I purchased one of those kits, it's on the right of the photo. The thing looked VERY Small once I had it up. In the end, I decided to raise more chickens than the two or so the kit recommended. So, I started literally throwing down boards and sawing, nailing and screwing. I had no plans other than a vague notion in my head. I was really pleased with the result.
I did 3-tab roofing, and made it so the top of the nesting portion lifts up. Once the chicks moved in, they really didn't go to the "kit" side ever. I was going to move it out of there and sell it, but a funny thing happened. The entire flock (six) all goes over to the kit side to lay their eggs. that's all they use it for. Try as I may, I try to make it inviting for the hens to lay in their nesting boxes on the left side, but they immediately scrape the pine shavings out of the boxes as soon as I put them there. so now my coop looks more like a hodge-podge, but that's ok by me.
One thing I would do different: I'm tall (6'4"), and since I didn't have a plan, I sure would have made it taller if I would've known I always hit my back on the top of the door jamb when I go in to clean the coop! the door is about 4 1/4" tall.. I think the roofline is at about 5 1/2 ft. tall. the "homemade left side cost me about $250 to make. Just 2x4's, a couple sheets of siding, thousands of nails. I have a three yards of crushed gravel around the outside of the coop, and play sand inside the coop which I scoop out like kitty litter.