and now that I'm building I've run into an issue for the roosting bars...
My understanding is that there's supposed to be 18" in between roosting bars and the first bar should be 18" off the floor and the second 36" off the floor.
You probably read this in this forum somewhere but I've never seen this.
The way I manage elevations is to determine the elevation of the coop floor, including bedding. Then position the human door and the chicken pop door high enough above that so the chickens do not scratch the bedding out.
Then position the nests. Some people put the nests on the coop floor. If you do this have enough of a lip so the chickens cannot scratch the bedding into the nests or eggs and bedding out of the nest. Some people put the nests up higher so they don't have to bend over to gather eggs. Do you have a bad back? Some have nests in rows, one row above the others. With 13 hens 3 nests should be enough, 4 certainly would be.
Then position the roosts higher than anything you do not want them sleeping on or in. This definitely includes the nests. Most chickens like to sleep as high as they can so give them enough room they can all be higher than anywhere you do not want them to sleep.
Some people use roosts all the same height. Some use ladder roosts as you described to help them get up there. Just make sure you have enough roost space above the nests so they can all sleep, even if it is not the same level. They will all probably want to sleep at the highest level but who gets to is determined by the pecking order.
I keep the roosts about 12" off of the walls and allow a minimum of 12" between any two roosts. It is possible a chicken's poop could hit a wall, but it will not build up on that wall to where it stays damp. I don't consider that a problem. Some people freak out at the thought of getting a spot of poop on a wall. Not sure where you fall in that. 12" between roosts at the same level give them enough room to use both roosts, at least it does mine. Witha ladder roost 12" horizontal separation keeps the higher chickens form pooping on the lower ones. If you want to use larger spacing you can. I consider 12" a minimum.
Also, they should not be located above or below a windows due to drafts.
In warmer weather they will enjoy drafts. In colder weather a strong cold breeze is dangerous. In summer I open my window and let breezes hit them on the main roosts. That is not a problem. In winter I close that window and have permanent year around ventilation over their heads.
What are your roof rafters, 6" or 8"? With your overhang I would not take the walls all of the way to the roof with anything solid. I'd fill in most of that area in with hardware cloth to provide excellent permanent ventilation well above their heads. That will keep predators out. That way you do not need to worry about the window being open or closed. That is the way mine is set up so I can close the window in the winter.