Hen Dyffryn Gwyn

Hen Dyffryn Gwyn is a significantly altered, 17th-century farmhouse, with later additions and alterations. The main house is a 1½-storey, three-window building constructed of roughly dressed rubble stone with larger quoins and boulder footings, now covered by a steep slate roof (replacing original thatch). A moulded stone eaves cornice runs along the top of the walls, and the left-hand gable has a coped verge. Original stone end stacks are present where one stack on the left-hand side is original. The entrance is positioned to the right of centre, featuring a round-headed arch with thin voussoirs leading to double boarded doors. Lower-storey windows have drip moulds. To the left of the entrance is a three-light mullioned window, above which is the inscription 'HP 1640'. Other windows are replacements within finely moulded surrounds, including a six-pane sash window to the left and a twelve-pane sash window to the right. Three hipped half-dormers are present, each with a four-pane horned sash window, incorporating re-used mouldings in their jambs.

A shallow rectangular bake-oven projection is located in the left-hand gable end, which also displays prominent boulder footings. An attic window to the right is set within a stone surround with a moulded lintel, likely re-used, while an inserted attic window is to the left. One of the quoins on the left-hand side bears the inscription 'Rice Hughes hoc fecit anno domini 1772'. On the right-hand side, the verge has the inscription 'EE mason 1893'. A one-storey projection, with a flat roof, extends continuously from the left-hand gable end, connecting to a two-storey former outbuilding—probably originally a brewhouse and farm labourers’ accommodation—constructed of rubble stone with a slate roof and featuring two-pane sash windows on each storey.

The rear of the main range has two hipped half-dormers with four-pane sash windows, and an inserted window below the eaves in the centre that provides light to a staircase added in the 18th century. The lower storey retains two original window openings with corbelled drip moulds; the right-hand one is a two-light mullioned window, while the left-hand one has been replaced by a narrower sixteen-pane sash window.

The entrance opens into a stair hall, containing a straight staircase. To the right and left are timber-framed partitions with reed-moulded studs, mirroring the detail on the inner sides of the door surrounds. Both screens appear original, although the left-hand screen has been partly removed and incorporates re-used materials. A fielded-panel door on the right leads to the parlour, while a replacement door on the left provides access to the kitchen. The kitchen features a central cross beam with ogee stops and reed-moulded joists. Its fireplace has a stop-chamfered timber lintel on coursed stone jambs; a shallow niche is cut into the right-hand jamb. To the right of the fireplace is the original stone fireplace stair. The parlour's fireplace has been filled, but a window seat with fielded-panel backing remains in the rear wall. Mullioned windows have ovolo internal mouldings.

In the upper storey, the room above the kitchen has an ogee door head and a post-and-panel partition with reed-moulded posts. The room retains an infilled fireplace, but its corbels and timber bressumer are still visible. Other first-floor partitions incorporate ovolo-mouldings and are likely from the 18th century. The roof structure includes two collar-beam trusses.