This is not as large of a change as it seems: single-family zoning was already (quietly) dead in California. Under SB13, almost all single family parcels can already have both an ADU (Accessory Dwelling Unit) a Junior ADU, for a total of three units per parcel.

SB9 increases that number from 3 to 4: you can now split the lot, and each half can have a duplex. This does give owners more options though: they can sell the other half of the lot, rather than having to rent out ADUs.