It is true, adding an ADU will affect your California property taxes, but your primary house will not be reassessed, and your property taxes will only increase based on the added value of your ADU.
If you have low property taxes because you have owned your home for many years, your base assessment will not be affected. The assess or can only review the value of the improvement.
Junior ADUs or interior conversions have much less (or in some case, no effect) on your taxes.
And for those of you who think about 1031exchanges, the portion of your primary home that is rented as an ADU could be eligible for this potentially significant tax savings.