It was the urge to avoid playing ping-pong in the dark that led Luke Irwin to make one of Britain’s most extraordinary archaeological discoveries in recent years. Without that compulsion, he might never have found out that he lives on the site of one of the biggest Roman villas ever built in the British Isles.

Irwin, a rug designer, and his wife had decided to convert an old barn on their newly purchased Wiltshire property into a room where they and their children could play table tennis, so they hired electricians to lay cables for lights.

“The electricians originally suggested stringing up an overhead cable from our house to supply the power for the barn, but I insisted it had to be an underground cable,” said Irwin. It turned out to be a fateful decision.

The electricians started drilling and hit a hard layer 18 inches below the surface. It was found to consist of pieces of mosaic. “We knew the significance of that straight away,” added Irwin. “No one since the Romans has laid mosaics as house floors in Britain. Fortunately we were able to stop the workmen just before they began to wield pickaxes to break up the mosaic layer.”

Irwin called in Historic England (formerly English Heritage), whose archaeologists confirmed that the mosaic had formed part the floor of a grand villa built between AD175 and 220, and had been remodelled several times before the fifth century.

The Irwins’ house, created out of two labourers’ cottages, was built in the centre of the old villa and rests on a large slab of Purbeck marble, which is probably of Roman origin. According to the experts, the discovery is of “national significance”.

“The rest of the site has not been touched since the house collapsed more than 1,400 years ago, and it is unquestionably of enormous importance,” said Dr David Roberts, an Historic England archaeologist.

An artist’s impression of what the villa would have looked like in Roman times. Photograph: PR

“This is a hugely valuable site with incredible potential. The discovery of such an elaborate and extraordinarily well-preserved villa, undamaged by agriculture for over 1,500 years, is unparalleled in recent years and it gives us a perfect opportunity to understand Roman and post-Roman Britain.”

It is believed that the three-storey structure, which would have dominated the valley, was similar to that found at Chedworth, one of Britain’s most important Roman villa sites. It is thought to have belonged to a family of extraordinary wealth and importance. “It is not just the size of the building – which is vast – but the other discoveries that we have made that reveal what a special place this must have been,” said Roberts. “We have found discarded oyster and whelk shells. To keep them fresh, they must have been brought in barrels of salt water from the sea, which is miles away, and that shows just how rich the villa’s owners must have been.

Other discoveries have included a perfectly preserved Roman well and the stone coffin of a Roman child. This had been used to hold geraniums until its significance was realised. Only a few test pits have been dug, but Roberts said it was clear the walls of the villa were probably still more than a metre high, although they are buried under alluvial sediment from a nearby river. In addition, the mosaic has been revealed to be of particularly high quality. “Everything about this villa suggests it was made of the highest-quality materials,” added Roberts. “We have identified bits of stone that have come from at least 13 different British quarries. This was the country house of a powerful, rich Roman. Doubtless he also had a city house in London or Cirencester.”

Intriguingly, the house was not destroyed after the collapse of the Roman empire, said Roberts. Archaeologists have discovered timber structures erected in the fifth century. Roberts said the remains from this period, between the end of Roman occupation and the completion of Saxon domination of England, could open a window into one of the least understood periods in British history. It could also reveal how people responded to the collapse of the Roman empire, the superpower of the age.

However, it is the sense of continuity that has affected Irwin. “Some of the oyster shells we have found have not been touched by another human for more than 1,500 years and now we have uncovered them lying around our house. It is a very powerful feeling,” said Irwin

Fishbourne palace, Chichester

Most of the palace’s sumptuously decorated rooms had mosaic floors. However, a serious fire in the late third century destroyed most of it and it was not rebuilt.

Chedworth, Gloucestershire

One of the largest Roman buildings in Britain, the villa was built in phases over 200 years and transformed into a palace arranged around three sides of a courtyard. Buildings included a heated west wing and two separate bathing suites, one for damp heat and one for dry.

Turkdean, Cirencester

Site of a major Roman villa complex which appears to have been occupied from the late second century until the end of the fourth. Coins, pottery, floor tiles, millstone fragments and a stone-lined water course have also been found, while aerial photographs have suggested a substantial complex.

Brading, Isle of Wight

All 12 of the villa’s ground-floor rooms survived. Artefacts include pottery and jewellery, while floor mosaics include images of Orpheus, Bacchus, a cockerel-headed man and gladiators.

Box villa, Wiltshire

The building probably contained at least 50 rooms, including a huge reception hall that was 40ft long, 20ft wide and 30ft high. Many rooms had central heating, most had glass windows and the interior walls were plastered, painted and decorated.