Hey pointyp, great to hear from someone close to Seattle!
My boyfriend and I just moved from Ballard to Whidbey Island the 1st of September. I'd gotten 3 three month old chickens 3 months before we moved and had them in my backyard there in a chicken tractor we built. When we moved the tractor to the front yard the week we moved, people were so fascinated by it! We had people in our yard all day long, just watching them. How fun! I'm so glad to hear of someone else in Seattle keeping chickens.
I agree with what the other BYC-ers have said about the tarp idea and the extra bedding. Lots of straw for the girls to snuggle up in. Pretty chickens too, btw! Where did you get them from?
Our temps got down to -8 this last week but I do have one heat lamp in the coop that keeps it about 30 degrees when it gets that cold. Sometimes dropping to 25. Now I learned and I put the heat lamp over the waterer and no more frozen water