A debate on what is the oldest building in a market town has been solved thanks to tree ring science.

Bishop Bonner's Cottage had been understood to be the oldest surviving domestic building Dereham and that has now been confirmed.

The Dereham Heritage Trust, which operates the museum on St Withburga Lane, has revealed research results - using a process called isotopic dendrochronology - that can officially age the building.

The results showed the mantle beam in the south cottage had 41 measurable rings, which means the timber was likely to have been felled between 1575 and 1600. 

While the north cottage's roof truss had 55 measurable rings, placing it between1680 and 1705. 

Dr Peter Wade-Martins announcing the results of the tree-ring dating to a group visiting the cottages last week.Dr Peter Wade-Martins announcing the results of the tree-ring dating to a group visiting the cottages last week. (Image: Dereham Heritage Trust) Dr Peter Wade-Martin, chairman of the group, said the process relies on tree-ring samples to measure the level of an oxygen isotope in each ring, which is able to give the age of the cottages more reliably.

Dr Wade-Martins said: "We tried standard tree-ring dating and brought in the Nottingham Tree-ring Dating Laboratory to drill two cores, one from the mantle beam over the fireplace in the south cottage and another from a roof truss in the north cottage but neither sample could give us a match.

"So the samples then went to another laboratory in Swansea for isotope dendrochronology."

Robert Howard, from the Nottingham tree-ring dating laboratory, drilling out the key sample from the south cottage.Robert Howard, from the Nottingham tree-ring dating laboratory, drilling out the key sample from the south cottage. (Image: Dereham Heritage Trust) The results bring the almost 500-year-old building in line with some of the oldest in the county, including Augustine Steward's House, which was built in 1530.

Dr Wade-Martin added the trust is still in search of a heritage centre for the town. 

He said: "Although the cottages are really important historic buildings, they are damp, too small, have no toilets and are not suitable as a place to tell the story of Dereham’s great past.

"The trust is still looking to the town council for help with this and further meetings with the mayor are planned."