Plas Tan-y-Bwlch including attached terrace walls to east and west

Plas Tan-y-Bwlch is an 18th-19th century mansion built in a castellated Gothic style. It is constructed from local stone with sandstone dressings and granite quoins, and has slate roofs behind embattled parapets. Tall stone stacks, featuring paired shafts, moulded drip-stones and cappings, are present throughout. The house is situated on a lofty site, with the main entrance on the east side elevation, opening onto a long south elevation designed for reception rooms. Service rooms, including the kitchen, are located at the rear.

The main south-facing range appears to have been built entirely in the late 19th century and is three stories high. The principal elevation has a prominent tower-like block to the right, incorporating a large full-height canted bay, and a symmetrical three-window range with an advanced stepped-gabled central bay. Mullioned and transomed windows, containing three and four lights, are prevalent throughout this range, with hood-moulds forming continuous string courses on the right-hand side. The southeast wing is of earlier character, and probably represents the core of the 18th century house. It serves as the entrance range, consisting of a three-story, three-window façade, with a heavy projecting porch featuring buttresses, an embattled parapet, and a moulded archway. Windows are hornless 12-pane sashes (with 9-pane in the attic storey), and string courses are stepped to form their hood-moulds. A grand gateway, dating from the later 19th century, is attached to the right side of this wing, leading to the rear service yard. It has an arched entrance with a faux portcullis and an embattled parapet stepped over a central gable.

A further range, added in the late 19th century, is located west of the main range and set back from it. This single-story range, built over a basement, features an embattled parapet, gable detail, and mullioned and transomed windows, including two projecting bays.

At the rear, parallel to the southeast wing, is another wing that also appears to be part of an 18th or early 19th century construction. This wing is three stories high and has sash windows similar to those in the earlier section. Further service wings from the late 19th century are situated alongside, to the rear of the main range's tower. These are single-storied with steep roofs; the wing to the east has lantern glazing. A small enclosed yard is attached to their west.

The house is closely integrated with its gardens on its steeply sloping site. Terrace walls continue the lines of the house to the east and west. To the west, a long section of rubble wall with raking ashlar coping curves around to the western entrance of the stable yard, topped with piers featuring ball finials and ornamental iron railing.

The internal layout reflects the late 19th century remodelling, creating a series of reception rooms arranged to the south of a central spinal corridor. A grand staircase is located to the north, alongside the service accommodation. The room layout remains largely intact, and there is some good surviving detail, including a series of fireplaces with elaborate overmantels.