Ystumllyn house is said to have been built at the end of the 16th Century by Ellis ap Cadwaladr (b.1557), who had acquired the property from the Crown. The house was later extended with the addition of a new wing.  

A date inscription of 1729 and a dated stone found in the garden (of 1720) are thought to provide the likely date for the additions to the house. Owners in this period were Rev. Humphrey Wynn (d. 1724) and Ellis Wynn (d. 1759).  

The original building is a 2-storeyed, large 2-unit house, with central service room, extended to the North West with a wing housing the stairhall and a further principal room.  

The main range has a central entrance in a roughly semi-circular cyclopaean archway. This aligns with a further entrance in the rear wall (the present entrance). 

A fireplace incorporates coats of arms in the panelled surround (those of Gollwyn ap Tangno and Owain Gwynedd) - the lower panels (German Renaissance reliefs), and the Delft tiled surround, were inserted c1946. 

Ystumllyn incorporates an important example of a 16th Century house, probably one of the first generation of storeyed houses in the region. The 18th Century extension and alterations also represent high quality vernacular work of the period.