Hi Edwin,
As a general rule in Los Angeles with SFR, a nice ADU can easily add $100k of value or more. I've been seeing more and more listings that have ADUs on the market, even including small multifamily, so it should be possible to find comps. However, with a duplex or any multi-unit, the valuation is based more on the income and specifically the NOI, rather than the comps. An appraiser would likely use the income approach for multi-unit, whereas they would use the sales comparison approach for single family.
So rather than looking for a rule of thumb, the real question is how much income could the ADU add to the bottom line? Let's say hypothetically the additional unit can rent for $1,200 per month, and after expenses it will add $10,000 per year to the property's overall NOI. If you spend $100,000 on the ADU, then that would equate to a 10% ROI in the first year.
Then, let's say you are assuming a 5% cap rate to value the property. So, a property with 50,000 NOI would be worth $1MM based on a 5 cap. If the ADU add can $10k per year, then it brings the total NOI to $60k, which at a 5% cap brings the property value to $1.2MM. So, the $100k ADU has effectively increased the value of the property by $200k. Of course this is a very rough example based on a lot of assumptions, but hopefully you get the idea.
I would start by researching the market cap rate for small multifamily in your area, and pull lease comps to get an idea of what the ADU could rent for. Then once you know the numbers you can work backwards to see what the value-add would be.
Good luck!