Ok, glad you found a solution. :) I wouldn't have room in the garage either, by the way, and nobody would want them back since we just kicked the latest batch of pullets out. Fortunately, my coop is withstanding the weather, and so are the girls. Update us in on the dog house when you start...
How big is your garage? How big is the pre fab coop? Is it the type with the tiny ity bity runs with the coop above? I've known people who have tractors that they over winter in the garage. Put a tarp down, layer it with some deep bedding (like hay- not straw- or shavings, or leaves) which you...