Farm buildings of Model Farm built c.1816-17 by Evan Thomas of Sully House with some later 19th century enlargements. A model farm of this size reflects the agricultural prosperity of the region in the 19th century and the associated group of rickstands indicates the volume of corn grown. The layout is designed for the most efficient distribution of feed from the barn to the animal sheds, and the yards are not enclosed in the Scottish manner but are open like in East Anglia.

Evan Thomas, son of Henry Thomas of the Llwynmadog Estate in Brecknockshire, was the estate farmer from 1811. He reduced the existing 5 farms and 4 tenements to 2 large farms, Cogan and Hayes and from 1814-17 farmed the whole parish and paid all tithes and rates. Evan Thomas died 1832 and JJ Guest, who became owner of Sully Estate and instituted further refurbishments, was related to Thomas' widow and proprietor of the Dowlais Ironworks - the source of ironwork of the roof including iron pegs. In a report on the state of agriculture in Glamorgan by Joseph Darby of 1885, Cog is described as a farm of 700 acres (283.5 hectares) of which 550 (222.8) were arable, the land at that time managed on 4 and 5 course rotation, with an evidently large proportion of arable to grass. Most units were converted into separate dwellings late 20th century. Before conversion, some food processing machinery survived, together with water tank in tower; the hammels had feed passages along the back walls and the rear barn had a stone tile roof.

Farm buildings of rubble stone construction, some limewashed and with slate roofs, some with corrugated roof replacements in the unconverted sections. The 2 yards are divided by a central north/south barn and the transverse buildings fronting the yards are divided into twinned hammels with cambered-arched openings and with a loft above to right range. The left hand yard has a long stable range at right angles closing the western side of the farm; this has been extended forward in later 19th century and includes a 2-storey gabled central bay with dovecote. Another addition is the 3-storey red sandstone gabled 'tower' block to front attached to the gable end of the central barn; this may have been an engine house with the more substantial construction being due to the housing of machinery for animal feed processing.

The right hand yard was formerly covered in by a modern shed and the hammel arches are partly blocked; there are loft openings with sandstone voussoirs above. A cartshed and granary close the eastern side of the farm with cambered-arched cart openings alongthe east side, with a loft above and loft steps at rear. At the rear is a further barn added in line to the main barn with access from the front courtyard created through hammel range and cart entry on the gable end; a later addition to west side, the 2 now forming parallel gabled wings.

The most significant interiors are those to the rear barn and the stable range both of which retain early cast-iron roofs. These are arched-braced trusses with diagonal struts and central vertical members that join the 2 triangular parts of each truss together. The rear barn was lofted, the later barn having simple timber trusses, the 'tower' A-frame trusses and a roof with staggered purlins to the cross range.
(Sourc; Cadw listing database) S Fielding RCAHMW 14/10/2005