I’m aware that a number of authors (even those on the bestseller lists) do not draw up a plot before they start writing a novel. Stephen King is probably the most famous such author and he says this in his book On Writing:
You may wonder where plot is in all this. The answer—my answer, anyway—is nowhere … I distrust plot for two reasons: first, because our lives are largely plotless, even when you add in all our reasonable precautions and careful planning; and second, because I believe plotting and the spontaneity of real creation aren’t compatible.