What was J.K. Rowling's first completed novel for adults?

J.K. Rowling's first completed novel for adults was The Casual Vacancy.

Published by Little, Brown and Company in 2012, The Casual Vacancy is a standalone novel about tensions in the fictional English town of Pagford. Its story begins after the sudden death of parish councillor Barry Fairbrother, creating a vacancy on the local council and exposing divisions over poverty, class, and public services.

The book was Rowling’s first major work after the Harry Potter series. It is not a fantasy novel and does not take place in the Wizarding World. That change of genre led many readers to approach it with expectations shaped by Harry Potter, even though the novel is written for adults.

Rowling later returned to crime fiction with the Cormoran Strike books, published under the name Robert Galbraith. The Casual Vacancy is therefore distinct both from her fantasy series and from her detective novels.