This is a female Black or Walnut Orb Weaver, Nuctenea umbratica. This spider hides away by day and occupies it's web after dark. Hence they are rarely seen, but they are a very common and widespread species and found in most gardens.
They can bite and it is comparable to that of the Noble False Widow, Steatoda nobilis. However, bites are extraordinarily rare and usually only occur when accidentally trapped between clothing and skin during shed or fencing demolition. I have a beauty on the side of my shed - I know she's there and that's where she'll stay.