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 Octagon Drawing Room is the principal room of the house.
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.
Behind the scenes: https://www.youtube.com/watch?v=sfI7nbH_Ocw