Ok....I'm going to weigh in here, for what it's worth (I don't have a lot of experience).....but...
Back in July I set out to build a coop that would handle 6 hens. My husband and I have no idea how to build anything. We are not handy like most other people seem to be here. So on this board I saw many people using a child's play house - purchased at Lowes - retrofitted to act as a hen house. We did the same (it was really easy to do) and bought a dog run to attatch to the coop as a run. Here is what we came up with in the end:
It easily manages the 5 hens (one died by way of raccoon before we finished covering the top) I have. They are quite content in there.
But then a funny thing happened. I thought to myself....Oooh, wouldn't it be nice to have some Wyandottes (so many colors!) and some White Rocks. Oh, and I just love those White Crested Black Polish! Oh, and I love those Orpingtons, too.
Bad news about the size of my coop and all the peeps I wanted. Very bad news. We spent $500 on the playhouse coop 6 months ago, and now it's too small. So now, we are working on converting an old shed into a coop that will let me keep about 30 hens! Yippeeee!
Now the playhouse coop will become my nursery for the 15 chicks I just ordered from Ideal. I'm really excited. My five girls can't keep up with the demand for eggs (all our friends, family, & coworkers buy from us) - we shouldn't have any trouble now!
My point for my long-winded tale is: go bigger than you think you need - or you'll probably be building a bigger one in less than a year. These silly birds are totally addictive.