Terrace, Wall, Grotto and Underground Garden about 5m to north west of Dewstow House

To the west of Dewstow House, there is a formal terrace featuring stone walls on its west, north, and east sides. The north wall includes blank arcading and a circular stone alcove at its east end. The low west wall has a circular turret with an alcove at its south end. In the southeast corner of the terrace, next to the west gable of the house, is a formal balustraded entrance to the underground gardens. This entrance is flanked by curved wing walls topped with ball finials and leads to steps down to a flat-topped entrance with a central Tuscan column. The only visible part of the gardens from above ground is the glazed roof located behind the aforementioned wall.

Upon entering, a narrow passage lined with artificial rockwork leads to a large chamber situated to the north of the terrace. Halfway along this passage, a branch passage leads to a blocked entrance flanked by Tuscan columns, which originally provided access to another chamber (now containing a swimming pool) and further to the basement of the house. At the entrance to the chamber, there is a Gothic doorway with the original oak door. The underground garden is illuminated from above through an iron framework disguised as rockwork, which supports glazing that is now significantly damaged. The garden features extensive artificial rockwork designed to resemble a limestone cave, complete with pillars, stalactites, and various pools at ground level housing goldfish, as well as tiered pools and numerous niches for growing ferns. Some formal elements, such as balustrading, are present, and the supporting walls on the north and south sides are made of coursed, undisguised concrete blocks. Originally, there were fountains and cascades flowing from pool to pool.

At the northwest end of the chamber, a passage leads to steps up to an arch that opens into an originally roofed tunnel. At the end of this tunnel, there are two blocked passages: one leading to the chamber to the west and the other to a demolished chamber to the east.