The current farmhouse is a traditional c.  early 17th century Welsh longhouse. Cefnmachllys appears in the written record in 1618 when a Jevan Thomas resided here. In 1629 he was  a churchwarden at Talachddu Church.  

It is possible that an earlier building once stood here - longhouses were known for periods of alternate rebuilding starting their life as timber-frame structures - the house was often re-built in stone, the byre remaining timber-frame and later the byre re-built in stone.  Indeed at Cefnmachllys the byre is later, c.1686 and the stonework not tied into the house - a projecting dripstone on the house within the byre loft suggests the original byre had a lower roofline.  

Positioned as it is below Llandefalle Hill and to the north-east of what was Talachddu Common, its location is a characteristic of the Welsh longhouse where cattle were overwintered and kept safe in the fold yard and byre attached to the house and grazed in the summer months on the higher ground of the hills and commons. This settlement pattern is testament to the site’s likely early medieval origins.

The longhouse plan-form seems to appear in Breconshire from about 1575. These longhouses were the dwellings of relatively prosperous pastoral farmers. They were built from the profits of the cattle-rearing economy which took advantage of extensive upland grazing. Where there was cattle raising, so there was cattle theft. Cattle theft was a hanging felony. Cattle theft increased during the course of the 16th century before it declined in the 17th century. The longhouse, especially the strong stone-built longhouse, was not only a convenience for over-wintering prime cattle but a prudent architectural response to cattle theft. 

The ground floor of Cefnmachllys comprised hall and possibly service rooms such as the buttery and pantry which were later made into a parlour: the essential addition to the 17th century vernacular house for anyone of status.

On entering the house from the byre the ‘meal house’ was contained in the passage and to the left was the hall with doorway to the side of the inglenook, to the right was the byre for tethering cattle with hay store above and an unheated farm labourer sleeping chamber. Only later in the building’s history was the front door inserted where it is today in what was a window opening for an oak diamond mullion window. The lean-to at the rear containing the dairy and scullery was a 19th century addition.