J.K. Rowling had the idea for Harry Potter in 1990, on a delayed train between Manchester and London. The gap between that idea and a finished draft ran far longer than the train-platform origin story usually gets credit for.

The Direct Answer
Rowling spent years developing the world and the seven-book arc before treating any of it as finished prose, then wrote inside a structure she'd already built.
She's described spending roughly five years developing characters, magical rules, and the overall story before Harry Potter and the Philosopher's Stone was done, and mapping the series before writing much of it.

Five Years Before Chapter One Was Finished

The idea arrived complete in a way most ideas don't. By her own account, when the train idea struck in 1990, most of the characters and the shape of the story arrived within that single ride. Turning that arrival into a publishable book took years anyway, because Rowling spent the time before finishing the first book building material that had nothing to do with prose style. Family histories. The rules that would govern how magic worked and where its limits sat. A timeline of the wizarding world stretching back centuries. None of it appears directly on the page of Philosopher's Stone, but it's the reason the world holds together for seven books without contradicting itself.

This kind of front-loaded worldbuilding is unusual for a first-time novelist, who typically doesn't have the patience to delay actual writing that long. Rowling has said the delay wasn't really a choice, more a necessity: she didn't feel ready to start drafting until she understood how her own invented world worked well enough to trust it. That's a different model from the write-to-discover method plenty of novelists use, and it's worth taking seriously if your own early chapters keep contradicting your later ones.

Planning Isn't a Separate Stage

Where Rowling's process breaks from a lot of standard writing advice is in how she treats planning itself: real structural work on the books, not a chore to clear out of the way before writing starts. The notebooks, family trees, and eventually the hand-drawn grids she built for tracking plot were construction work on the books themselves.

The clearest evidence is the seven-book structure. Rowling has described knowing the shape of the whole series, including where certain characters would end up, well before she'd published the first installment. That's only possible if outlining counts as writing rather than something writing waits for. It also explains a detail careful readers have noticed for years: objects and characters that seem minor in early books turn out to matter later, because those threads were plotted from the start rather than invented under deadline pressure in book six or seven.

See how it fits together in the full J.K. Rowling writing routine.

What You Can Steal