Plas Newydd is an early seventeenth century mansion. Subsumed into Glynllifon Park (NPRN 86377) in the 1830s, it was derelict by the end of the century. It was largely restored in about 1900, when the garden enclosure and service range (NPRN 31431) were constructed. The house would presumably have been a subsidiary dwelling in the park, or else a very elaborate summer house.
A three storey house with attics and a cellar. The walls are of stone rubble, sometimes rendered, under a modern slate gabled roof. The transomed and mullioned windows are largely original.

The house faced north-east where is a two storey porch and a massive projecting chimney stack. The doorway gave onto a passage between the hall and service rooms or parlours, with a full height staircase wing opening off the far end. This may have been added. The first floor would have contained the Great Chamber and retains some heavily restored original panelling, as does the upper storey, although this might be re-set. There are graven and moulded armorial bearing over the first and second floor fireplaces, dated 1637 and 1632 respectively.

Between 1889 & 1900 a portico and projecting stair was added to the stair wing, depicted on the OS County series 2nd edition (Caernarvon. XX.7 1900), effectively reversing the house. The 3rd edition of 1918 shows the house enclosed in a rectangular walled court, with a moat crossed by bridges about the eastern side. The service wing of 1900 encloses two sides of a yard on the north-east side. None of these features are shown on the earlier editions. Balled yews were planted along the moat.

Sources: RCAHMW Caernarvonshire Inventory II (1960), 183-4 No. 1199 fig 122
CADW Listed Buildings Database (3683)
Register of Parks & Gardens in Wales, 210-11

John Wiles, RCAHMW, 28 June 2007