In all establishing shots of the Huxtable house, it seen as being in the middle of a group of houses, with fences on either side of the front steps. In episode #2.21 ("An Early Spring") some scenes take place on the Huxtable's front steps. In these scenes, there is a driveway the the left of the steps, and to the right of the house there is an alley.
While Mrs. Westlake is sitting on the steps talking to Claire, the rims of her glasses reflect light, but not the lenses. Obviously, there are no lenses!