Tan-yr-Allt (in English `below the hill?) is a two storey, Grade II* house in Bangor, Caernarvonshire, built in 1755 for John Ellis, the Archdeacon of Merionedd on a portion of land belonging to his family. It was built in the Palladian style popularised at the time of its construction by Inigo Jones and his contemporaries, and is believed to have been planned by one such Master builder, though there is little remaining evidence to support this supposition. The house was originally situated outside of Bangor, set within extensive formal gardens sloping downhill to the River Adda, with pathways connecting the property to Bangor Cathedral and Bishop's Palace. However, the house and grounds were purchased by Bangor University in 1928 as part of its post-First World War expansion, along with adjacent land that the university required for its planned development. The house has since been encroached by university buildings, and more recently by the PONTIO Arts and Innovation in Bangor development, obscuring its original rurality.

Tan-yr-Allt was visited by Royal Commission investigators in the early 1950s while preparing the Caernarvonshire Inventories, which were published in three volumes between 1956 and 1964. A plan of the property was made, and photographs were taken of key features of the house and its interior, which was by that time in use as Bangor University Student's Union.

Records at the Commission indicate that at the time of its initial survey the interior features of the building had survived well. They date the main staircase, windows, fireplaces, cornicing and ground floor panelling to the building's original construction in 1755. Particular attention is given to the main staircase, which the final inventory notes is `a good example of the local Chinese Chippendale style?. It ascends from the main entrance of the property to the first floor in a single flight, with a railing on the landing and fluted reveals to the window at the head of the stairs. It features a latticework design, with two contrasting patterns repeating in sequence to form the balustrade. The stair rail on the landing uses two different but equally contrasting patterns across the span of the landing. In addition, the tread end of each step is carved with a stylised wave design. Although the staircase is now painted white, the photographs from the initial 1950s investigation show the original staircase as being dark wood, though it is not possible to ascertain from the images whether its appearance is inherent in its materiality (i.e. whether it is made from mahogany or similar), or whether it is a result of wood staining.