger proper greenhouse, I'm torn between the cheaper and easier normal rectangular design with a pitched roof, and how cool it would be to build a geodesic dome. For glazing I'm trying to source ETFE film (As used in the eden project, near indestructible and self cleaning), but I can only find 200m rolls.