Luke Irwin, from Wiltshire, was laying an electricity cable in his barn when he uncovered a mosaic underground.
The rug designer, who was making the alterations so his children could play table tennis, took a photo and sent it off for more information.
Mr Irwin said: "I sent a photograph to the council and within 24 hours they were here with archaeologists to see what we'd found.”
Experts from Historic England and Salisbury Museum carefully began excavating the site, and realised the mosaic formed part of the floor of a grand Roman building.
The find has been proclaimed by Historic England as “unparalleled in recent years”.
After an eight-day dig, archaeologists uncovered more of the ‘elaborate’ and ‘extraordinarily well-preserved’ villa, thought to be one of the largest ever found in the country.
Dating from between AD 175 and 220, the grand home is thought to have been three storeys high, similar to those found at Chedworth in Gloucestershire.
The team also discovered hundreds of discarded oyster shells which were artificially cultivated and carried live from the coast in barrels of salty water.
The dig unearthed "extremely high status pottery”, brooches, coins and the bones of wild animals which had been hunted, as well as a suckling pig.
All the evidence suggests a family of high importance and wealth lived at the villa, possibly a Roman emperor.
Dr David Roberts, an archaeologist from Historic England who helped work on the excavation, said: "We've found a whole range of artefacts demonstrating just how luxurious a life that was led by the elite family that would have lived at the villa.
In pictures: 12 amazing archaeological discoveries
In pictures: 12 amazing archaeological discoveries
1/12 Ancient forest, discovered in February 2014
Ancient forest revealed by storms. The recent huge storms and gale force winds that have battered the coast of West Wales have stripped away much of the sand from stretches of the beach between Borth and Ynyslas. The disappearing sands have revealed ancients forests, with the remains of oak trees dating back to the Bronze Age, 6,000 years ago. The ancient remains are said by some to be the origins of the legend of ‚Cantre‚r Gwealod‚ , a mythical kingdom now submerged under the waters pif Cardigan Bay
2/12 Medieval royal palaces, discovered in November 2014
Archaeologists in southern England have discovered what may be one of the largest medieval royal palaces ever found – buried under the ground inside a vast prehistoric fortress at Old Sarum. The probable 12th century palace was discovered by archaeologists, using geophysical ground-penetrating ‘x-ray’ technology to map a long-vanished medieval city which has lain under grass on the site for more than 700 years
3/12 The Dead Sea Scrolls, discovered ca. 1950
The Dead Sea Scrolls are almost 1,000 biblical manuscripts discovered in the decade after the Second World War in what is now the West Bank. The texts, mostly written on parchment but also on papyrus and bronze, are the earliest surviving copies of biblical and extra-biblical documents known to be in existence, dating over a 700-year period around the birth of Jesus. The ancient Jewish sect the Essenes is supposed to have authored the scrolls, written in Hebrew, Aramaic and Greek, although no conclusive proof has been found to this effect
4/12 Diamond, discovered in March 2014
This rare diamond that survived a trip from deep within the Earth's interior confirmed that there is an ocean’s worth of water beneath the planet’s crust
5/12 Whale skeletons, discovered in February 2014
Chilean and Smithsonian paleontologists study several fossil whale skeletons at Cerro Ballena, next to the Pan-American Highway in the Atacama Region of Chile
6/12 Complete mammoth skeleton, discovered in November 2012
The first complete mammoth skeleton to be found in France for more than a century was uncovered in a gravel pit on the banks of the Marne, 30 miles north-east of Paris. Picture shows experts at work making a silicon cast of the mammoth's tusk
7/12 Million-year-old human footprints, discovered in February 2014
Photograph of the footprint hollows in situ on the beach as Happisburgh, Norfolk
8/12 Terracotta warrior, discovered in June 2010
Chinese archaeologists unearthed around 120 more clay figures in June 2010 excavations at the terracotta army site that surrounds the tomb of the nation's first emperor in the northwestern Shaanxi Province
© Jason Lee / Reuters
9/12 Neolithic 'lost avenue' - prehistoric stone circle, discovered in September 1999
The discovery of a Neolithic 'lost avenue' was described as one of the most important finds of the last century. Since the 1700s, archeologists and historians have argued over the existence of the huge sarsen stones, which were unearthed at the site of the world's biggest prehistoric stone circle at Avebury in Wiltshire
10/12 Byzantine mosaic, discovered in February 2007
Plans for a walkway at the centre of the furious dispute over Jerusalem's holiest site were delayed by the discovery of a Byzantine mosaic
11/12 Ancient gold, discovered in March 2014
Gold fitting for a dagger sheath (around 1900 BC.) found near Stonehenge
12/12 Rosetta Stone, discovered in 1799
The Rosetta Stone is a basalt slab inscribed with a decree of pharaoh Ptolemy Epiphanes (205-180 BC) in three languages, Greek, Hieroglyphic and Demotic script. Discovered near Rosetta in Egypt
"It's clearly not your run-of-the-mill domestic settlement."
A perfectly preserved Roman well and the stone coffin of a Roman child, which went unnoticed and was being used as a flower bed, were also among the finds.
Dr Roberts added: "The site has not been touched since its collapse 1,400 years ago and so it's of extreme importance.
"The large scale and complexity of the site present a unique opportunity to understand Roman and post-Roman Britain.
"Without question, this is a hugely valuable site in terms of research, with incredible potential.
“It's one of the best sites I have ever had the chance to work on."
The site has now been protected to preserve it.
Mr Irwin said it was “unbelievable” to have stumbled across the rare find.
He said: "The thought of the footsteps we are following in. I have always been fascinated by history ever since I went to Pompeii as a child.
"But to find it 20 yards from your own front door - and then the 20 billion to one shot that you design luxury rugs for the Roman aristocrats of today. It's mind blowing."
Further research will be continued when more funding becomes available.