The way this works with ADUs is a blended approach. The existing home's tax status is protected by Prop 13 and the ADU as a structure is appraised. While adding an ADU adds considerable resale value to a home, tax assessors don't typically value them that highly. It's also worth noting the ADU also becomes protected under Prop 13 after the initial assessment (https://maxablespace.com/granny-flat-tax-implications/)