Adding an accessory dwelling unit or finishing a basement is the highest-leverage move in house hacking — a whole new rent check on land you already own. Model the build, the financing, and the rent, and see the payback and the return on the money you put in.
Estimates only — ADU costs and rules vary enormously by city, and many places restrict them. Confirm zoning, permits, and contractor bids before you count on any of this. Not financial advice.