Of course you have to build a new pen for them..how many pens do you have right now? Had to ask! I have three pens..one is at the end of the main run though, does that count? I have two coops. One is much smaller, for my little bantams, only going to have two. I put babies in there once they are old enough at times. Helps them get used to other chickens, and they can see the other big girls when they are out free ranging.
I have 2 pens and 2 coops now. And a brooder pen. Going to make a couple of breeder pens too. We just got done with soccer season for my youngest son. I was assistant coach so we have been very busy lately. So now I have a little more time to do more.