We used an extra large doghouse - Saint Bernard, I believe - that we got for free from Craigslist. That worked for our whole first year with four, and then seven and then nine chickens. At which point we knew a lot more about them than when we started, though still never enough!
At that point we built a coop we still love.
Our climate's cold, and yours may be cooler still - but the chickens were just fine that first year. They had the doghouse, lots of bedding, their lightbulb, fresh water and food, and nary an illness or complaint.
Might be worth holding off on building til later, when you know more about what you like?