The discovery is one of the largest and best-preserved Roman villas yet discovered in the country.
Shaped like a church, the building was discovered on the Isle of Wight, and has been likened to a medieval hall.
Its remains were discovered at the site of another Roman villa in Brading, and are believed to have been constructed 150 years before the other building.
The later Brading villa’s remains had disappeared from sight until 1879 when a couple of local men stumbled across them by chance.
Its ornate decorations are unrivalled in Britain and the building may have belonged to Allectus, who in AD293 murdered his predecessor Carausius, a Roman army commander who had proclaimed himself Emperor of Britain.
The discovery is comparable in scale to the Bignor Roman Villa, near Pulborough, and the hall of Fishbourne Roman Palace, near Chichester, both West Sussex.
Its remains, around 3ft below ground, are so well preserved that the standing structure, masonry and many roof tiles have survived.
Sir Barry Cunliffe, Emeritus Professor of European Archaeology at Oxford University and head of the excavation, said: “It’s a very impressive building, absolutely magnificent. It could have been seen for miles around.”
The residential part had under-floor heating and walls plastered and painted with mock marble patterns while the communal end would have been used for meetings and legal matters such as boundary disputes and payment of dues.
The Victorians explored this part of the site in the 1880s, although they dismissed the remains as a barn.
A team of 30 archaeologists from America and Europe are now involved in the excavation.
The new site will now have to be covered up however, with Sir Barry warning they would disintegrate in two winters.