Sharon Dogar found writing her latest book Annexed “particularly demanding”. This is not surprising, since she was trying to describe the horrors of the Holocaust to teenage readers. It would be difficult enough if she had stuck to the facts, but she has been brave enough to turn the revered diaries of Anne Frank into a novel, reimagining the events through the eyes of Anne’s friend Peter.

Written by an adolescent Jewish girl hiding from the Nazis in a Dutch attic, the diaries have been translated into more than 50 languages and are compulsory reading for many schoolchildren as part of Britain’s National Curriculum.

Ms Dogar said: “I have re-read the diary every year since I was 11. Until I was in my early thirties I thought the only way to see the annexe was through Anne’s eyes. Her perception of Peter is that he’s boring, always wingeing; that he would never get out of bed. When I re-read them more recently, I realised that he was depressed.

“I thought how interesting it would be to look at it from his perception, rather than hers.”

She thought around the problem for 15 years before beginning her story. Such is the sensitivity about fictionalising anything to do with the Holocaust that the publishers of her previous two books declined it, and she had to find a new publisher.

She said: “There is nervousness about portraying the Holocaust in fiction. You have to be absolutely secure and accurate about the facts because if you are not, there is a feeling that you might be playing into the hands of Holocaust deniers. It has to be handled with care.”

It took her three years to write, partly because she came to a halt at the point when Peter goes into the Nazi death camps — an experience that Anne’s diary did not record.

“I really didn’t want to think about what might have happened to him, and I started to wonder whether it was possible to describe it responsibly and well.”

But she came across a comment by Primo Levi, who wondered whether survivors had the right to tell what happened, because the story was not about survival, but death.

“I think he was saying that there is only one way to do this, and that is to imagine what it was like to die. When I had that thought, it enabled me to continue, and the final 70 pages came quickly.”

Her decision was vindicated by the judges of the Costa Book Awards, who described it as “a brave re-imagining of a harrowing story and an iconic figure”, shortlisting it in the children’s category.

When I met her in her Summertown home, she was still coming to terms with the Costa nomination. She said: “It’s wonderful. You spend your time as a writer in your own world and when something like this happens you realise that there are readers out there, listening.”

Ms Dogar, 47, has wanted to be a writer since she was four and was encouraged by teachers at Ivanhoe Middle School in Blackbird Leys, including Rod Hunt, of the Oxford Reading Tree books; and Philip Pullman, whose wife Jude later encouraged Ms Dogar’s writing. Before she moved to Kennington at 13, Ms Dogar lived in a council house in Blackbird Leys, which she loved. “People underestimate how wonderful it is belonging to a community where you can play in the street. There was lots of space and we roamed around in groups of all ages, having lots of fun and getting into innocent trouble.”

After studying English literature and comparative religion in London, she travelled round Pakistan on her own, trained as a hearing therapist and then as a psychotherapist. She came back to Oxford to work as a counsellor in local schools, and has lived here since, marrying and raising three children.

She is keen to emphasise that her first book, Waves, was picked from the slush pile, and that she did not use her friendship with Mr Pullman to get published, though her publisher did solicit a comment from him for the jacket.

She works two days a week as a psychotherapist with young people, supplementing her writing income, which until now has been uncertain. Will that change with the Costa nomination?

“That’s the difficulty with success. I would not change that moment when I heard, but you have to manage it properly,” she said.

* Annexed is published by Andersen Press at £12.99. The Costa children’s winner will be announced on Wednesday, January 5.