I do not have plastic under the sand in the house and to date have had no problems. The floor is plywood. The hen house is weather tight so I don't have issues with water getting in. My run is on a hillside, and is pretty well drained soil so I have no intention of amending that surface. The run is about 20' x 20' with 11 birds and I have not noticed any need for any kind of cleanup in the run, except occasionally raking pine needles. The house floor gets kitty litter scooped every day, but it only takes 3 or 4 minutes. As for temperature, as long as the sand is dry it shouldn't freeze or anything. The hen house has dropped into the 20's inside on a number of occasions and it hasn't been an issue. I feel like wood shavings would be less cold to the touch, but don't actually know if that's true.
new2thebrood, I'm thinking living in San Diego will make some very happy chickens.