Mary and Mike Hudd cut down a tree blocking sunlight to their cottage in Bincknoll, and found a Norman chapel believed to have been built around 1209.
Archaeologists have since confirmed the approximate age and origins of the building, whose whereabouts had been unknown for hundreds of years. The chapel measures 19 by 52 feet.
Experts believe its significance goes back even further than then, having found an Anglo Saxon room inside which they believe was used to conduct services. The Anglo Saxon period lasted between 400 and 1066.
The village of Bincknoll is first documented on paper in the Domesday Book in 1086, suggesting the chapel was a key part of public life from its very beginnings.
Mrs Hudd told the Daily Mail: "How we have managed to miss it in our garden I don't know. We had a drive put in in the 1970s and it missed the southwest corner by inches.
"The east corner just misses the road."
Once excavation finishes, the site will be covered up to preserve it for future generations.
