Unless I missed something on this thread I have not seen any mention of roost space which is also very, very important. 4 sq. foot per bird floor space is fine if you leave them on the floor? If you want 20 birds you need at least 8 to 10 inches of roost space per bird inside your coop. That means you would need at least 2- 8 foot long 2X4's for your 20 birds or 4- 4 foot long 2X4's about 20 inches off the floor. Hope this helps with your plans.
