The dig at Auckland Castle's walled garden revealed a pioneering pinery-vinery, created by Bishop Richard Trevor in the 1750s.
This utilised furnaces and flues to allow the cultivation of exotic fruits, particularly much coveted pineapples.
It is believed to be one of the first of its kind in the North East and set the standard for following ones.
The dig, by archaeologists from Durham University and volunteers from the Auckland Castle Trust, was in advance of a Â£10m scheme to grow fruit and vegetables under glass domes in the walled garden to supply a new on-site restaurant.
Heating systems to grow fruit have been uncovered at the former private palace of the Prince Bishops of Durham.