A few observations about outlining, gathered from watching too many novels stall in the planning stage:
The outline you have before you start writing is less important than the outline you make after chapter five, when you've learned something about your story that you didn't know going in. The first outline is a guess. The second one is informed.
Agatha Christie reportedly said she worked out the solution to her mysteries before she wrote the opening paragraph, then wrote the novel working backward from what she already knew. Her readers thought she was discovering the mystery alongside them. She was staging a performance of discovery.
There's a version of outlining that's really just procrastination with extra steps. Color-coded index cards, three-ring binders, plot software with seventeen fields per scene. If you've spent more time organizing your outline than writing prose from it, you already know what I'm talking about.
E.M. Forster's distinction between "story" and "plot" is still the most useful framing I've found. A story is "the king died, and then the queen died." A plot is "the king died, and then the queen died of grief." The outline is where you turn your story into a plot. Without it, you often don't know which scenes are connected by causality and which are just adjacent.
An outline is a hypothesis, not a contract. The draft is the experiment.
Most writers outline the external story and forget to outline the internal one. Your protagonist's external journey: from the wrong side of town to the manor house. Your protagonist's internal journey: from believing they don't deserve a life to believing they do. Both need to end at the same scene. If you've only outlined one of them, you've only outlined half the novel.
The writers who say they don't outline usually have the clearest sense of where they're going. They've internalized the structure they claim to reject. Ask them enough questions and you'll hear a full three-act breakdown come out in conversation, described as though they just stumbled onto it.
Your outline is most useful at the scene level, not the chapter level. "Chapter 12: conflict between protagonist and mentor" is too vague to help you write anything. "Scene: protagonist finds mentor's draft letter resigning from the project; mentor doesn't know she's seen it" is specific enough to sit down and generate the actual scene, because now you know who wants what, who knows what, and what's at stake in the room.