The ending of the Haunting of Hill House is very open ended. It's assumed that Eleanor commits suicide by crashing her car. It's interesting because throughout the novel Eleanor constantly remind us about how she wants to belong somewhere. She doesn't truly have a home for herself. The manifestation produced by house  to entice Eleanor could have been part of Eleanor's imagination. Her will to find a home was so strong. Once she was rejected by Theodora she may have found there was no other option other to kill herself and supposedly become a spirit of hill house. Also, her desperation may have been caused by the fact that no one has ever really wanted her. She alludes to this throughout the text. The house seems to have an attraction towards her and Eleanor appreciates this. Of course no one knows for sure what exactly happened but the author does tell us that hill house will never be the same again.