2. Ventilation (important to chicken health), with helpful links to coop designs organized by climate (because what is just right for my New England hens won't work for my brother's Arizona chickens)
3. Farmers Almanac on Building Coops (Includes size requirements!) Allowing plenty of space for chickens is really important because, even as chicks, they will start pecking each other or plucking their own feathers, or become unhealthy if they are overcrowded. Here is a link to Colorado State Extension's publication on space and temperature requirements for chicks as they age.