T-plan agricultural range at Gelli-Dywyll Farm (also known as Cwmgelli Farm)

Gelli-Dywyll Farm, also known as Cwmgelli Farm, features a large agricultural building with a T-shaped plan, consisting of a lofted former cow house and stable in the main range oriented north-south, and a barn in the rear wing on the east side. The structure is built of coursed rubble stone, showing traces of limewash on the north-south range, and has a corrugated iron roof. It faces the farmhouse to the west, with openings that have wedge stone lintels.

At the uphill (north) end, the former cow house has three original doorways; the central doorway has a replaced boarded door, while the outer doorways have been replaced with metal-framed windows. Ventilation strips flank the outer doorways, with additional strips for the loft and stone steps at the left end. To the right, at the downhill end, the stable features boarded doors with metal-framed windows to the left side of each doorway, along with two small loft windows beneath the eaves. The uphill gable end includes a loft doorway, and both gable ends have small round pigeon-loft openings beneath the apex. The downhill gable end has an added lean-to that incorporates a kennel. At the rear (east) of the stable, there are external steps leading to a boarded door, with small stable windows on either side. The back of the cow house has an added outshut.

The barn has its threshing bay offset to the east side, featuring one tall ventilation strip on the east side and two on the west side. The entrance on the north side has a full-height opening, while the exit doorway on the south (downhill) side has a shallow segmental head with voussoirs and retains wooden brackets from a former canopy. The east gable end includes ventilation strips and a small round opening beneath the apex for a pigeon loft.

The roof is supported by pegged collar-and-tie-beam trusses, with chamfered beams at the upper level, which was formerly lofted, and sawn joists at the lower level.