That coop will not house 8 chickens. Like I said previously , I have that coop. They calculate by using the square footage of the henhouse (aka the coop), the nest area, and the run. Chickens need 4sqft/bird in the henhouse not including the nest area. This is because they might need to stay...
I'm doing exactly the same thing. Im leaving the henhouse floor intact making it a 2 story coop, therefore more floor space. See my progress in my thread "Altering my terrible TS coop". I hope you post pics too!