This is the Missing Sector Orb Web spider, Zygiella x-notata. This is a very common and widespread species and loves to make its web over windows and uses the corners if the frame and the sill for its refuge. I doubt there is a home in the land that does not have this species in residence.
They are completely harmless and through the year they will catch a good deal of insects and other invertebrates that are considered pests. They are also one of the few species that happily feed through the winter given available winter insects.