Photographs discovered in archives at Bristol Library will aid an important National Trust restoration project.
The sepia images, snapped by local photographer H Robert-Lewis, show how the Tyntesfield estate in North Somerset looked in the 1920s.
They depict the mansion and gardens during a period for which there had previously been no photographic record.
The images have helped establish where Tyntesfield's tennis court was located and show the lake in its heyday.
From them, experts will also be able to glean new details of the gardens and see a set of lodge gates which no longer exist.
Bristol City Council, which owns the collection of photographs, plans to make the pictures available online in a digital database.
The estate was bought by the National Trust in 2002 and was opened as an attraction this month.