My house has a newer adjacent part with its own roof, pretty isolated from the old house. But I had mice on both attics. Finding their way from one attic to the other was the easiest part. But I needed infrared camera surveillance to figure where they entered the building. From the footage it was pretty clear that they never stayed long in the new attic. They entered through the new building, climbed up to the attic, than traversed to the old attic where they probably did all kinds of mouse things including raising their offspring.