The foundations were uncovered along with a large Norman building at an undisclosed location on the Mendip Hills.
Saxon keys and a 13th Century jug were also among the finds.
Project leader Pip Osborne, said: “There’s no written record of a building here but ever since I moved here I’ve been intrigued by the field.”
The find was made by a community archaeology group on a plot of land near the centre of a Mendip village.
The land, according to Ms Osborne, was given to the Abbey of Jumieges in Normandy in France by William the Conqueror in about 1080.
“I had a hunch about this empty field. I ran the machines over it and there was an image of something quite strong on the geophysics and I thought this has to be investigated,” she said.