I was still living in California when they changed rules to allow for it and the cost to build an ADU was prohibitive at us$200k. I dont know how you make it cheaper, but if home owners can not afford to build, it will never get done.

EDIT : After thinking some more there are a few ways to encourage it. Zero interest loans or gov subsidy to help pay for construction. Expedited permit process. It would have to be coupled with rent controls though, otherwise the property owner is just profiting at everyone else’s expense.