The location used for the foyer and staircase of Three Gables is Wrotham Park in Hertfordshire. The same location was used for the film Gosford Park (2001), written by Julian Fellowes, who also wrote the screen adaptation for Crooked House.
One of the two Doberman Pinschers (Dobermans) in the den/library has cropped ears. In England it Has been illegal to crop the ears of a dog for at least 100 years.
Ruth Ellis is mentioned in dialogue. In 1955, she was convicted of murdering her lover, David Blakely. She was the last woman hanged in the United Kingdom.
The name of Charles' secretary, Miss Ackroyd, is a nod to another of Agatha Christie's most famous and influential works "The Murder of Roger Ackroyd."