Basildon Park is a country house in Berkshire, between the villages of Upper Basildon and Lower Basildon. It is owned by the National Trust and is a Grade I listed building. The Staircase Hall is at the centre of the house and is considered to be one of Carr's "most monumental interiors.
This Georgian mansion was used for the interior shots of the Grantham House, the Crawley family's London residence. The exterior shots where done by Bridgewater House on Cleveland Row in London.
Basildon Park was used as the location for Netherfield Park in the 2005 film Pride & Prejudice, and more recently the 2016 film Pride and Prejudice and Zombies and as a location for the 2006 movie Marie-Antoinette.
Behind the scenes: https://www.youtube.com/watch?v=sfI7nbH_Ocw