The Norfolk Medieval Graffiti Survey (NMGS) group claims to have found 8ft (2.4m) building plans etched into the stonework of Binham Priory, near Wells.
The drawings, which are too faint to photograph, seem to show how the building’s west front was built.
“It was difficult to believe what I was seeing,” said NMGS’s Matthew Champion.
“Most graffiti inscriptions tend to be relatively small and modest. This was just so big that it wasn’t really possible to see exactly what it was until we had surveyed the whole wall surface.