A teenage girl stabbed an elderly woman to death after writing a detailed murder plan while in custody which she passed off as a plot for a crime novel, a court was told yesterday.

Kemi Adeyola's 18-page plan to force a "defenceless" woman to hand over her fortune before killing her, dismembering her body and disposing of it was found by guards at a young offenders' institution - but she claimed it was nothing but fiction.

Within months Ms Adeyola, then 17, had stabbed her former neighbour Anne Mendel, 84, to death, the Old Bailey heard. When she was arrested she told police the notes found in custody were a draft of the sort of thriller American author James Patterson writes.

Sir Allan Green, prosecuting, told the court that Ms Adeyola had been detained for three months at a young offenders' institute in Essex at the end of 2004. During a routine search of her room 18 pages of handwritten notes, entitled Changes for a happy start, were found. She outlined a plan to make ?3m, the first step being to rob and murder an elderly woman.

"Look for a fairly rural, well-off area," the notes said. "She must be wealthy, quite elderly and defenceless" and have property worth "over ?3.5m".

The notes suggested visiting the woman disguised as an A-level student doing a geography project to check the layout of the house, then hiding in the woman's drive until she came home, adding: "She pulls into the drive, climbing out of the driving seat. Run lightly and silently behind her and cover her mouth with a gloved hand. Make her so scared she cooperates. Keep calm, composed and silent. She must cooperate or take a knife to her throat."

The notes spelled out clothing and equipment needed, including two automatic guns and a silencer, a Taser gun, knives, bulletproof jacket, black plastic bags and gloves. She also needed drugs to "stop a heart attack, make someone unconscious, make them paralysed".

Sir Allan said Ms Adeyola planned to order the woman to turn off the house alarm, check there was no one else in, and handcuff her blindfolded to a radiator.

The notes continued: "Inject her with a tranquilliser. Remove her credit cards and house keys. Get the card details, mother's maiden name and code to the safe." She planned to force the woman to write a note saying she was leaving her husband for another man and pack bags with the woman's clothes before laying her out on a plastic tent on the living room floor. "Remove her head. Wrap in plastic to catch bleeding. Detach limbs one by one and wrap in cling film. Place bits in plastic bags. Take bags to car and drive to busy area. Take bags to large bin one by one till they are gone." The notes added: "The job must be done by February 2005."

After the notes were found, Ms Adeyola was interviewed by a senior member of staff and a psychologist. She allegedly said: "I want them back. It's a story. The prison staff had no right to take them." She had walked out of the room when the psychologist asked why she had written about such things.

On March 14 last year Mrs Mendel's husband, Leonard, 80, left the couple's house in Golders Green, north London, to buy tickets for a holiday in Israel. When he returned an hour later he found the body of his wife, who was 1 metre 47 cm tall (4ft 10in), covered in coats in the hallway. She had been stabbed 14 times.

Ms Adeyola had lived next door a couple of years before the murder and traces of her DNA were found at the murder scene, it was claimed.

After her arrest last May, Ms Adeyola, of Camden, north London, was heard in secretly recorded conversations trying to come up with a false alibi, the jury was told. She considered trying to bribe a bus company employee to say she was at a depot at the time of the murder or altering the time on a shop receipt.

She then tried to explain the traces of DNA by saying Mrs Mendel had scratched her hand as she helped her cross a road the day before she was killed. Ms Adeyola, now 18, denies murder and two charges of perverting justice. The trial continues.