Would the medium size of
this dog house work? I'd just need to add a secure door.
When you are looking at housing, a good rule is 4 square feet floor space inside for each bird, and 10 square feet ground space for each bird outside. This gives them enough room to avoid some of the problems of crowding - fighting/pecking, being kept away from feed/water, illness/other injury.
You want to plan that they have enough room inside the coop to be stuck in there for days at a time in bad weather, or if something happens and you were unable to let them out for some reason, like you were in the hospital and no one remembered to take care of your chickens. That is why it is recommended to give them so much space. You can cram any number of chickens into a small space but you're gonna wind up with problems at some point when something happens in life you didn't plan for.
I also recommend having food and water available both inside and outside of the coop - this keeps more subordinate birds from being scared away from food/water and also ensures that if it is pouring rain or the birds are forced to stay inside, they still have access to food and water. Sometimes you may not get home in time to let them out or into their coop. So I recommend that you plan to have enough room for the birds inside as well as food/water.
The recommended amt of floor space is not floor/roost space combined either. If you have roost space, that will make them happier, but you want them to have enough space on the floor/ground to be able to mill around without invading each other's space.
That medium dog house is roughly 3 ft x 2 ft or not quite 6 sq ft of floor space That will cover 1 large fowl (not bantam) bird. If the birds you have are on the smaller side and get along well, you might be able to safely squeeze 2 in there, but that does not leave room for an in-house nest box and you would definitely have to alter that house to provide a nest box that hangs out of the house.
Many people are fine with providing a very small space for their birds, but I don't recommend it - particularly not for someone who is not able to let their chickens free range a large area all day. It is one thing to house them "snugly" when they can range far and wide during the day to alleviate their boredom, but for birds that can't have a large free range area, trying to cram too many birds into a coop can get dicey. It sucks to not plan ahead and then wind up with birds that do not enjoy being so close together and then you have to scramble because you have bloody heads/faces from pecking, low-weight birds because of constant stress, etc.
