Gate-house, with gate-piers and retaining wall to courtyard, Plas Rhianfa

The gatehouse at Plas Rhianfa is designed in the French Gothic style and is constructed from Penmon limestone rubble with freestone dressings, topped by a fish-scale slate roof. It is two storeys high from the courtyard level but slopes down to the garden level below. The building has a square plan and features details that echo the main house, including miniature turrets at each corner on the first floor.

The west-facing elevation includes an archway leading to the stable yard on the right, with a triangular oriel window above that cuts through the eaves. To the left, there is a porch that projects as a square turret. The north (roadside) elevation has a narrow window, while the south side features paired narrow lights. Dormers with traceried lights, nail-head decoration, and cresting are present in the roof on both the north and south sides. A later 20th-century flat-roofed extension on the east side disrupts the original design.

Adjacent to the gatehouse on the north side are a pair of square gate-piers topped with tall pyramidal copings. The gatehouse is connected to the main house by a castellated wall, which also serves as a retaining wall for the courtyard and includes a staircase that descends through it.