I started with basic measurements, I wanted it up in the air, so the chicken would have a spot to go in wind, rain and snow. Having a front foyer was a fun touch. With no plans, it was a build as you go, using my past experience as a plan. I put a window in the back, so the chickens would wake up early, also, put in some power and three outlets. In the winter, they get a light that comes on early for a couple hours. I insulated the walls and ceiling. Didn't want to take up space with sheetrock, so I used heavy cardboard over the insulation in the walls, and painted it with grey cement paint. Also painted the floor with cement paint, then added some linoleum, yes, I know it is slippery but I put a LOT of shavings down over it.
