The series is filmed at Highclere Castle, set on 1,000 acres in West Berkshire--a landmark estate owned by the Carnarvon family since the late 17th century. It's open to the public during the summer for $27 admission fee and can also be rented for weddings and other events. The kitchen and servants' quarters, however, are shot in a London studio because those parts of Highclere have been modernized.
The real estate that "plays" Downton Abbey, Highclere Castle, has been the home of the Carnarvon family since 1679. In 1922, George Herbert, the 5th Earl of Carnarvon, co-discovered the Tomb of the Egyptian pharaoh Tutankhamun. On the show, the names of the Earl of Grantham's beloved dogs, Isis and Pharaoh, are nods to the real castle's connection to Egyptian history.
As a guest on Late Show with David Letterman: Episode #19.88 (2012), Michelle Dockery revealed that, given her working class background, had she been affiliated in any way with society depicted in the story's time frame, she would most likely have been her housemaid Anna rather than the aristocratic Lady Mary Crawley she portrays so convincingly in the series.
Julian Fellowes' inspiration for his original story came from authors such as Edith Wharton ("The Buccaneers") and close friend Henry James's general research of the novel's time period and subject matter. He also sourced the 1989 book "To Marry an English Lord". Elizabeth McGovern's character Cora, was the first one that he developed.
When the first series was broadcast, Julian Fellowes, the creator and executive producer, was annoyed at newspaper articles which printed viewers' criticisms of anachronisms (e.g. TV aerials and double yellow lines in shot), especially the newspapers' assumptions that *all* such criticisms were valid - for example the word "boyfriend" was wrongly thought to be an anachronism when in fact it appeared in print in 1889, long before the time when Downton Abbey was set. At the time, he said "They think to show how smart they are by picking holes in the programme to promote their own poshness and to show that their knowledge is greater than your knowledge". However in an article in Radio Times on 17 September 2011, coinciding with the start of the second series, Fellowes apologised and commented "I behaved rather badly by getting the hump".
In the 2012 book "The Chronicles of Downton Abbey: A New Era," authors Jessica Fellowes and Matthew Sturgis clarify that although Cora's late father (who has been identified on the show as Isidore Levinson, a dry-goods magnate from Cincinnati, Ohio) was Jewish, her mother, Martha, was Episcopalian and raised Cora likewise.
STV, the Scottish counterpart to ITV, irked viewers for refusing to show the first series, instead ordering a six part mini series of long running show Taggart. Scottish viewers complained and STV eventually showed the first season with the second immediately afterwards.
Julian Fellowes revealed in an interview that the story of Pamuk dying and being dragged back to his room was based off a diary entry found in Highclere Castle during one of his stays at the house. He is good friends with the current Earl and Countess.
During the second season, the Abbey is turned into a recovery house for troops. In real life, the Countess during World War I turned the house into a hospital. Unlike Cora Crawley, the Countess eagerly opened her home and there are letters in some of the rooms from former patients thanking her for her hospitality.
The trivia item below may give away important plot points.
Julian Fellowes admitted in an interview that he didn't know about Dan Stevens' intentions to leave the show until it was too late into the Mary/Matthew storyline, so the only option he had was to kill him to write him out of the show.