Helena Coggan explains that it was her teacher at primary school who first speculated that she might write a novel: “When I was six, I thought the pinnacle of human achievement was writing a book – not knowing about the Olympics. I promised myself I would write a novel before I was 13. And when I turned 13 I thought: ‘I may as well attempt this now.’ There is a seat at our dining room table, and for two years I’d sit there with my dad’s laptop until – my apologies! – it broke.”

The apologies are directed at her father Philip, who is sitting in on the interview. And we are here because the novel Helena started at 13 is, remarkably, about to be published.

The Catalyst is a phenomenal achievement: a fantasy novel with a cast divided among the Gifted and mere mortals, the Ashkind. The 15-year-old protagonist, Rose Elmsworth, is one of the rare “hybrids” who become monsters at times not of their own choosing. Rose and her adoptive father (also a hybrid) struggle with their secret identities, keeping their monstrous selves under lock and key to limit damage. The narrative is assured, frightening, action packed. Nothing about it, except the age of its heroine, suggests it was written by the teenager in front of me.

It is a winter afternoon, in the offices of Hodder & Stoughton publishers, high in a London tower block. The PR for the book is keeping us company on the off chance that, at 15, Helena might need defending. But she needs neither paternal nor professional support. She is her own person: spirited, with an alert face and flowing dark hair. She has a racing intelligence, but also a steadiness that bypasses any conceitedness she could be forgiven for feeling.

She relives, with gusto, the moment she heard Hodder was publishing the novel. Georgina Laycock, at John Murray, read an early draft and passed it on to Hodder’s Kate Howard, now Helena’s editor: “We met on 20 November 2013. I don’t think I’m ever going to forget anything about that day for as long as I live. We came in, and Kate must have been bemused because, even when people know a 15-year-old girl has written the book, they don’t expect to see a teenager. Kate said she’d pitch it to the board. The next Tuesday she called Dad, who was in Washington. My mum was picking me and my sister up from school. He called us in the car and we put it on speaker and Dad said: ‘They want to publish the book.’ And I can quite confidently say I did my first proper teenage-girl scream and nearly burst the eardrums of everyone in the car. And then he said: ‘Not only that, it is a three-book deal.’ And we actually had to stop the car and get out and run up and down the street a little bit.”

I had thought Samantha Shannon, Bloomsbury’s accomplished novelist who published her first book at 21 (and also secured a three-book deal) was as young as a young adult (YA) writer could get. Then came 19-year-old Nigerian Chibundu Onuzo, who made her debut with Faber. But Helena wins the contest hands down, while acknowledging the limitations of her youth. When I talk about the sophisticated moral question she raises – whether it is possible to be innocently evil – she stresses: “I was trying to ask the question, not answer it. Any life lessons I can pass on are extremely limited. I am 15,” she says, laughing. “I don’t think my understanding of the world is inherently limited by my age, but I didn’t write the book with an agenda; I wasn’t trying to address a societal ill. Abdication of responsibility interested me.”

There is not much abdication of responsibility possible in her own teenage life. She has not neglected her schoolwork, though she says: “I hate studying for GCSEs.” She attends a London school and has not yet decided what she will do at A level or beyond – she is as interested in sciences as arts. “The world, as I see it, ends on 20 February [publication day], although any idea of myself as an author is still alien.”

‘I wasn’t trying to address a societal ill. Abdication of responsibility interested me’: Helena Coggan. Photograph: Pål Hansen

Helena was born in and grew up in London and lives in a “house filled with books”. Aside from the inevitable JK Rowling, she praises Derek Landy, whose Skulduggery Pleasant series – especially its fight scenes – influenced her. Philip Pullman, to whom you might suppose her indebted, almost put her off writing: “I loved His Dark Materials. But in terms of motivation to write, that series is terrifying because he explores deep things about theology and the human condition and, seriously, is that what you have to be saying to write a book?” The book she most wishes she could have written is Douglas Adams’s Hitchhiker’s Guide to the Galaxy. And she hails a “wonderful” book called How Not to Write a Novel, by Sandra Newman and Howard Mittelmark, “the closest thing I had to a holy text. It tells you that if the reader knows the plan, the plan must never work. It can’t – otherwise the suspense has gone.”

She also has a theory about teenage girls and the way they are “betrayed” by YA fiction. She launches into it, gathering speed as she talks. There are, she maintains, three girl types: “I like to call them the ducklings. They are: 1) the ugly duckling who becomes acceptable to society; 2) the naive duckling who knows nothing about her world; and 3) the antisocial duckling, who is not socially able.” Reading about these girls when she was 10 or 12, she thought: “Hang on, I’m sensing a common theme. The idea of teenage girls having to be ugly, naive or antisocial baffled me. I thought: what if you are fine? There are not many stories out there about a girl who is fine and confronted by things other than falling in love.”

She adds: “The world is usually saved in YA novels as the teenage girl figures out who she is and falls in love.” When I ask whether she has ever been in love, Helena replies: “The Catalyst is fantasy after all, and the whole point of the genre is to write about things that may not correlate exactly to your own experiences… If that’s all right, I would prefer to leave that one; it’s just a little too personal.”

But she goes on to say that reading such books as a teenager, “the overall impression is you can’t possibly know who you are because, whatever you think you’re like, you’ve not experienced enough to know the truth. And when that is being thrown at you – I’m sure none of the authors intend this – but when you read it over and over again, it undermines your self-confidence. [So you think]: ‘Is what I’m like OK?’”

Rose in The Catalyst is fine if you discount her hybrid flips. And there is no conventional love story, just one memorably nightmarish and humiliating date. To me, Helena seems more than fine and has a charming way of championing her supportive family. When I read the book I was playing a guessing game, wondering what her parents did for a living, imagining they must be high-powered. It was not until the final acknowledgements that I saw that they are journalists (her father works at the Economist; her mother Sandra is a radio producer at the BBC in the current affairs department). It would be easy to assume this was the clincher in getting the book published. Helena says: “I wouldn’t say it helps that they are journalists, but they are incredibly clever people. That is the useful thing.”

But she is aware that her father’s acquaintance with Georgina Laycock got the book read initially. And she acknowledges her good fortune, because getting anyone to read a first novel is always hard (whatever age you might be). But from my own experience of working in publishing, I’d say that while an introduction can be a lucky break, once that has happened you are on your own: writers must sink or swim according to their talents. “My parents have been fantastically supportive. And they were useful as primary editors before I had an actual editor because they are kind and insightful.” For the benefit of her listening father, I joke that I might have to tone this down in print. Helena protests: “Come on, you have got to put on the record how awesome my parents are!”

What most intrigued me is the book’s dedication to her younger sister Catherine, who sounds as remarkable as Helena. In it, Helena predicts Caroline will one day be “curing cancer or implementing world peace. Catherine is 12 and incredible,” Helena says. “I meant every single word – I couldn’t ask for a better sister.”

Catherine will have to get used to long stretches of having a sister typing away at their dining table. A sequel to The Catalyst is under way, and a standalone third novel will follow. “Please don’t ask me what it will be about,” says Helena. “That is a post-GCSE problem.” You can see what a stressful collision of demands there now is on Helena. But she reassures me she does not write or study all the time – she loves playing the guitar and composing sad songs. Having said that, sad songs are not getting a look-in now because, she says: “I’m so happy, I can’t imagine being happier.”

This seems a natural ending to our conversation, but she wants to tell me one more thing. “That English teacher I mentioned at the start? I told her about the book a few months ago. She was astonished. She said: ‘Oh! When I said I thought you could be an author, I meant when you were 30 or 40…’” And that surprised Helena, because she could not imagine putting any prediction on hold. Besides, by the time she was 13 she felt there was no time to lose.

The Catalyst is published on 19 February (Hodder & Stoughton, £14.99). To order a copy for £11.99, go to bookshop.theguardian.com. Helena will be at the Dark Societies event at Waterstones Piccadilly on 11 February at 6.30pm. For tickets call 020 7851 2400