ADUs, JADUs and SB 9: adding units on small residential lots

For the smallest infill deals — adding units to an existing house or duplex lot, or building new small-lot product — the controlling law is the state ADU/JADU statute, recodified in 2024 at Government Code §§ 66310–66342. It is aggressively preemptive: local standards that conflict with the state floor are void, and a non-compliant local ordinance means the state standards apply directly.

SB 9 (Gov. Code § 65852.21 and § 66411.7, in the Zoning Regulations and Subdivision Map Act) layers on ministerial two-unit projects and urban lot splits in single-family zones. Together these tools can turn one single-family lot into as many as four to six sellable or rentable units — but each has conditions that change the underwrite.

Ministerial approval within 60 days of a complete application, no public hearing, no discretionary design review. Cities must approve at least one ADU and one JADU per lot with a proposed or existing single-family dwelling, and prescribed numbers of ADUs on multifamily lots (including conversion of non-livable space such as storage rooms and garages, plus detached units).

State standards cap what locals can demand: no minimum lot size, maximum unit-size floors the city must allow (at least 850 sq ft / 1,000 sq ft for 2+ bedrooms), 4-foot side and rear setbacks, no replacement parking when a garage converts, and no parking at all within a half mile of transit.

• Underwriting watch-outs:
• Impact fees: prohibited for ADUs under 750 sq ft; larger ADUs pay fees only proportionate to the primary dwelling (§ 66324 area). School and utility connection charges have their own limits.
• Owner-occupancy: barred for ADUs permitted through 2024 and constrained after; JADUs still require owner occupancy — a JADU on a rental asset ties the owner to the property.
• Separate sale: ADUs generally cannot be conveyed separately from the primary residence, except the § 66341+ pathway for qualified nonprofit sales and condo-style structures adopted by local ordinance — do not underwrite condo exits without confirming the local ordinance exists.
• Rent restrictions: state law does not require ADU affordability, but AB 1482 caps and just-cause rules can apply to the rental units — see the rent-regulation guides.

SB 9 requires ministerial approval of (a) two-unit projects on single-family lots and (b) one urban lot split creating two roughly equal parcels, in urbanized areas, subject to objective standards, short-term-rental bans and a lot-split owner-occupancy affidavit. Stacked, the tools allow up to four primary units on what was one single-family lot — before ADUs are added.

• Underwriting watch-outs:
• Charter cities: a Los Angeles trial court held SB 9 unconstitutional as applied to charter cities (Redondo Beach v. Bonta), and appellate practice is still settling. In charter cities, treat SB 9 capacity as upside, not base case, until counsel confirms current status.
• Sites with recent tenancies, rent-controlled units, or Ellis Act withdrawals are excluded — the same protected-housing screens as elsewhere in state housing law.
• Objective local standards still apply (size, setbacks, design) so long as they do not preclude two 800 sq ft units.
• Lot splits carry a 3-year owner-occupancy intent affidavit — an investor buying to split immediately does not fit the statute.

The combinations matter more than any single statute: an existing single-family lot can host the primary home, a converted-garage ADU and a JADU ministerially; a multifamily lot can add up to two detached ADUs plus conversion ADUs inside non-livable space; an SB 9 split can double the lots first. For 5+ unit new construction, the Density Bonus Law and the by-right statutes in this topic's other guides usually dominate instead.

Because every one of these paths is ministerial, they also avoid CEQA — see the CEQA guide on ministerial approvals. That is often the decisive schedule advantage for a small developer.