The House in the Cerulean Sea reads like a gentle, low-conflict book, and it's easy to assume gentle books come from a loose, unplanned process. T.J. Klune's actual method for writing it was close to the opposite.

The Direct Answer
With extremely detailed outlining, driven by his ADHD, that combined the plans for Cerulean Sea and its sequel into a single document running close to 100,000 words before either book was drafted.
Klune has said he plots out ridiculous amounts, including practical story details and personal facts about characters that never appear on the page.

An Outline Nearly as Long as a Novel

Klune has said he "plots out ridiculous amounts" because of his ADHD, and that the combined outlines for The House in the Cerulean Sea and its sequel, Somewhere Beyond the Sea, ran to roughly 100,000 words before he considered either draft ready to write. That's close to the length of a finished novel spent on setup before a single scene got drafted.

The outlines go beyond plot mechanics. Klune has said they include details with no direct function in the finished book, a character's birthday, their favorite food, small facts that exist purely so he knows the people he's writing well enough to keep them consistent across hundreds of pages. That groundwork gives him room to write warmly on the page precisely because the mechanics are already solved before drafting starts.

A Floor, Not a Cage

Even with that much preparation, Klune has described staying open to new ideas mid-draft, restructuring earlier material if something better occurs to him partway through. The heavy outline works as a foundation solid enough to build differently on top of, not a rigid cage, without the whole project collapsing. For the fuller picture of how this fits into his daily schedule, see the full T.J. Klune writing routine.

What You Can Steal