Thus, SB9 increases the limit from 3 units to 4 units, not 6.
https://leginfo.legislature.ca.gov/faces/billTextClient.xhtm...
“Notwithstanding Section 65852.2 or 65852.22, a local agency shall not be required to permit an accessory dwelling unit or a junior accessory dwelling unit on parcels that use both the authority contained within this section and the authority contained in Section 66411.7”
However, it does not appear that the law prevents an ADU from being built without any additional rezoning, just that local cities may choose to approve/disallow it as they see fit
https://leginfo.legislature.ca.gov/faces/billNavClient.xhtml... (section 4).
Do the lot split first, then the duplex construction, then construct 2 ADUs. 8 units where a SFH was before.
SB 9 is paradigm shifting: every lot in California that’s single family zoned can a) be split into two lots AND/OR b) each resulting lot can now have up to 2 units AND c) if the lot is compliant with all other criteria, each lot is still able to have an ADU built in addition to the extra unit, per statewide ADU law passed two years ago
So you can potentially turn a single lot into 6 units. Or 3 without splitting the lot.
All of this is by-right, too. Design approval and local councils can’t stop it, and I believe they must be approved within 90 days, too.