The more heat you have, the more important ventilation would be, IMO. Also, You will want to have a nice big roof overhang to provide shade. Were you planning 5 x 5 for your coop size? Does space availability demand that size? Unless you builder is using scrap material from an other job, or has a plan to use the extra material from the cuts, that is going to be quite wasteful of material, thus increasing your overall cost. You might want to ask him to look at the site, and throw out HIS idea of size and shape to make best use of materials, and give you the biggest bang for your buck in terms of size. Also, if you are planning to use second hand materials (windows and doors) you'll want to have those available before he even gets to the framing point. So easy to frame a wall when you know what size the windows are, and where you want to have them put. Much more expensive to say, I'm going to use a window that's such and such a size, frame it up, and then buy the window to fit.