Renovate the basement first or buy the building that already has the second unit

I gut and rebuild for a living, so I lean toward buying the cheaper house and building the rental unit myself. A basement apartment on a house I already own costs me materials plus my own weekends, and I control the finish level. The problem is the timeline. Nobody pays rent while the framing inspection is pending, and permits in my county have taken anywhere from three weeks to five months with no pattern I can predict.

The other path is paying up front for a duplex or a house with a legal second unit already in place. More money at closing, higher payment, but a lease can start the month you move in. If you want to count that rental income toward qualifying, an existing unit with an existing lease is a much easier conversation with a lender than a plan on graph paper, and the newer Fannie Mae approach to counting projected income on one-unit owner-occupied purchases has its own conditions that you need confirmed in writing by whoever is underwriting you.

For a first-time buyer with limited cash and no construction experience, I honestly don't know which one I'd pick. Build cost overruns are real and I've watched people run out of money at drywall. Overpaying for finished units is also real, and you can't renovate your way out of a bad purchase price.

Curious where the room lands on this, because I suspect the trades people here and the buy-finished people here are going to split hard.