Aart is a great source for information on this for sure, definitely check out the ventilation article he links to. I live a little bit north of you now, central coast, rural SLO county area, but I used to live just south of you in Orange County. Very similar weather depending on how close to the ocean you are. That being said, the coop I built was 3 sided with the 4th side completely open, but covered with hardware cloth. I believe it was 1/4 or 1/2 inch to prevent any curious raccoon hands from making their way through it. I think as long as you have the opening protected from the rain you should be ok in that climate. Wind could possibly be an issue too so take into consideration which way the open side will face. Post some pics when you get things going! Good luck.