Brightwood by Tania Unsworth

Eleven year old Daisy has never left the grounds of her home, Brightwood mansion, where she lives with her mother.  She has never even spoken to another person in her life.  She has no idea what the outside world is like at all. Her mother is a hoarder, filling a box a day with things to help her remember and the house is so full that it  has become a maze.  One day, Daisy's mother goes out to the store, and does not return.  Then, a man shows up at the house.  He claims to be her cousin.  He is not a nice man.  Can Daisy survive?  Will her imaginary friends help her?  This is an intriguing mystery for middle grade children.