Might I suggest, instead of a lamp, instead using a heating pad? I raise all my batches of chicks with Mama Heating Pad, even brooding them outside in the run from the start, and have been absolutely thrilled with the results. Heat lamps scare the peewadding out of me....chicks put out so much dust and dander that before you know what's happened the entire lamp is covered in a fine, grey powdery layer. Chicks not only tolerate a lot of cooler space, they need it! With a heat lamp everything gets heated - the walls, the floor, the brooder box, the feed and water, even the air and that dust settled on top of it. It's very difficult (although certainly not impossible if the brooder is big enough) to provide that cooler space. With Mama Heating Pad, the only thing that gets warm is the chicks when they scoot directly underneath into the cave. And I love that the chicks learn from day one that nighttime is dark and for sleeping, daytime is for playing, eating, and learning to be chickens! They sleep all night, from sundown to sunup! I think it makes for calmer chicks with less feather-pecking and noise, not to mention more sensible use of chick food because they aren't eating 24/7.
https://www.backyardchickens.com/t/956958/mama-heating-pad-in-the-brooder-picture-heavy-update https://www.backyardchickens.com/a/yes-you-certainly-can-brood-chicks-outdoors There is a coop section tab in the upper section of the home page. And check out the Learning Center, also tabbed up there. We built a 6X8 shed style building and used cattle panels for the run. Love it...no problems at all. You can click on the My Coop link under my avatar, but there are so many other great coops and designs out there that should work very well for the number and size birds you are starting with. So the only advice I'll offer is to build as big as you can. If you're like most of us, those 9 chickens won't stay being only 9 chickens for long - you'll see a pretty chicken and think, "Oh, I need to get a couple of those..." Before you know it, 9 + just a couple of those = 20 chickens!!

I lied. I do have another piece of advice for you, Decide on a plan and start building before your chicks even get there. I didn't. I got the chicks first, long before I'd settled on a plan for a coop and started the building process. I ended up with 22 chicks in my house for 5 weeks and darn near butchered them to pass them off as Cornish Game Hens before we got the coop going and I evicted them. Some folks love having chicks in the house for weeks and weeks. I DON'T and I'll never do it again. Good luck with your new adventure!!