My plan is enough research to not have to "add stuff" when I am done. With the advent of the internet there is no reason I can not come close to getting it right the first time.
(right down to screws to attach the lower siding to keep it from coming loose or animals trying to get in)
When I hear this, I'm always reminded of the line adapted from Robert Burns' poem, To a Mouse :
"The Best Laid Plans of Mice and Men, often go awry..."
But, you are right - there is little reason why you can't come close. I will guarantee that you will miss the mark somehere along the line, too. But it should be minor and we are always here when you do. Chickens are not a crisis, what ever happens there is no reason to panic.