DH built me 4 breeding/grow out pens under our deck.
There are 4 pens, each are 3'x5'. After we are done pairing up couples and have hatched the chicks, these then become temp grow out pens for the chicks after they have moved out of the brooders. We are still in the process of dividing the space under the deck outside the pens, into two sides, so we can let two pens outside at a time.
We also built this duplex brooder box, and we did use it for bantam breeder pairs late last winter. Each side is 3'x3'.