Tŷ unnos (plural: Tai unnos) (One night house), is an old Welsh tradition which has parallels in other folk traditions in other areas of the British Isles.
It was believed by some, that if a person could build a house on common land in one night, that the land then belonged to them as a freehold. There are other variations on this tradition: that the test was to have a fire burning in the hearth by the following morning; and that the squatter could then extend the land around by the distance they could throw an axe from the four corners of the house.
From a period spanning the seventeenth to the beginning of the nineteenth centuries, the expansion of the Welsh population combined with poverty brought about a series of incidents of squatting on isolated patches of land in the most rural parts of Wales. The practice arose because of the pressure of the lack of land due to the land enclosures of the period, and the taxation laws established by landowners. Family units paid taxes based on the land they inhabited, so families with adult and married children faced either paying additional taxes on a second home, even if it was on the same land.
Tŷ unnos has no status in English common law (the legal code which applied to England and Wales in this period), although there is some tradition of legal discussion about the point at which land occupied by squatters without title may be regarded as a legitimate possession. This legendary belief may bear some relation to genuine folk customs and actual practices by squatters encroaching on common or waste land.  The tradition may have provided squatters with a sense that their actions enjoyed some legitimacy conferred by an older code of laws more in tune with values of social justice than the supposed "Norman yoke". Interestingly, many modern day commentators and local historians accept the claim that the tŷ unnos is a tradition enshrined in law.
Many localities in Wales and England have a house or houses which may be identified as a one night house in local folklore. These may in fact be properties that were originally built by squatters and may be constructed in a vernacular building tradition using locally available materials. The Ugly House (Ty Hyll) is a celebrated example in Snowdonia.
Many of these legends seem to be passed on in ignorance of the broader tradition of the one night house and may feature picturesque details based on variants of the traditions noted above. These legends generally take the form of a prominent member of local society proposing a wager with a landless family that members who could raise a house in a night and a day could keep the property. Some versions of these legends may emphasise that the family may cheat and win out over the complacent authority figure by building a very small hut or by simply building a hearth and chimney.
A good general account of one night house traditions is provided in a recent book by the British anarchist and writer on housing issues, Colin Ward. Ward considers the one night house tradition in the context of squatting and other informal systems of occupying and using land and relates accounts from England, Ireland, Scotland and Wales (he observes that similar traditions exist in Turkey, France, North and South America), which demonstrate clear parallels in different folk traditions.
Very little is known in detail about the building of these structures, their numbers or inhabitants, and no accurate representations survive.
Most Tai Unnos were originally made of turf and soil, with a roughly thatched roof. Once established, the walls were often replaced with local materials, including clay and stone. An experimental construction in Carmarthenshire in 2006 demonstrated that a rudimentary structure could be assembled quickly. The squatters, may not have depended exclusively on agriculture and in some areas may have worked in quarries and mines. This development led to dispersed settlement patterns seen in the Welsh landscape today. Materials from early stages of construction may have been replaced by higher quality timber and slates, available via the new railways. Single storey Ty Unnos cottages were modified by raising the roofs and enlarging the windows.
- Iorwerth C. Peate, The Welsh House (Brython Press, Liverpool, 1946)
- Colin Ward, Cotters and Squatters - Housing's Hidden History (Five Leaves Publications, Nottingham, 2002)
- Eurwyn Wiliam, Hand Made Homes: dwellings of the rural poor in Wales (National Museum of Wales, 1988)
- Eurwyn Wiliam, The Welsh Cottage (Royal Commission on the Ancient and Historical Monuments of Wales, 2010)
- http://history.powys.org.uk/school1/agriculture/land.shtml Victorian Powys
- WILIAM, Eurwyn: The Welsh Cottage. Royal Commission on the Ancient and Historical Monuments of Wales. 2010
- "A 'one-night' house for America". BBC Wales. 2009-04-15. Retrieved 2009-04-15.
- Ty Unnos as an inspiration for low cost modular housing systems