Take a look at my BYC page, you will see the interior of my coop. It's about the size you are talking about (give or take a ft or 2). You can see the roosting rods (made from tree branches) that are nailed to 2X4's graduating up toward the wall. I have a chain that I use to lift them up when cleaning the floor, makes it easy to clean under them. I have 12 nesting boxes (made from scraps) but they only use the lower 8 for laying and roost in the upper 4 (lots of poop in those every week). 8 (4 upper, 4 lower) on the opposite wall and 4 shown in the pic. These have been attached to the wall which they weren't in the pic. I don't have electricity in mine.
Once we had the house built, we fenced it in using the lower 3 ft as chicken wire and the upper as used cattle wire. Works great! Never had one to "fly the coop". Used lots of scrap lumber and old windows. We had lots of used landscaping tembers we used as fence posts. Very low cost for the size of the coop and lot. Even bought the paint from the paint store as rejected custom color! $3 a gallon! Using older/used materials is a very cost saving way to go! This is a chicken house after all. I think the only thing new we bought was the chicken wire for the lower 3 ft of fence. The rest of the materials we either already had or got from people who wanted to get rid surplus.
My chickens seem to be very happy with their "condo". Also not pictured is the feed storage bend that I keep in the house too. The roof is one slant (10 ft to 7 slope), and I collect rain water in buckets for easy watering of the birds (don't have to tote water too/from the coop as much). My friend gave me a gutter, I just need to install on the low side so I can use a large (new) trash can to collect more water than my 2 - 5 gal buckets do.
The shelves of your shed might be easy make the nesting boxes (bottom of the shelf as the bottom of the box) if they are not too high.
Good luck!
