Grade II* Listed Building

The house was formerly known as Greenfields and one of two substantial houses about 100m apart, both probably dating to the 1700s. The other was known as Lledfair Hall. At the end of the 18th century, Greenfields was owned by John Edwards, a solicitor at Machynlleth, who died in 1789. One of his wives, Cornelia Owen, brought the Garth estate at Llanidloes and the Van lead mines into the family's ownership as her dowry. The estate passed to his son, also called John, who managed to acquire Lledfair Hall. In the 1840s, he demolished the rival mansion to enhance the surroundings of Greenfields. His daughter, Mary Cornelia, married George Henry Robert Charles, Vane-Tempest in 1846. Thus the Plas family came to own 27,000 acres in Mount Steward, Ireland; plus 23,000 acres in Wynard Park, Plas Machynlleth, and elsewhere in England and Wales, including Northumberland-Durham coalfields.

The Vane-Tempests carried out major alterations to Greenfields. The east elevation or present day frontage was altered to have a portico supported by five ionic columns. The north wing was added when Pentrerhedyn Street was realigned in the 1850s. A single storey conservatory was built onto the south central section and demolished in the 1950s. The former stables, coachhouse and kennels survive to the southwest, as do the two lodges ? West and North Lodge, both built in 1840.

When the Royal family stayed here in July 1911, the South Wales Daily Press stated that Y Plas was:

`A spacious mansion, replete with every comfort... Its front is square and three-storied, but many additions have been made at the sides and back, and a long dining and drawing room built out at each side of the central hall. A verandah runs along the front, which looks to the sun rising, and a conservatory and sunk garden, with a fountain in its midst, are at the southern end of the house. A large kitchen garden is at the other side and the drive running from the main entrance in the Aberystwyth Road to the one opening into Maengwyn Street, also goes round to the back to a large stables and coach houses, the stable yard being shaded by an enormous poplar planted by Sir John Edwards, its trunk being eight feet in diameter.... we enter a hall, comfortably furnished as a living room... many heads of deer are on the walls, and a board holds the model of a Norwegian salmon, weighing 53lbs, and killed by Lord Herbert. On the left is the drawing-room, its large south window opening into a conservatory, now furnished as a smoking lounge and looking out into the garden. Its furniture is covered with pale blue satin; amongst its pictures are a fine Gainsborough and a Kneller, and its ornaments include some large vases of antique china.?

The house was given to the townspeople in December 1948 under the stewardship of the then Machynlleth Urban District Council. Various local government re-organisations saw it pass first to Montgomeryshire District Council, who in 1995 converted it into the Celtica Visitor Centre. Powys Council took over Celtica and the house when it was formed as a unitary authority in 1997. The centre was successful in attracting tourists, school groups and conferences for a number of years, however initial visitor number predictions proved to be too ambitious and Celtica closed in March 2006. On 1 April 2008, Machynlleth Town Council took ownership of Y Plas, its parkland and facilities. It has reopened the cafe and the 1st and 2nd floors of the main building are rented out as office space. Medium sized meeting rooms and conference space are also offered for hire.

Maritime Officer, RCAHMW, June 2015.