In his 'Author's Note', Evelyn Waugh is at pains to emphasise that the characters of Brideshead Revisited are fictional, although it has often been suggested that events and characters in the book are based on real-life situations and individuals.
For example, Sebastian Flyte bears a resemblance to Waugh's Oxford friends Alastair Graham and Hugh Lygon.
The Lygon family's ancestral seat is Madresfield Court near Malvern in Worcestershire, which is believed to be a model for Brideshead Castle (as is Castle Howard in Yorkshire), and the Art Nouveau chapel at Madresfield is almost certainly the model for the chapel at Brideshead. Various events in the novel match those in the lives of the Lygon family, and Hugh Lygon's elder brother and father may have been a partial inspiration for the characters Bridey and Lord Marchmain.
However, it is clearly important to bear in mind the complexity of the process by which a novelist may transform experiences in the real world into fiction. As is noted in Selina Hastings' biography*, Evelyn Waugh was irritated by people's tendency to see real-life figures as the basis for his characters. After commenting that the trade of the novelist is the only one where its practitioner has to face people coming into the workshop and playing with the tools, Waugh goes on to say that one of the most mischievous forms which this interference takes is the attribution to him of living models for his characters.
*Selina Hastings, Evelyn Waugh: A Biography (Sinclair-Stevenson, 1994) p.172.