ADUs seem to be really popular in California, depending on location. 

It depends on a variety of factors. 

Depends.

@Harsh PoshtiThis will be dependent on the area and I would recommend you perform a market study to determine which project would yield better results and if either is even viable.

Depends on your goals.

@Robert FrazierThis really depends on the market. 

It depends on the goal, are you trying to build wealth or cashflow.

Depending on how many properties you have that may be overkill.

Congrats on at least taking action.You've got nothing budgeted for vacancy that I can see; it depends on how much turnover there is in your market and what your prices are like compared to the rest of the market, but most people would budget 1 month of vacancy/year/unit.

Of course, with all things, the answers to all these matters will depend on the circumstances.California tends to have more laws on the books and requirements and restrictions that it can be a good idea to form a CA LLC for out of state property so that you as a CA resident are covered, and to try to have your contracts fall under the purview of CA courts.