Experts are urging property owners in Wales to help them discover buildings with historical value to preserve the nation's heritage.
The hall is said to be one of the best examples in Wales
The call came after a couple from Hengoed in Denbighshire discovered one of their outbuildings was originally a 15th century medieval hall house.
Historians have dated the building using dendrochronology - the analysis of tree rings in timbers - to 1447.
Experts said the find was "extremely rare".
Anthony and Helen Rose bought their 21-acre smallholding near Ruthin in 2003.
Mr Rose said although they did not realise its true significance, the building was one of the things that had attracted them.
"We were told it was the original farmhouse, but it wasn't listed or anything... you could tell it was a substantial building, it was quite dramatic," he said.
The couple discovered the building's history when they had become concerned about the leaning trusses and called in a structural engineer, who had alerted a local architect.
Mr Rose said many original features of the hall house were still evident, including the remains of a window.
"The important part is the woodwork, the five crucks are still there. What is impressive are the size of crucks, they are very big, heavy substantial cruck trusses (a roof truss composed of two curved or angled pieces of timber).
"It's a great honour to have this on your doorstep.
"In time, maybe it would be nice to make it into a dwelling again, but keeping as much like it would have been in the 15th Century.
"We're just custodians. It's a big part of history, it's part of the culture, so it's got to be preserved."
National Trust archaeologist Emma Plunkett-Dillon said the find was "extremely rare" as hall houses were built by wealthy people, and at the time, Wales was a relatively poor country.
But she said it was not uncommon to find examples of domestic dwellings which were built slightly later in the 17th or early 18th Century.
Ms Plunkett-Dillon said one clue was if the doors or windows of a building had different alignments, which could signify an "interesting history".
"The other giveaway is interesting roof timbers. If you've got a roof built with large old beams... you might be looking at something that's much older, but the only guarantee is to get an expert in."
Judith Alfrey, from Cadw, the assembly government's historic environment service said: "It is a rare find... it's quite exciting really, it's pushing on our understanding of the development of architecture.
"Every new discovery is adding to the total stock of our knowledge, it really is a direct line to understanding of how people lived in the past," she added.