Is an SB 9 duplex multifamily under Title 24?

No. Part 6 defines a single-family building to include a Group R-3 building with two or less dwelling units, so a duplex follows the single-family standards and uses the CF1R forms. The multifamily rules start with Group R-2 buildings.

Does an SB 9 unit need solar?

A new detached house or a new duplex does, as a newly constructed single-family building, unless an exception applies. A unit built as an addition to the existing house does not, because PV is not required for additions.

Does splitting the lot change the climate zone?

No. The climate zone follows the location, so the new lot normally shares the original one. What changes is the project name and address on each certificate, which must match that permit's plans.

Are SB 9 units treated like ADUs under the Energy Code?

The Energy Code has no category for either. HCD treats SB 9 units as primary units, distinct from ADUs, but for energy purposes both are classified by how they are built: new construction, addition or alteration.

SB 9 Lot Splits and Title 24

SB 9 decides whether a second home or a lot split must be approved; the Energy Code decides how each unit is built, and it looks at the building, not the parcel. A new detached house is new single-family construction, a new duplex is still single-family under Section 100.1, and a unit built onto the existing house is an addition.

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SB 9 lets an owner in a single-family zone add a second primary home, split the lot in two, or both. It is a planning law, and the HCD fact sheets that explain it deal with zoning, subdivision standards, tenancy and review times, not energy. The energy rules arrive later, with the building permit for each unit, and they turn on what is built rather than on how the land is divided. This guide takes the two halves in the order you meet them.

Which SB 9

Two different bills carry the name. The one people mean is SB 9 of 2021, Chapter 162 of the Statutes of 2021, codified in Government Code sections 65852.21 and 66411.7: ministerial approval of up to two primary units on a parcel in a single-family zone, of a split of that parcel into two, or both. HCD has had authority to enforce those sections since 1 January 2024. A second SB 9, Chapter 510 of the Statutes of 2025, amended state ADU law on how local ADU ordinances are submitted to HCD, and took effect on 1 January 2026. It has nothing to do with lot splits.

What the 2021 law allows, briefly

Units. Up to two primary units on a qualifying parcel. After a lot split, up to two units on each new lot, and an ADU or JADU counts toward that two. Without a split, the two primary units can sit alongside ADUs and JADUs allowed under ADU law.
Size and setbacks. Local objective standards may not prevent units of at least 800 square feet. Side and rear setbacks are capped at four feet, and none may be required for an existing structure or one rebuilt in the same location and dimensions.
Review. A complete application for a housing development or lot split must be approved or denied within 60 days of receipt, ministerially, without a hearing.
Conditions. Rentals must be for terms longer than 30 days. A lot split applicant signs an affidavit of intent to live in one of the units as a principal residence for at least three years, unless the applicant is a community land trust or nonprofit organisation. Among other exclusions, the law does not apply where the project would demolish or alter housing occupied by a tenant in the last three years.

Your planning department applies these rules to your parcel, and HCD's fact sheets give the detail. For ADUs added alongside, see California ADU rules and Title 24.

How the Energy Code classifies each unit

Part 6 sorts residential buildings by occupancy group and number of dwelling units. Its definition of a single-family building includes a Group R-3 building with two or less dwelling units, and a townhouse; a multifamily building includes a Group R-2 building. The Energy Commission's multifamily manual puts it plainly: single-family homes, duplexes and all townhouses follow the single-family requirements. The usual SB 9 layouts therefore sort like this:

A new detached house, beside the existing one or on the new lot, is a newly constructed single-family building. It meets the mandatory and prescriptive or performance requirements for new homes, including newly installed solar PV unless an exception applies. See new home Title 24.
A new duplex, two units under one roof, is also a newly constructed single-family building, documented on the CF1R forms rather than the multifamily set. See does a duplex need Title 24.
A second unit built onto the existing house adds conditioned floor area and volume, so it is an addition under Section 150.2(a). Solar PV is not required for additions. As a new dwelling unit it needs whole-dwelling ventilation sized on its own floor area (§ 150.2(a)1Cic), and CMC 311.4, which bars one unit's return air from being discharged into another, keeps it off the house's ducted system.
Dividing the existing house into two units within its existing conditioned space is an alteration, judged component by component under Section 150.2(b).
A third dwelling unit under the same roof takes the building past the two-unit line in the single-family definition, so confirm its occupancy group with the building department before design. See triplexes and fourplexes.

What the lot split changes, and what it does not

Nothing in the energy classification depends on parcel lines. The climate zone comes from the location, not the parcel number, and a building's type comes from its occupancy and unit count. What the split changes is paperwork. Each certificate of compliance must identify the project name and location consistently with the plans submitted for that permit, so make sure the energy documents use the same name and address as each set of drawings. And if the two units go in under separate permits, each is checked against the code in force on the date its own application is made.

Solar on two new houses

Each newly constructed single-family building carries its own PV requirement. Equation 150.1-C sets the minimum size from conditioned floor area and dwelling-unit count, using two factors from Table 150.1-C, capped by the solar access roof area (SARA). That area excludes roof with less than 70 percent annual solar access, and for steep roofs, shading from existing permanent obstructions such as trees, hills and adjacent structures counts. An addition, by contrast, carries no PV requirement. Exceptions are on residential solar requirements.

Order of work on an SB 9 project

Confirm eligibility, and the split if you want one, with the planning department.
Settle each building's type: new detached house, new duplex, addition or alteration.
Commission energy documents for each permit, named to match its drawings.
Build, test where required, and pass a separate final for each permit.

The Title 24 timeline covers the stages inside each permit.