T-Shaped Range to rear of Pengwern Hall (including Vaulted Undercroft)

This building, a T-shaped range, stands to the rear of Pengwern Hall and incorporates a vaulted undercroft. The core of the range dates to the 13th and 14th centuries, connecting the main house to a parallel agricultural range likely built in the 17th century. It is speculated that the medieval section may have been constructed by Lord Iorwerth of Pengwern in the early 13th century, potentially during his tenure as Seneschal to Prince Madog, with subsequent alterations probably occurring around the 17th century. The agricultural range is notable for the incorporation of reused Gothic features, likely from Valle Crucis Abbey, which were probably inserted during the remodelling of approximately 1770 by Sir Roger Mostyn.

The building is constructed of rubble with undulating slate roofs. A red brick chimney stack is visible on the medieval portion, with some of the northwest side hidden by the house's kitchen range. A brick staircase with stone treads leads to a boarded door with a chamfered freestone surround. The southeast side, overlooking the garden, features two narrow splayed openings and a larger former doorway, buttressed with brick on the right; a staircase has been removed. A small casement window is positioned below to the left. The agricultural range is situated at a right angle, its garden-facing front having freestone surrounds to all windows; the first-floor windows have sub-medieval mullions removed. The gable end, with a stone parapet, has a 3-light window inserted into a smaller opening, featuring cusped ogee lights and partial replacement of one mullion in timber. One stone is initialed "R C D." The opposite side of the cross range has a 2-bay rubble front incorporating diamond-shaped ventilators and brick cambered arches over stable doorways and windows. The rear elevation has a cusped lancet window adjacent to a first-floor boarded door; further to the northwest is a 2-light sub-medieval window with mullions removed, with later brick openings positioned below. A long barn range steps down to the northwest. Stone gate piers mark the southeast corner.

Inside the cross range is a 9-bay barrel-vaulted undercroft with pointed and chamfered stone ribs, a stone-flagged floor, and stop-chamfered jambs to the doorway. Springers above suggest a former, now removed, vault, and the interior is now supported by trenched purlin trusses. The other range has a 6-bay roof with overlapping purlins.