In the late seventeenth and early eighteenth centuries shell hoods were a fashionable enhancement of a main doorway. They were not used on the biggest houses but did find favour on smaller town houses and those where the main door was on or adjacent to a street. They were seen as giving stature to the house by drawing the eye to the entrance. The example above, at Castle Hill House in Monmouth, is on the street that leads to the grandest house in the town which is next to the remains of the medieval castle. The "shell", very typically, rests on console brackets above the transom light and the door. These hoods can never have provided much protection against rain: ornamentation was their main purpose.
photo © T. Boughen Camera: Lumix FZ1000 2