Plas Mawr is an Elizabethan house comprising a gatehouse facing High Street, behind which is a lower courtyard and steps up to the main house, which originally faced Crown Lane. Rubble-stone walls are lime rendered, although freestone dressings remain exposed, and steep slate roofs are on a cornice and corbel table. Consistent use is made of distinctive crow-stepped gables on moulded kneelers, incorporating diagonally-set apex pinnacles. Windows have stone mullions and transoms, many of them renewed, with diamond-leaded glazing. Doorways mostly feature continuous mouldings and hood moulds.
The gatehouse, which is distinguished from the earlier main house by its marked Renaissance influence, is 2 storeys and attic and has a 3-bay symmetrical front to High Street. The central projecting porch has a higher eaves line than the main gatehouse. It has rebuilt stone steps to an entrance with freestone surround. This has an elliptical arch with keystone, spandrels with shields bearing raised eagle and stag's heads. Iron gates are C20. Above is a tablet with the Garter arms held aloft by a lion and dragon (symbolising England and Wales). It is framed by Tuscan pilasters and pediment, the cornice of which is carried over the crown of the Royal Garter. Inside the porch is an elliptical arch with 2 orders of chamfer, capitals with foliage spandrels and faceted keystone and capitals. The pediment is inscribed, some of it weathered, with Robert Wynn's motto ('bear: forbear') in Greek and Latin. The studded boarded door has strap hinges, with secondary door cut in between the hinges. Side walls of the porch have transomed windows. In the 1st floor is a pedimented 3-light transomed window, and transomed side windows. The attic also has a 3-light transomed window, and single-light windows in the side walls. Outer bays have pedimented 3-light transomed windows, shorter to the 1st floor. The attic has crow-stepped dormers treated in a similar way to the porch, with 3-light windows, above which is a string course to the gable.
In the R gable end, facing Crown Lane, are pedimented 3-light windows similar to the front. The attic has a string course below a shoulder-headed doorway boarded up, beneath a pediment. The rear is dominated by external lateral stacks with gables and tall shafts. In the central bay, above the elliptical passage arch, is a 1st-floor cross window. On the R side, where there is a raised terrace, is a 1st-floor entrance inserted in the C18, with studded door under a timber lintel.
The main house faces Crown Lane, a sloping site that incorporates a basement in the bay (S wing) at the downhill end. It is 2 storeys with attic, and has a near symmetrical 5-bay front. End bays (the gable ends of the N and S wings) are brought forward, and the central bay has an C18 porch with higher eaves line than the main house. The porch is whitened rubble-stone in the lower storey. It has a segmental arch and steel gates. Inside, the original entrance has an elliptical arch with continuous moulded chamfer, foliage spandrels and weathered moulded cornice with central heraldic shield. It has a studded door with secondary door cut in between strap hinges. In its L side wall is a small window. Above ground floor the porch is timber-framed in large panels. The 1st floor has paired wooden cross windows with single cross windows in the side walls, and the attic has a similar arrangement of shorter 2-light windows. The L-hand bay, at the downhill end, has a 6-light transomed window to the ground floor, with small transomed window in the R-hand return, and 2-light 1st-floor window. The attic has a round corbelled and transomed oriel window. In the basement is a segmental-headed doorway to a studded door with strap hinges. It is flanked by 3-light mullioned windows with iron bars, in weathered moulded surrounds. In the main range the bays flanking the porch have 4-light transomed windows in ground and 1st floors, and 2-light casement windows in gabled timber-framed dormers. The (earlier) R-hand bay has a corbelled and pedimented 6-light transomed oriel window to the parlour. The pediment incorporates a raised shield, relief foliage and inscription 'RWdG' (Robert Wynn and his wife Dorothy Griffith). The R-hand return has a transomed window, the L return a pedimented 2-light oriel. The 1st floor has a similar 6-light corbelled and pedimented oriel window, the pediment of which is dated 1576. The L-hand return has a 2-light oriel and the R-hand a cross window under a weathered cornice. A round corbelled oriel is similar to the attic window in the L-hand bay.
The SE elevation of the S wing, facing the lower courtyard, was built in 1580 but was adapted to become the main entrance in 1585. It is 2 storeys and 3 bays, with 4-light ovolo-moulded transomed windows. Upper R is an additional small 2-light corbelled oriel window. The inserted entrance is L of centre. It has a 4-centred arch with shields in the spandrels bearing the monogram RW in raised letters. The hood mould has blank shields above label stops. The hood is carried over a renewed tablet with shield bearing the date 1585 in raised numerals.
The rear of the main house faces the upper courtyard. It has doorways with basket arches and boarded doors with strap hinges. The 2-window main range has a central entrance and cross windows. A central lateral stack has a tall shaft over a gable. To the R and L are polygonal stair turrets in the angles with the N and S wings. The R-hand (S) is higher, under a pyramidal roof with weathervane. In the ground floor is a doorway. Above are 3 tiers of stepped stair lights, and similar lights to a chamber at the top. The L-hand (N) turret also has a pyramidal roof and pinnacle. It has 2 tiers of 3 stepped stair lights, and lights around the upper landing.
The upper-courtyard elevation of the S wing has a doorway on the R. On the L side is a doorway with shields to the spandrels under a square hood mould. A single cross window is in each storey, including a gabled dormer. The upper-courtyard elevation of the N wing has an entrance on the R side. Windows are 4-light transomed, of which the upper storey has RW in shields over the label stops. Upper L there is also a small window under a hood mould, and 1576 in shields over the label stops. The attic has a gabled dormer with cross window.
The NW elevation of the N wing now faces a gallery. On the L side is an external parlour stack with tall stone shaft over a freestone moulded cornice. The attic has a central gabled dormer, offset L of centre, with cross window and string course above it.
Between gate house and main block is the lower courtyard. Facing Crown Lane is a whitened rubble-stone courtyard wall, which has a stepped freestone coping. It has a segmental-headed entrance with restored studded door. It is flanked by segmental-headed openings with wooden latticework, behind which is studded boarding. The NW side of the courtyard has a raised terrace in front of the main house, of rendered rubble stone and corbelled parapet. In the centre of the wall is a weathered stone tablet. Quarter-turn stone steps lead up from the lower level, with flanking coped walls. To the L of the steps is a tunnel-vaulted recess to a 3-light basement window with unmoulded mullions. An C18 terrace on the SW side of whitened rubble stone has 3 tunnel-vaulted recesses, and a parapet with weathered stone coping.