Dulasau-isaf has a classic north-west Wales unit-system arrangement. There are two storeyed houses of Snowdonian type set corner-to-corner. The principal house has the characteristic regional plan with large hall and twin outer rooms (parlour and service) on either side of the central entrance. An added rear kitchen wing has an C18th inscription on the fireplace beam: T. P. Esqr. 1735. The secondary and smaller house (now used as a store) has a two-unit plan with projecting end chimney heating the hall. The first floor is now approached by an external stone stair which, with the tall end chimney, gives the house a tower-like appearance. However the dwelling is a diminutive version of a Snowdonian house and the present stair obscures the original plan.
(RFSuggett/RCAHMW/2011).

[Additional:] Tree-ring dating commissioned by North-West Wales tree-ring dating project in partnership with RCAHMW in 2010/11.
Seven timbers from the main east-west range matched together well. Two timbers were felled c1570 and in winter 1587/88, and probably represent stockpiled timbers, whilst four timbers were found to have been felled in winter 1592/93, with a fifth timber having a likely felling date range incorporating this date. It seems most likely therefore that construction of the main range took place in 1593, or within a year or two after this date. Full report available in NMRW. (NJR, 01/07//2011)

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

x. PENMACHNO, Dulasau-isaf (SH 8218 5259) Main Range: Felling dates: Winter 1587/8 and Winter 1592/3
Transverse beams 1592(35C, 38C), 1587(39C), 1569(43?C); Tiebeam 1592(52C); Purlins 1592(14C), 1572(H/S). Site Master 1412-1592 DYLASAU1 (t = 9.2 LYDBURY; 9.0 LNYDLOS2; 8.9 CLNGFRD)
Dulasau-isaf (sometimes Dylasau-isaf) has a classic north-west Wales unit-system arrangement. There are two storeyed houses of Snowdonian type set corner-to-corner. The principal house has the characteristic regional plan with large hall and twin outer rooms (parlour and service) on either side of the central entrance. An added rear kitchen wing has an C18th inscription on the fireplace beam: T. P. Esqr. 1735. The secondary and smaller house (now used as a store) has a two-unit plan with projecting end chimney heating the hall. The first floor is now approached by an external stone stair which, with the tall end chimney, gives the house a tower-like appearance. However the dwelling is a diminutive version of a Snowdonian house and the present stair obscures the original plan.

Tree-ring dating showed that the principal house was built towards the end of the C16th, but the added C18th kitchen wing failed to date. Further sampling of the secondary house (considered by the Caernarvonshire Inventory to be the older house) is needed. Plan and account in RCAHMW, Caernarvonshire Inventory, Volume I: East (1956), pp. 172-3. ). Dating commissioned by NWWDP in partnership with RCAHMW. (RF SUGGETT/RCAHMW/JULY 2011)