JK Rowling's first novel aimed at adults will be a "blackly comic" tale of small town politics that at first glance couldn't be further from the magical world of Hogwarts, her publisher has revealed.

The Casual Vacancy, perhaps the most hotly anticipated book of the year, will be published on 27 September and is set in a small town called Pagford, described by her new publisher Little, Brown as an English idyll "with a cobbled market square and an ancient abbey".

The story is set in motion by the unexpected demise of Barry Fairweather, a stalwart of the town's parish council who dies in his early forties.

Pagford's chocolate-box façade hides a town riven with strife, and the struggle to replace Fairweather "becomes the catalyst for the biggest war the town has yet seen", with "teenagers at war with their parents, wives at war with their husbands, teachers at war with their pupils," the publisher said.

As the election to find his replacement unleashes "passion, duplicity and unexpected revelations", the novel puts Pagford's rivalries under the microscope.

The subject matter raised eyebrows in the publishing industry; there had been speculation that Rowling might be entering the genre of hard-boiled Edinburgh crime fiction after discovering that her editor, David Shelley, counts Dennis Lehane, Val McDermid, Carl Hiaasen and Mark Billingham among his authors.

Neill Denny, editor of the Bookseller, said it was an "interesting move" from Rowling: "It's not the niche people were steering her into," he said. "This is clearly not a traditional crime novel, it sounds quirky, it sounds interesting – and ambitious. It's a niche with little else in it, which perhaps only she can fill.

"A lot of people will be curious to see if she can move from writing young adult titles to writing for adults," he added.

Booksellers will be hoping this comedy of British provincial life can match the commercial might of Rowling's books for children, which have helped her to amass a personal fortune estimated at more than £600m. The seven titles that make up the Harry Potter series have sold 450m copies worldwide, and have produced eight films, which grossed more than $7bn. The boy wizard is also the focus of a theme park in Orlando, Florida and now a tour at Leavesden Studios just outside London.

Rowling has spoken of moving into a "new phase" of her writing life. She has always said it was "highly unlikely" she would write more Harry Potter novels, though she teased fans by telling Oprah Winfrey in 2010 that she could "definitely write an eighth, ninth [and] 10th book ... I think I am done but you never know".

Earlier this year Rowling said her latest book would be "very different to the Harry Potter series", though she "enjoyed writing it every bit as much. The freedom to explore new territory is a gift that Harry's success has brought me", she said.

Author Michelle Paver, who followed her bestselling Wolf Brother series of books for children with Dark Matter, a ghost story for adults hailed in the Guardian as "a spellbinding read", said making the transition into adult fiction was "a scary experience".

"But it's not as if adults are any more critical than children," she continued. "In fact you could argue that children are the harshest critics – they're not going to keep turning the page if it's not gripping. You've got to try to find a story that you believe in and then whether it's for children or adults isn't that important."

For Denny, the transition from fantasy adventure to comedy of manners is just as daunting as the switch to an adult audience. Fans of the Harry Potter books will cite the Weasley twins as evidence of Rowling's comic talents, but for Denny "comic writing, as a genre, is a tricky one to pull off. If you look at commercial authors writing comedy, they're not massive sellers".

Denny said he is confident that The Casual Vacancy will sell more than 1m copies in the UK, citing the Harry Potter off-cut The Tales of Beedle the Bard which topped the bestseller charts in 2008, because the Rowling name "is so powerful".

"The bestselling book typically sells 20% more than the number two title," he said. "But a bestselling title from JK Rowling will sell 20 times more than other titles. The scale is freakish. The success of the films has created a new audience for her writing – the film franchise feeds back into the book franchise."

More than a decade after the first publication of Harry Potter and the Philosopher's Stone, readers' appetite for Rowling's work seems undiminished, with electronic editions of the series amassing sales worth more than £1m in just three days after their release last month.