Plas Isaf, Llansanffraid Glan Conwy, is an important site, in the ownership of the Williams family (of Cochwillan) and probably the birthplace of John Williams (1582-1660), Archbishop of York and Lord Keeper under Charles I. Of the early house, a sub-Medieval parlour wing of the third-quarter 16th century, and an early 17th century stair block addition survive, though the (presumably Medieval) hall was gone by c.1800 and replaced by a low bakehouse/brewhouse to the north-east; the house appears to have been subsidiary to the (later) Plas Isa farmhouse since at least the turn of the 19th century and was used for storage and services.

It is a T-shaped complex of three main phases of construction. Built of random rubble with slate roofs and stone-coped and with kneelered gable parapets to the north-west and south-west gables. The parlour wing is central and is rendered externally (except for the farmyard-facing gable end). There is a lateral chimney to the south-west face with rebuilt upper courses in old brick.

Internally, there is a stone fireplace to the ground-floor parlour, with semi-octagonal slate-stone shafts supporting rounded corbels. The fine ceiling is framed in four stages, its main and subsidiary beams broach stopped and its joists finely stopped-chamfered. There is a post-and-panel partition screen with a (restored) shuttered serving hatch and a former service bay beyond with stopped chamfered ceiling beams and plain joists. There is a pegged oak, chamfered doorcase through to staircase range. The former solar has one arched-braced collar truss with cusped diagonal struts, together with a contemporary partition truss; there is a blocked original 4-light wooden mullioned window (visible from the inside). 3 brick ovens with cambered heads are in the wash/bakehouse with a further fireplace opening as before.