You know, I know someone who took a couple (3) of rose/bulb racks (when empty & with permission) that a local store had, used some plywood for the upper half, added a 'floor (again for the upper half) and fenced the lower half, and had a decent 'coop' for his chickens (he only had a few)
Think: rack is about 3' wide by 18" deep (the ones he used, some are only about 12-14" deep). Took the sides off one. put the other 2 facing each other, with the back of the 3rd between them on the far side, screwed them together (and he attached it to the side of his garage, so there was that back side wooden support for strength, too) He put plywood around the upper half 3 sides (back, left & right), and on most of the 'floor' of the coop, which was at the middle height wise of the racks.( across both of the middle shelves of the 2 sides, and between the 2 sides for the back half). Put secure hardware cloth on the bottom half - left, back & right. Pulled apart the 'extra' shelves and the 2 sides from the back rack. Used that to create a slightly slopes 'roof', added plywood and hardware cloth as needed (on the angled elevation- -for ventilation, rest plywood) Put sloping 'ladder' created from spare lumber so chooks could access bottom half from coop area. Used an old storm window - frames out with scraps from racks - as light and access to coop @ front at coop height. Created door from remaining scrap for bottom half ('run part') of coop - with more hardware cloth. Presto! a 3' deep by 6' (3' + 18" + 18") wide coop plus an equivalent amount of run space underneath. He had to buy a roll of hardware cloth (at a discount, since he works at TSC), and one sheet of plywood (he had some, and bought some) and hinges, door latches, screws. He had some tar paper left over from his own roofing - so made 2 layers of that on the roof to protect the plywood roof.
(Note, he did a few additional things, like trap door to secure coop proper from bottom 'run' half, and semi box off one back corner for a 'nest box'., etc. But, main point, the majority of it was FREE!! He only has 6 bantams, so 18 sq. foot coop is plenty of space. I think the run is a bit tight, but they do get to free range when either he or his partner are home - and is it 36 sq. feet between the two (up and down), which is reasonable for 6 bantams - not spacious, but reasonable - especially with the free ranging.
He is thinking of grabbing another rack this fall, laying it 'front down' and attaching it to the present run, so they will have an additional 3' DX 5'L by 18" H space. This he plans on using just hardware cloth, and using a 4X6 cheap tarp(dollar tree carries them -$1.25) for snow protection in the winter.
The plus with his set-up? There were 3 'shelves' Middle one became coop floor. The other 2 on each rack = roosts for coop & for run - build in! So, he had ready-made 6' of roosts both up & down!!
Where he lives, it works perfect for him! I would be concerned since we have bears and coyotes and such that it might not be sturdy enough for me, but he is in a fairly developed part of town, and, again, he screwed the back side to his garage, which added significant strength to it.
So, I hope you can all follow the imagery of how it went together. I understood it completely when he was describing it to me - but I also knew exactly what the plant racks looked like - so that helped.