H M Prison Dartmoor: Polygonal buildings, former officers' mess and former steward's house, now admin building and visits building

The former officers' mess and former steward's house, now an administrative building and visits building, are two polygonal buildings dating from the initial construction of H M Prison Dartmoor between 1806 and 1809, designed by Daniel Alexander. These buildings have been subsequently altered and extended. A significantly altered mid-19th century block now stands to the north of the northern building, and a 1909 block is attached to the south of the southern building.

The buildings are constructed of coursed granite rubble with replacement ashlar window surrounds and replacement window frames. The roofs are metal, with pyramidal shapes over the polygonal sections, featuring finials.

The polygonal buildings are positioned to the north and south of the former marketplace entrance. Their polygonal ends face east, overlooking the former marketplace, while the buildings extend westwards to squared west ends. Both buildings have later additions: a large block, originally an officers' mess, is attached to the south of the southern building, and a significantly altered block stands to the north of the northern building.

The buildings sit on granite plinths. The polygonal sections feature large, triangular-headed windows on the first floor, overlooking the former marketplace, likely reflecting their original design. Other windows are smaller. The southern building has entrances from the northeast and south. The northern building has blocked openings on its south-facing elevation, with a new secure entrance incorporated into a lean-to addition against the west wall of the former steward's house. A late 20th-century extension projects westward from the north end of this building. The former officers' mess, to the south, is a two-storey building with four pairs of basket-arched windows with granite surrounds, and a central chimney.

The interior of the northern building has been opened up, removing internal divisions between the polygonal section and the northern range. It is believed few historic features remain, although the ends of roof timbers are visible in the polygonal section. The interior of the southern building was not inspected.