The Duchess filmed at Holkham Hall, Holkham. The vast Marble Hall of this Norfolk house serves as the entrance hall of Devonshire House in the film. On screen it's Devonshire House.
Probable Named in published production coverage; the coordinate is the place itself, checked against a gazetteer.
Planning the trip? The United Kingdom travel atlas covers money, language and getting around
Venture keeps every scene location on one map, sorted by how far you'd have to walk. Open it in Holkham and it tells you what was shot around you, how to get there, and whether you can actually go in.
Download Venture for iOS Android