Thornston is situated at the southern edge of Bosherston, on the east side of the minor road to St. Govan's Head. In the 1970s, when it was visited, it still took the form of an 18th century, 2 storey building, built of rubble with a south wing. It had recessed casements, glazing bars, a plain central entrance and has a modern porch on the north-west side. The main feature of the house was an extremely large, square chimney, of which there were once a number of examples in Pembrokeshire. Central to this was a very large oven extension, with a slate conical roof. Thornston appears to have been extensively modernised, as the 2000 OS Landline 1:2500 map indicates that the site plan is now a regular rectangle, as opposed to the irregular shape indicated by drawings of the building and the previous OS 25" to the mile maps of 1908 and previously.

(Sources: P Smith, Houses of the Welsh Countryside, 1988, p. 21, 286).
Ian Archer, RCAHMW, 23rd March 2005