Plas Dinefwr, including SW Screen wall

Plas Dinefwr, including SW Screen Wall with attached Summer House

This castellated mansion is built with snecked grey-shale facings and pale sandstone dressings with redstone patterning. The symmetrical entrance front has three storeys over a basement, with splayed angle turrets and a massive central porch. The openwork quatrefoil parapet is corbelled, while the turrets are machicolated and crenellated with inset red stonework. The front elevation features five top-storey windows with shouldered and chamfered openings set into segmental outer arches, each with paired casements. The lower floors have pointed relieving arches to 2-light timber cross-windows, and heraldic shields decorate the tympani of the ground-floor openings. Corbelled sills sit above segmental basement openings. Low coped parapets flank a robust Gothic porte cochere with tall chamfered openings; its outer face is tripartite with hoodmoulds and quarter-round shafts, while the inner doorway has a trefoil-headed surround.

An extension from 1896 extends to the left with one storey and an attic, featuring decorative loops to the side elevation and a link corridor. This section has a hipped slate roof with a panelled parapet, five windows with pointed relieving arches, and quatrefoils matching the main building. The modern roof includes a new chimney and dormers.

The west elevation is more elaborate, with fine tall chimney stacks topped by crenellated and corbelled cornices set behind the parapet. It is slightly asymmetrical but retains most of the detailing from the entrance front, with additional openwork corbelled balconies to the first floor on the left and an elaborate two-storey venetian Gothic stone verandah to the centre. The side walls are topped by tall pinnacles and flying buttresses, with an openwork parapet over a 6-light glazed balcony. Red dressings are inset to the quatrefoil tracery. A three-bay arcade at ground level is rib-vaulted with quatrefoil columns and foliage bosses. A tall boundary wall runs from the south-west corner of the house and returns west to enclose the south side of the walled garden.

The interior retains its plan-form including a thick spinal wall and large parts of exceptional late 17th and 18th-century interiors, though some later alterations have been made. The entrance hall features a columned Doric screen and a 19th-century ribbed and bossed ceiling; the right-hand wall was rebuilt with a modern concrete beam. The old dining room to the right has a splendid 17th-century coffered ceiling enriched with low plaster relief mouldings including guilloche, acanthus and egg-and-dart patterns. It retains a dado rail, architraves, and raised fields to door panelling, though the chimney piece has been removed. The old drawing room to the rear displays rich 17th-century plasterwork including a pulvinated frieze bearing rosette bands and a coffered ceiling patterned with a centre oval containing a bay leaf design. The room has lugged architraves and panelling with foliage sprays on plain fields. A 1911 dining room occupies the rear.

A splendid 17th-century openwell timber staircase features some 19th-century additions, an open balustraded handrail, plaster foliage patterns to the soffits, and fine early classical detailing to the strings and cornices. Nineteenth-century rose pendants and finials have been inserted at the newels. A half-landing has a ribbed archway opening to a Gothic balcony with a glazed room and ornamental iron brackets. Nineteenth-century top lighting to the staircase features foliage trails in the coffering.

The upper floors retain good quality 18th-century fittings including panelled dados, lugged architraves, low-relief plaster ceilings, and closets set within angled turrets. The north-east bedroom, reached diagonally through the thick spinal wall, is almost polygonal in shape and is linked en suite along the entrance front to a room overlooking the main porch. This latter room has a very rich coffered ceiling with foliage and guilloche cornice band, and includes a large bed recess. The topmost floor sits beneath an oak trussed roof with morticed principals for the purlins of the former roof with dormers. A remarkable bedroom on this level retains a deep Georgian bed recess with a wide, flattened and keyblocked arch.

The stone-flagged basement contains vaulted cellars, a strong room, and slate shelving to the wine store.