I have a brooder that sat in the garage with four chicks for about 8 weeks, then I moved them outside to a teenage "annex" which is basically a small run. Our brooder was three very large plastic storage containers which had holes cut into the sides, then Velcroed together--like a three caboose train. When I moved the chickens outside, we moved one of the storage containers into the annex run and it will act as a coop for the next few weeks.
Our annex is just a simple 4x4 cubed chicken wire box with a hinged top that lifts up. It sits right up next to the run so that the old flock and the new flock can stare at eachother through the fence over the next few weeks before I start to integrate them together. I had my DH built this annex to fit exactly over one of our raised beds in the veggie garden, so that after the two flocks are integrated, I can use the cage to protect my veggies!
Additionally, the cage could be used if any of my hens needs a time-out or separation for medical attention.