An open verdict was recorded in the inquest of 17-year-old Lauren Eley, who died in 2004, in Northampton.
Coroner Anne Pember was told the police notes were destroyed after seven years.
The Eley's family's legal representative, Sefton Kwasnik, called the way police had handled the case a "disgrace".
Mrs Pember said she would be writing to Northamptonshire chief constable Adrian Lee, to say "when an inquest is still live original notes should be retained".
A Northamptonshire Police spokesman said: "An officer's notebook was stored and destroyed after seven years in line with national guidelines.
"It is regrettable that, due to an oversight, a copy of the notes made within that book that relate to this investigation were not retained with the case papers.
"We accept this would have assisted at inquest."
Police did not offer any apology to the Eley family.
The officer involved in the case attended the inquest, but said he could not remember a huge amount of detail because the events had happened so long ago.
He said his note book had been passed to the police administration department.
Miss Eley died at Northampton Hospital on 11 December 2004 having fallen from the Grosvenor centre multi-storey park.
The inquest was told police could not rule out her death was assisted by another person.
And Mrs Pember said: "I cannot presume that her death was accidental."
Mr Kwasnik said : "I think it's a disgrace the way the police have conducted this investigation from the beginning.
"The family had to endure being told it was most likely a suicide, from the start."
The police spokesman said the force had considered several possible causes of death, including "that of accident, misadventure and the involvement of a third party".
The destruction of police notes on the death of a teenage girl who fell from a multi-storey car park 10 years ago, was "regrettable", police said.