1. Take it from me, its better to build too big then too little. It doesn't cost much to make it bigger.
---- For the price of only 2 more sheets of plywood (50 bucks?), and a few 2x4's you can make the coop 4 feet deeper. Plus a little more roofing I suppose.
2. Remember with your coop, you are NOT trying to keep your chickens in the coop. You are trying to keep predators OUT. Build it from that viewpoint.
---------- When I took the time to ratproof the coop and spill-proof my feeder, I saved probably 60-70 percent on my feed. Do that it will pay for your expansion many times over.
3. I wish I had made my coop big enough for a dedicated "broody" space. Where my broody silkies can brood, and raise chicks. They keep trying to use nesting boxes, and I only have 2. I built it much too small.
4. Don't build on the cheap. I used cheap plywood, and it didn't hold up well. Go to home depot, spend 30 bucks and buy some cheap paint, and slap it on there. YOu will thank me in 3-4 years.