Machen House including attached outbuildings and curved screen wall

Machen House is a small country house of two storeys, rendered in painted stucco with close-eaved slate roofs and stuccoed rebuilt chimneys—one at each end, one on the front ridge and one on the east end of the rear range.

The garden front faces south and comprises four bays. A full-height canted bay on the left has a three-sided hipped roof and contains three 12-pane sash windows on each floor, fitted with blind boxes. The three-window main range to the right has 12-pane sashes above and two broad French windows with external shutters replacing similar sashes on the ground floor, also with blind boxes.

The east entrance front displays a two-gable facade, formerly of three gables. The left gable is blank; the right gable was originally central before removal of a gable to its right in the mid to later 20th century. A stuccoed square Tudor porch with battlements, angle buttresses and chamfered depressed arches on all three sides (lower on north and south) contains a moulded pointed arched doorway with panelled doors. Small pointed windows with Gothic glazing bars flank the porch interior, and above it on the first floor is a large Gothic sash window with a pointed hoodmould.

The rear was remodelled after removal of a parallel service range. It is now parallel to the three-bay main part of the front range, with an arched first-floor landing light re-using a fanlight from the old back door and a square lead-roofed lantern-light on the ridge. The rear of the west end section, without the parallel range, has one small first-floor window; the west end is windowless. A curving castellated stuccoed screen wall runs from the northeast corner and has been extended across the site of the demolished rear range, incorporating a broad vehicular opening. It originally linked the house to the churchyard wall, screening the outbuildings from the forecourt.

An attached outbuilding to the rear, a former larder, is linked to the house by 20th-century garages. It is L-plan and single-storey with a slate roof hipped to the east and at the northwest corner and gabled to the south. The walls have rounded corners, and the eaves feature a band of cast-iron pierced ventilation panels. Two pointed doorways facing south into the re-entrant angle have panelled doors, and a pointed window facing east has Y-tracery. There is a plain casement pair window on the north side right and west side left.

The front entrance hall contains Tudor Gothic decoration and is divided axially by moulded cast-iron piers carrying three Tudor arches with quatrefoils in the spandrels. The broader centre arch frames an exceptional spiral timber staircase, cantilevered from a central column said to have been a ship's mast. The staircase has pierced Gothic spandrels under each tread, thin octagonal balusters and a remarkable continuous handrail that snakes upward in spiral before looping back around the upper landing. A pointed recess is situated behind. The top lights of the front door contain armorial stained glass with the CASM monogramme and dated 1831, remade to the original model in the later 20th century, and a window to the right has armorial glass of circa 1831 in its head.

Panelled shutters and reveals appear throughout the interior, with deep skirtings and square-headed six-panel mahogany doors. The first two front rooms feature later 18th-century Adam-style timber chimney-pieces removed from Ruperra Castle after the fire of 1941, dating from the refit by T. Hardwick following the fire of 1785. The southeast morning-room has two original Gothic bookcases with details matching the staircase, although their cornices are missing, flanking a chimney-piece from Ruperra. The centre dining-room has a deep Adam-style frieze, restored cornice and floral ceiling border, and a chimney-piece from Ruperra. The long transverse southwest drawing-room features a fine late 18th-century timber chimney-piece said to have been brought by Lord Thorneycroft from his family home in Tamworth, Staffordshire. This room displays particularly fine plasterwork, with an enriched cornice bearing paterae and a ceiling in diamond and square panels. Elliptical arched recesses appear on the north and west walls, and two mahogany six-panel doors are on the east wall. The first floor has six-panel doors in plain reveals. From the landing is a small section of the former stair to the service range, fitted with a similar balustrade to the main stair.

The former larder to the rear has a plastered vaulted roof, slate slab shelves and a flagstone floor.