Do I need a written exit plan, or is a rough idea enough to start?

There's no legal requirement for a written exit plan, and a rough idea is genuinely enough to start having conversations, doing preliminary preparation, or talking to an accountant and lawyer about your options. Nothing forces you to formalize anything before you're ready to.

The nuance is that "enough to start" and "enough to execute" are different thresholds. A rough idea works fine for early exploration, but once you move toward an actual transaction — whether a sale, a succession, or a merger — you'll need real documents regardless: a shareholders' agreement addressing the transfer if there are co-owners, tax and structuring advice specific to your situation, and eventually a formal purchase agreement or succession documents. Treating a rough idea as sufficient all the way through tends to leave people improvising important decisions under time pressure once a real buyer or successor appears.

A reasonable middle ground is writing down even a simple outline — your target timeline, your preferred path, and the handful of things you know need fixing — and revisiting it periodically with a business lawyer as it firms up, rather than either over-formalizing early or staying vague right up until a deal is on the table.