This kind of project doesn’t have to be as complicated or as expensive as it sounds. “Many houses are well-suited to be modified without too much expense to include an autonomous rental apartment," said Joanne Cleaver, author of The Career Lattice. "The rent is great extra income, and the homeowners will save on taxes through pro rate write-offs of certain expenses. Just be sure to check with your home insurer first about maintaining coverage or find a new insurer that will meet your new needs.”
Also remember to follow zoning and other laws.
If you aren’t crazy about dealing with the hassle of managing a rental unit, you might want to consider hiring a property management company. You’ll sacrifice some of your profit, but it could eliminate a lot of headaches.