Experts believe they have discovered the remains of the largest-ever Roman building found in Leicester.
The dwelling, thought to be a second century town house is 230ft long - equivalent to 15 terraced houses.
Archeologists believe it could have been a hotel for Roman officials visiting the city.
Alternatively, it could have been a large home for a wealthy family.
The discovery was made in Vine Street, in the city centre - yards from the former St Margaret's Baths site, where archaeologists recently found the skeletons of 1,300 people in a medieval cemetery.
Evidence of Roman existence in the area was first reported in May last year, when experts thought they had found the remains of a wealthy family's townhouse.
But the excavation has proved much larger than originally thought.
Richard Buckley, director of University of Leicester Archaeology Services, said his team had not expected such a large discovery.
He said: "The surprising thing is that we didn't expect any Roman activity here in this side of the city. We certainly did not expect this sort of density of population.
"Until this development, this part of Leicester is blank on Roman history and this shows there was industrial, residential and commercial activity over this part of town.
"The good thing is that it remains in good enough condition for a good analysis."
The town house is at the junction of two streets with rooms arranged around a central courtyard, served by several corridors with some containing fragments of mosaic pavements.
One of the rooms is equipped with a hypocaust, an ancient central heating system, thought to be part of a small bath suite with a plunge bath.
Another large Roman building, 98ft long, from the third century was also discovered. The unusually thick walls, around 1.2 metres , suggests it could be a public building used for storage.
Mr Buckley said: "We also found two lead seals marked with the initials of the sixth and 20th legion in both areas.
"No doubt this will help us to understand the era and period a bit more and this is a significant find for the city.
"The site is still being examined and we believe it will still be a few months before we have finished our dig."
The area of the discovery is to form a new multi-storey car park for the £350 million Shires development.
Chris Wardle, a city archaeologist, said: "We knew Leicester was a Roman city but the findings suggests that part of town was intensely occupied.."