Plas Llwynonn including coach house and service range to rear.

Plas Llwynonn is a large, 18th-century gentry house of Neo-Tudor style, accompanied by a coach house and service range at the rear. The main house is constructed of granite rubble with limestone dressings, and has a roof of ornamental 'fishscale' slates, featuring sawtooth ridge tiles, stone copings, kneelers, and finials. Rectangular stone stacks are present. A single-storey wing with an attic extends to the rear, continuing as a lofted coach house and tackroom. Stables and a gate lodge were built parallel to the house across a yard, incorporating a stone archway and enclosing walls. Rear wings of the main house have been altered to form entrances to individual dwellings, linked by lean-to extensions.

The main L-plan block is two storeys with attics, with the entrance on the right (east) gable return. A projecting gable of two storeys with an attic is positioned to the left of centre, containing an entrance porch with a pointed, chamfered arch. A 4-light chamfered mullioned window is on the first floor and a lancet window on the attic floor. To the left of the porch is an offset chimney breast, and to the right of the porch is a ground floor cross window and a 2-light mullioned window on the first floor. A 2-storey bay window with 4-light chamfered, transomed, and mullioned windows is at the right end, surmounted by a raking dormer window of 3-lights. A central stone stack is also present.

The south-facing principal elevation features three gables, with paired gables to the left slightly advanced. Each advanced gable has a 2-storey bay window with 3-light chamfered, transomed, and mullioned windows, hood bands, sill strings, and parapets with ball finials. Between the twin gables is a ground floor 3-light transomed and mullioned window with a hood mould, a 2-light window to the first floor, and 3-light mullioned windows with hood moulds in the attic. The right-hand gable is similarly fenestrated. Three rectangular stacks are positioned, one to each end and one to the right of the advanced bays.

To the left of the main block is a 2-storey, L-plan range, previously used as servant's quarters, built of similar materials, including a gabled bay with an attic to the left. A 3-light chamfered mullioned window is on the ground and first floors, with a continuous hood string on the ground floor; the attic floor has a blind slit window. A two-window range connecting the main block to the right has 2-light chamfered mullioned windows and a door on the right. A lofted domestic range and coach house are positioned to the rear, featuring three brick cambered arches to the right, a slate-roofed verandah supported on tapering columns, and a tackroom to the right end. This tackroom has a single storey with an embattled parapet, a through passage with pointed arched openings, and a single 16-pane sash window. Set at right angles to the rear of the main house and the domestic wing are five single-storey wings with a mixture of pointed and elliptical arches to doorheads, with plain hoodmoulds.