Mark Haddon: 'I don't just write books for children'

"Curious feels like it doesn't belong to me anymore," says Mark Haddon. "I've read it too much, I've talked about it too much... It's gone flat." The writer is talking about The Curious Incident of the Dog in the Night-Time, his crossover novel that became a word-of-mouth sensation for kids and adults when it was published.

It's a relief that Haddon has mentioned Curious first, even if it is to make clear his dissociation. I am under instruction not to focus on his most famous novel because he is apparently sick of talking about it. Except that he isn't. Haddon, a one-time children's author, is sitting in a swanky west London hotel, feeling underdressed for not having worn a swanky suit to match, and is happy to talk about anything at all, not least his debut adult novel – which brings us back to Curious. In 2003, the book became a global bestseller, winning 17 awards including the prestigious Whitbread Prize. Nearly a decade later, the story of the suburban maths whizz-kid (wrongly identified as an Asperger's sufferer on the dust-jacket) is to be staged as a play at the National Theatre this month and there are plans for a film adaptation, too. The play will star Luke Treadaway, Una Stubbs and Nicola Walker, and has been adapted by Simon Stephens. The book's phenomenal success, reflects Haddon, allowed him to take his time to write his next novel (A Spot of Bother, in 2006), with a poetry collection and a stage play in between.

Now comes a third adult novel, The Red House, about a family trip to a country cottage that brilliantly gets into the heads of all of its characters, from an eight-year-old boy to two teenage girls and an unfaithful husband. Even now, he doesn't have to churn out conveyor belt novels just to pay the rent. Recently, before beginning The Red House, he went as far as throwing away 150 pages of a half written novel (plus reams of notes) because it wasn't panning out.

"It was too purple, too clever by half. It was an awful realisation that dawned on me slowly. It was like ending a marriage, not that I ever have, but that sense of a relationship getting worse and worse, not doing either of you any good. It's a great relief when you're free of it.

"One of the freedoms you get if you earn a lot of money from a book is to throw away what you want. And if you throw a lot away, the good stuff always comes back, nothing is lost. One of the binding themes of the book I threw away was of children who have died which emerges as a major aspect of The Red House."

At 49, Haddon is a mix of middle-aged father ("I just have to make a call to get the child-minding sorted out"), big kid ("I feel slightly out of place here"), nerd ("I'm anti-Kindle because of the typeset for the en- dashes and em- dashes are exactly the same") and intellectual ("I like to re-read Ulysses from time to time").

These numerous sides to Haddon correlate with an almost childlike zeal for numerous artistic passions: he is a cartoonist and illustrator and was on the verge of including illustrations in The Red House before pulling the plug ("they weren't quite right"). When he's not a novelist, he's a poet, TV script-writer or stage dramatist (his TV scripts have won Baftas and his play, Polar Bears, was staged at the Donmar Warehouse). Soon he will stage a solo exhibition of paintings, too . He recently turned to portraiture and put together images featuring fellow artists. "Years ago, an interviewer called me a Renaissance man, and my wife rather tartly said, 'More like a Jack-of-all-trades.' I am a Jack-of-all-trades."

Or he could be the grown-up embodiment of the Agent Z series in which a gang of boys create a club which "battles against boredom". It's more a case of prevarication than boredom, he says. He tends to switch to another medium when the present project is not going as smoothly as it should – he reaches for the paintbrushes when the novel is stalling . The boundaries are easily blurred, as his art studio is also his study.

The son of an architect, his early interest lay in maths until he swerved into the arts and graduated from Oxford with an English degree. His father's profession nevertheless left him with an obsession with houses and he regards the eponymous "red house" of his novel as seriously as he does a living character. "I'm very aware of different houses. My parents now live in a house with thick stone walls, they are like burrows. I thought I'd live in a Modernist house with lots of glass – that's what I grew up thinking adults lived in. But what I live in is a late Victorian period house because the Modernist house would only look good if the children's pants weren't all over the floor!"

Haddon wrote his first children's book in 1987 as a stepping stone to adult literary fiction. Fifteen books later in nearly as many years, he was still struggling to make the leap. He likes to remind interviewers – and maybe also himself – that he wrote unpublished novels in this period. Would they be worth reviving now? I ask. No, he says definitely not, or only as examples of how not to do it.

When the idea for Curious came to him, his agent suggested it be marketed for children as well as adults – an idea which dismayed him at first, given how hard he had striven to change direction.

"It was a publishing arrangement [for the book to be simultaneously published by Jonathan Cape and David Fickling] or a stroke of genius because if a book has two ISBN numbers, it can go into two different parts of a bookshop.

"Initially, I was slightly depressed by this as I was trying to write an adult novel and get away from children's books. I felt like I'd dug a tunnel for years and come up in the commandant's office."

It was published at the height of the Harry Potter frenzy, when JK Rowling had suddenly "made it acceptable for adults to read children's book in public". Yet in spite of the similarities in its crossover appeal, Haddon's book was never written for kids and adopted by parents. He set out to make his story deliberately different from what was out there. The "sheer oddity" of the book, as well as its postmodern qualities, are sometimes overlooked, he says.

What is marked in his latest novel is the narrative democracy between characters; the voices of the children and the voices of the adults are equally interesting and equally dramatic. In this respect, it is his truest crossover work of fiction. "It began with the story of the [mother] Angela, but I became just as interested in the lives of the children."

What The Red House also has in common with his other two adult books – Curious included – is its focus on the dark side of family life – relationships going wrong, teenage angst and sexual frustrations, as is the case with the teenage daughter, Daisy, who may or may not be a lesbian in this latest novel.

When I suggest that the novel encapsulates the social and psychological mores of middle-class life, he agrees, but balks at the way in which the "middle-class novel" has come to be seen as less authentic than one with a more gritty urban, working-class or immigrant experience. "A book about the life of a Bangladeshi woman living in a small community in Coventry is seen as more real."

Yet there is enough grit in Haddon's books to keep them from twee destinies. Where A Spot of Bother dramatised the onset of dementia, gay sex and self-harm, this novel draws out the equally hardcore themes of mourning a stillborn child, sibling estrangement, teen sexualities and the emotional stasis that stalls some marriages.

Ironically, he gets the greatest pleasure in creating characters who are least like him, though this is formally more of a challenge. "[In writing this novel] I wanted to be equally fascinated by everyone but I was more fascinated by the teenage girls. I don't have any teenage daughters and they are the least like me so they were the hardest to conjure into being, but the most interesting for me. It was the same with James, my favourite character in A Spot of Bother, who was gay.

It took him a while to realise that family life was what he was really keen on writing about. "I'm really interested in the extraordinary found in the normal. Hopefully, my books don't take you to an entirely different place but make you look at things around you. I realise I write about families. Even Polar Bears [a play with a mentally ill character] is about families. Madness doesn't happen to someone alone. Very few people have experiences that are theirs alone."

Then there is his own, real-world family life: married to Dr Sos Eltis, an Oxford University don, he has two sons aged eight and 11. The boys have read some of his books, he says evasively.

Haddon seems to keep himself in check with a strict sense of modesty. If he meets a big fan of his books then "it's a bit disconcerting. It's nice people like them but if people think I'm mildly famous then that's different..."

His ethos for parenting is not to impose his interests but to let the children discover their own passions. His oldest son, Alfie, has naturally drifted towards writing and drawing, just like his father, while his younger son says he wants to be a teacher (Haddon also teaches once a year at the Arvon Foundation, a writing charity), yet he has pushed neither into those choices.

"Before I wrote Curious, I felt like I was going to be one of those pushy parents. But afterwards, I realised I had satisfied myself. Some parents want the kids to be what they haven't been able to be. Now, I have two rules for the kids: I want them to be happy and to be kind. More than that and you're doing it for yourself."

'The Red House' by Mark Haddon is published by Jonathan Cape, £16.99