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.

Cardiff council has issued travel advice to drivers, with roads shutting from 06:30 BST and reopening at about 12:30.
East Tyndall Street, Lloyd George Avenue and Bute Place are among those being closed for the triathlon.
Drivers heading to the Extreme Sailing event before midday have been advised to follow signposted routes to the bay.
The council said the best route to get to the sailing would be the A4232, exiting at the junction for Mermaid Quay and the central link road.
A cycle diversion is also in place around Britannia Park, between Cardiff Bay and the barrage.
Roads closed during the triathlon:

David Miller, 56, died at his house on Patterdale Walk, Northampton, in the early hours of Tuesday.
Northamptonshire Police said the woman was 39 and of no fixed address.
Four others have been charged with murder: Zena Kane, 35; Ian Cuthbertson, 49; Michael Hallett, 37 and Joseph Catlin, 30, who were remanded in custody at Northampton Crown Court.
Heating systems to grow fruit have been uncovered at the former private palace of the Prince Bishops of Durham.
[NEXT_CONCEPT]
Road closures have been announced ahead of Cardiff Bay's triathlon and sailing events on Sunday.
[NEXT_CONCEPT]
A woman has been arrested on suspicion of the murder of a man who was stabbed to death at his home.