Plas Isaf formerly occupied a site at the north end of Poplar Grove/Plas Isaf, on the opposite site of Station Road from the North Llanrwst Railway Station (NPRN 96158). It is best known as the home of the humanist scholar and translator William Salesbury (before 1520?c.1584), most famous for his 1567 translation of the prayer book and New Testament (with Richard Davies and Thomas Huet), who moved to the house from Cae Du, Llansannan, (NPRN 26904) prior to 1540, Plas Isaf having been the residence of his father and brother.

The house likely dated from at least the mid- to late-fifteenth century as it was acquired by William's grandfather, Thomas Salusbury Hen, who reportedly died in the Battle of Barnet in 1471 (although he possibly lived until 1490). The house was demolished early in the twentieth century, possibly around 1910, and late-nineteenth- and early-twentieth-century commentators notes the dereliction into which the house has fallen. A drawing which appeared in Cymru, vol. 10 (1896), p. 116, shows a building constructed of rubble stone with a tall chimney projecting prominently from the main structure. The building was unroofed, with beams and rafters visible, and there was a gabled cross wing to the rear. A contemporary photograph which appeared in W. Bezant Lowe's The Heart of Northern Wales (p. 293, fig. 152), shows much the same view although the building is heavily covered with ivy, obstructing most of its surface.

An early-twentieth-century painting by Nancy Thomas shows largely the same view but includes several more modern elevations to the rear of this structure. From the plan of the building given on the 1st and 2nd edn 25? O.S. Maps, the derelict structure depicted prominently in these drawings was the westernmost elevation of a larger structure with more modern elevations to the east. One of these was likely shown in an early-twentieth-century postcard depicting `The Home of ?William Salesbury??. This was a two-storey rendered structure with a slate roof, sash windows and a central gabled porch.

(Sources: NMRW Archive, Site File, Denbighshire/Domestic/SH76SE; Dictionary of Welsh Biography, s.v. Salisbury, William; Oxford Dictionary of National Biography, s.v. Salisbury, William; Ibid., s.v. Salusbury [Salesbury] family; Nancy Thomas, `Plas Isaf, Llanrwst?, oil on canvas, early C20, National Library of Wales; `Plas Isaf Llanrwst? (Graphic), Cymru, 10 (1896), 116; Welsh Newspapers Online: `Hen Gartref Wm. Salisbury?, Y Cymro, 22.09.1898, 5; `News and Observations?, Aberystwyth Observer, 27.10.1898, 2; [No Title], North Wales Express, 18.10.1898, 5)
A.N. Coward, RCAHMW, 15.05.2019