Cornwall Council are pushing ahead with the Truro Eastern District Centre (TEDC) despite finding the remains of a one-off prehistoric enclosure thought to be 5,500 years old on the site.
The TEDC is planned to include 97 homes, 1,350 park-and-ride spaces, a Waitrose supermarket, foodhall and recycling centre on an area of former farmland.
Archaeologists, led by Cornwall Council’s Historic Environment Service, suggest the eastern end of the site may have been a "causewayed enclosure" dating from the early Neolithic period (3800BC to 3600BC.)
This is the first find of its kind in Cornwall.
Now the team will catalogue their findings, take samples and, "in line with national guidance," re-bury the site to "protect it for future generations," a council spokesperson said.
Dan Ratcliffe from the Historic Environment Service said: "While it is important that we take the opportunity to learn more about our findings now, best practice is for the site to be preserved for future generations of archaeologists who will have better technologies to understand it than we do today.
"Scientific analysis of evidence recovered during the excavations is expected to take some years after the sample excavation has concluded.”
Initial surveys of the site were carried out in 2009, with a condition of the planning approval being to carry out further archaeological research. This work was commissioned by the council’s Transportation Service.
Tim Wood, Cornwall Council’s assistant head of transportation, said: “The proposed development has sufficient flexibility in the design to ensure that the construction above does not interfere with the archaeological remains.
“Following recommendations from the council’s archaeological advisor, we will reflect the archaeological significance of the site including installing interpretation boards.”
Around 80 sites with evidence of causewayed enclosures are known across southern Britain.
The find at Truro is the first to be discovered to the south west of the border between Dorset and Devon although the "tor enclosures" at Carn Brea and Helman Tor are thought to have been built at the same time and may have served similar functions.
Mr Ratcliffe said: "A causewayed enclosure was a large circular or oval area enclosed by a large bank and ditch.
"These sites date to the early Neolithic period - a period which also saw the first introduction of agriculture to Britain, the domestication of animals, the manufacture of pottery, and the first appearance of large communally built and used ceremonial monuments. Both the construction of the site and the activities within and around it probably served to bring communities together."
Investigations at the site, on land owned by the Duchy of Cornwall between the A39 Newquay Road and the A390 Union Hill, are expected to be completed at the end of November.
The excavations are a requirement of the planning conditions placed on the TEDC development.