Dyffryn Mymbyr is a small storeyed gentry house built of rubble on a part-boulder plinth and has a renewed small-slate roof with rubble gable parapets and a squat nineteenth-century end chimney to the right. There is a two-window front with off-centre entrance, which has an original cyclopean lintel with a Tudor-arched opening. In the former hall there is a very wide fireplace with a massive flat bressummer and a crisp framed ceiling with deeply-chamfered cross-beams and stopped-chamfered joists, although a few have been replaced. There is mortising evidence for a former post-and-panel screen to left of the entrance and this survives in part, re-located on the first floor. There is no visible evidence for an opposing entry (forming a cross-passage), though this was probably destroyed when the rear extension was put on. There is a modern stair to the first floor. This has modern partitioning, although the trusses are visible. It is 3-bay with original pegged collars and queen struts, with post and panel partition to the right bay, re-located, apparently from the ground floor and an ogee-headed doorway to the centre.

Noted with plan in RCAHMW Caernarvonshire 1: East, monument 334. Additional note in Caernarvonshire III, page 112a, drawing attention to ogee-doorhead.

Tree-ring dating commissioned by North-West Wales tree-ring dating project in partnership with RCAHMW in 2011.
Two timbers (collar and ceiling beam) retained complete sapwood, and the felling date ranges for these two timbers are therefore given as 1553?55, and the other timbers have likely felling date ranges that would seem to be in agreement with these, making the most likely date of construction of this house 1553?55. Full report available in NMRW. (NJR, 07/04/2011)

[Additional:] Tree-ring dating and site description reported in Vernacular Architecture 42 (2011):

x. LLANDEGAI, Dyffryn Mymbyr (SH 695 573) Felling date range: 1553-55
Principal rafter 1521(h/s); Collar 1522(h/s+31NM); Ceiling beam 1526(15+26NM); Purlins 1513+5NM(h/s), 1523(3+18NM), 1525(8+19NM), 1531(7+2NM). Site Master 1383-1531 DYFMYM (t = 6.6 BDGLRT17; 6.1 STUBLEY; 5.9 PENGWERN).
Dyffryn Mymbyr is a small stone-built storeyed house of Snowdanian plan-type. The S. doorway retains a cyclopean three-centred lintel. The hall has a wide fireplace with a massive flat bressummer and a framed ceiling of deeply-chamfered beams. The partition beam at the former cross-passage has mortices for a post-and- panel partition, apparently partly relocated to the first floor with the double-ogee doorhead. The collar-beam trusses have raking struts. The mid-sixteenth-century date is consistent with the absence of a fireplace stair and the presence of a double-ogee doorhead. Noted with plan in RCAHMW, Caernarvonshire Inventory, Volume I: East (1956), fig. 105 (monument 334), where an early-seventeenth-century date is conjectured. Dating commissioned by NWWDP in partnership with RCAHMW. (R.F. SUGGETT/RCAHMW/JULY 2011)