Didn't mean to offend, and I did read the OP's winter temps, but my post was meant to address your generalized statement that (all) California has mild winters, not that you made a wrong statement about the OP's winters. I think it is funny that I didn't even see in your profile that you are from Calif!
I still feel it important to have a general idea of location. If the OP is in the Bay area, then summers are mild as well. I can think of plenty of areas here that are in the 30s in the winter and 100+ in the summer. More fog and humidity than dryer areas, but not Florida humidity.
An open air coop would probably work well there. A good roofed run with large overhangs, because that area can get some pretty good rains, and a three sided shelter at one end. Maybe a removable panel or two in case there is an odd hard freeze.
I have something like a miniaturized version of this coop:
https://www.backyardchickens.com/threads/texas-coop-build-pic-heavy.1371038/
Mine is well roofed, too. Not because we have much rain in So Cal, but because the only legal location for my coop was where I had a shade house for my orchids. So I had to make sure the coop stays dry even as I water the orchids.
I’m in northern CA and Have had soooo much snow, I’d definitely make the coop not too airy... then buy a few nesting boxes from 
Another thing to consider, assuming you have a truck, is calling around to look for throw away shipping crates! usually they are recently built and only used once!
unfortunately chickens CAN freeze to death- even In California... especially if only 7 chickens are housed in a large coop with too much draft And no supplemental heat...May as well prep for what may come- don't ya think?
you’re pretty funny, I like that.
