Police were called to the house, in Furness Drive, Illingworth, at around 12.45pm on Saturday.
She was treated at the scene by paramedics and taken to Leeds General Infirmary by air ambulance with a serious head injury.
She remains in a serious condition following surgery.
Police said an 18-year-old male was assisting with their inquiries.
Neighbours spoke of their shock at the incident and said there was a heavy police presence in the area.
One neighbour, who did not want to be named, said: “It’s awful. There was lots of noise and lots of police cars about.”
Another described the incident as “heartbreaking”.
Detective Inspector Gary Stephenson, of Calderdale District CID, said: “Enquiries are ongoing to determine precisely how this girl has come to be injured.
“Our investigations suggest she was not intentionally injured and an 18-year-old male is assisting us with our enquiries.
“Specially trained officers are working to support her family as our investigations continue.”