I decided to build my coop to hold up to 15 or so heavy layers, so 8 X 12 is what I ended up building. The construction method I used is the same as is used on the prefab sheds you can buy at the lumber companies. I customized mine by making the roof a salt box style, and adding vents and large windows. I saved about $200.00 from what one of those "kits" would have cost, though the foundation added additional cost to the project. I wanted my coop a foot or two off the ground to keep out mice and other varmits, and provide shade for the birds from the hot sun. I built the foundation just like you would a free standing deck using pressure treated lumber for the posts which were set in the ground below the frost line, and joists. (See pictures that follow). Regular 1/2 plywood was used for the floor, and screwed down every 2 foot on center into the 2X6 joists. I then made my frames out of 2 X4's, nailed them in every 2 feet on center then used regular 4X8 sheets of rough sawn siding for the walls, and regular 1/2 inch 4X 8 roofing for the roof.
Note the snow! This is what we Michiganders have to put up with in April!