Previous owner had a semi-enclosed sand box (20 x 20) for his kids, with one side being an A-frame swing set. When I bought the house I pulled out all the swing set stuff and eyeballed it for a couple of years, wondering how I could turn it into a chicken run. Eventually I just started building and I'm very happy with the way it turned out. Here's the build -

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Swing set frame. Entire enclosure is 20 x 20.
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Putting in frame for floor, roughly 12" off the ground. Roughly 8x8, and nothing is square.

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Floor is exterior grade OSB.
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I planned to have the lower half plywood walls, and use recycled polycarbonate panels for roofing.
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Nest boxes going in

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Ends on

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Recycling some polycarbonate roofing.

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Human door

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Pop door. Exterior painted/.

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Human door on
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Trimmed out

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Interior wood/OSB floor sealed with poly sealant.

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Happy Henhouse automatic door

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First batch of chicks checking out the great outdoors for the first time.
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Since these are the first chicks, I used the entire coop as a brooder.

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Exterior getting enclosed with wire fencing.

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Growing flock
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Roost bar from a tree. Barred rock and production reds in the foreground are the survivors from first batch of 8 chicks. Ameroucanas and Black Lace Wyandottes in the back are 6 additional chicks to shore up the dwindling flock.

Next addition was two strands of electric wire around base to keep racoons and possums out. Also had to modify the pop door so racoon hands can't get under it.

My chickens did great for about 6 months, free ranging during the day. Then they started getting picked off, one per week. I put them all in the run and came home one day to find a hawk in the run eating one. So completely enclosed the run to make it hawk proof, while doing that work, I lost another one. So now, a year later, I have 3 hens remaining out of 14 I started with.