We are using a modified freebie dog house for a growout coop. It's about 40 inches square and was a Mother's Day gift from my kids. Gotta love them when they think of me!
Pat is right... drilling round holes is not enough ventilation. That was our first attempt and did not work.
Ventilation was a big concern when we started converting it to a coop. First, I enlarged the door so I could squeeze my head/arm inside for cleaning and working. Then we got an awning window very cheaply so I put that on the end opposite the door. It does almost replace the entire end of the doghouse and was installed from the outside. The awning opens to the bottom so wind and rain really don't come in the coop. I covered the entire inside of that end with hardware cloth tacked down with boards ( from the inside- not really fun
) I have not closed the window at all yet. Not even sure I will close it for the winter! The chickens seem to roost facing the window.
A roost was placed with 2x4s perpendicular to the openings about a foot off the floor.
And yes, it was put on a frame and posts to elevate it about 24 inches above the ground. ( THAT was a fun project moving it up there!) It is easier squeezing head/shoulders into it when it is raised, so I suggest you do that first.
I have two interchangable doors. Both slide on a top/bottom track to little more than cover the opening. They are secured with two long nails through small holes in both the door and the wall. We store the nails in similar holes in the upper track when the door is open. One door is a piece of plywood-- used in cold and rain to keep drafts down. The other door is a plywood framed piece of hardware cloth to have open ventilation for spring and summer.
Food is on a hanging feeder on the left of the door. Water is hanging to the right of the door.
Chickens have been in it for about 5 months now. Improvements that we need: Space-saving feeder/waterers of PVC pipes along the walls instead of round ones. I also need to make an outside nest box SOON! I am figuring on a 30 inch/2 box unit on the side where there is no door or window. There is just no room inside for a nest IMO. I will make the nestbox first then attach to outside with some sort of cleats and have the top open to retrieve eggs. Come spring it will be our marans breeding coop

I don't really draw plans, just wing it

Suggestions: drill and screws or a nailgun work easier in tight spaces. And hey, it's a chicken house, not the Taj Mahal. If it's crooked, they won't complain!