On the California Central Coast, is adding an ADU the only way an investment property can cash flow?

For the Central Coast specifically, that's close to true. Inland Central California pencils more easily because prices there sit lower relative to rents. Decide up front which game you're playing: a cash-flow play or an appreciation play. They get underwritten differently. In expensive California markets, most properties won't cash flow as a standard long-term rental unless you bring a large down payment, run a short-term rental where that's allowed, or add income with an ADU. An ADU can tip a marginal deal positive, but it's no magic bullet. The result hangs on three numbers: the rent the unit will actually command, the true construction cost, and your down payment. The common mistake is overestimating the first and underestimating the second, which turns a spreadsheet winner into a money pit. We did a full podcast episode on ADUs for exactly this reason. So do genuine due diligence before you count on one: - Get real build quotes, not per-square-foot guesses. - Verify what comparable ADUs actually rent for in that market. - Confirm local zoning and permitting before you write an offer. And nobody can promise appreciation, so don't rely on future price gains to rescue numbers that don't work today.