We bought a rental property with a ADU 17 years ago and it has paid off well. Especially now that I just retired the beginning of this month, we sold our primary home and moved into the rentals main unit. We are making enough off the ADU to cover property taxes, homeowners insurance, utilities and more!
If you plan to own this property for a long time, it would be a worthwhile investment, including raising the resale value!
The permit process is sometimes a pain to go through. For instance, if the house is older, will you be required to have fire sprinklers and will they make you install them in the whole house? Things like that come up depending on your county requirements.
Being your own GC is a huge plus and will save you a lot of money if you have time to do some of the work.
Good luck!