Purpose of 就...了 in 这套房子下午就没有阳光了吧？
You can see component in your sentence 下午就没有阳光了(There has been no sunlight on afternoon) as [particular situation]就[other situation]. It denote [other situation] occurred after [particular situation]. In this component [particular situation] is 下午 and [other situation] is 没有阳光了. So 没有阳光了 occurred after 下午 has begun.

You can see component in your sentence 没有阳光了(There has been no sunlight) as [situation]了. It denote [situation] has happened. In this component [situation] is 没有阳光 so 没有阳光 has happened.