Looks good! With your proposed number of chickens being up to 15 (12 RIR + 3 rare breeds), you may need to think about your roost bar length or having more than 1 roost bar. In addition, I see that the window they propose in the picture is right at roost level. Ventilation is great, but needs to be where the birds will not be in a draft. Also, many people on BYC recommend having nesting boxes lower than the roost bar. Chickens like to roost at the highest spot and will do so on the next boxes if they are the highest (leading to poopy eggs). My nest boxes are at floor level, but I have a ladder-style roost, so the floor is the only thing lower than the lowest roost bar.
Have you looked at the articles area on BYC? there is an area for coop builds, and you can see a wide variety of coops people have built.
Good Luck!