Nannau was the ancestral seat of the Nanneys family and from the late 18th century, the Vaughans. In the early 17th century Huw Nannau Hen built a new house in lavish style, much praised by his contemporaries. There is no record of this in the 1660s hearth tax returns so it is possible that the house was a casualty of the Civil War. Col. Huw Nannau rebuilt the house c. 1693 and a subsidiary domestic range behind the present house probably relates to this; it now lies in ruins. Nannau eventually passed to a relative, Robert Hywel Vaughan. He built the present restrained Georgian house c.1795. To the three-storey primary was added storied pavilion wings in c.1805, to the designs of Joseph Bromfield. These additions were demolished in the 20th century.
It is a 3-storey 5-bay house in restrained Georgian style, of a square plan, built of large dressed blocks of local slate-stone with sandstone dressings and shallow, hipped slate roof with plain chimneys. There is a recessed, wide entrance bay to main front and an elegant entrance porch with Ionic columns and simple moulded entablature. Good decorative and heraldic lead hoppers survive, 2 of which are original (one dated 1795) and 2 of which bear the date 1872. There is a plain parapet with a moulded cornice and balustrading above the entrance bay.
Adjoining the right side of the house, there is a a triple-arched section of walling, a surviving fragment of the pavilion wings.
Internally, there is a stair on 3 floors with swept mahogany rail and decorative iron balusters, 6-panel mahogany doors, marble fireplaces and an Adam style plaster ceiling.
