Well, it is based on a true story to some extent. Shirley Jackson took the concept from the stories she came across about spooky old houses. The story in Hill House plays with the psychological state of the characters and the mystery surrounding the house, which is often the case in real - life haunted house stories. It takes those elements and weaves them into a fictional but somewhat based - on - truth narrative.
The Hill House is purely fictional. The story and its elements were imagined by the writers to provide a thrilling and engaging narrative for viewers. There's no real-life basis for it.