It's a Zygiella species, or missing sector orb weaver. Except this one must be quite young as it seems to have built a fairly complete web. Usually, 2 sectors are missing the concentric strands.
Presence or absence of the missing sectors is influenced by the angle at which the spider approaches the web. In a window frame the angle is very shallow - so the spider is likely to brush up against the web as it approaches it. In this case the sectors are likely to be missing. But in a bush the angle is much greater and the sectors are more likely to be complete. Hope that makes sense.