Overshadowing is listed as a negative impact in Section 14.10.2.4, First Floor Extensions. It should also be listed as a possible negative impact for all ground floor extensions.
Consider a typical modern pair of semi-detached houses with an east-west axis, where each house has a kitchen occupying half the width of the rear of the house. A ground floor extension to the house on the south side should not be allowed to overshadow the house on the north side.
See PDF for further discussion.