SB 9 Explained: California's Lot Split and Duplex Law for Homeowners

What Is SB 9 and Why It Matters for Homeowners

Senate Bill 9, also known as the California HOME Act (Housing Opportunity and More Efficiency), took effect on January 1, 2022. It is one of the most significant housing laws California has passed in decades. The law gives owners of single-family residential lots two powerful development rights: the ability to split their lot into two parcels and the ability to build a duplex (two units) on each parcel. Combined, that means up to four housing units on what was previously a single-family lot.

For Bay Area homeowners sitting on valuable land, SB 9 opens a path to significant property value increases, rental income, and long-term wealth building. Whether you want to build a second home for family, create rental units, or sell a newly created parcel, SB 9 provides the legal framework to do it.

What SB 9 Allows: The Two Main Options

SB 9 provides two distinct development pathways. You can use one or both on the same property.

Option 1: Urban Lot Split

An urban lot split divides your single-family parcel into two separate lots. Each resulting lot must be at least 1,200 square feet, and neither lot can be smaller than 40% of the original parcel. The lots must be roughly proportional; you cannot carve off a tiny sliver and keep the rest.

Once you complete the lot split, each new parcel receives its own title and can be sold, developed, or held independently. This is a fundamental difference from ADU construction, where the accessory unit remains tied to the primary property.

Option 2: Duplex Development

SB 9 also allows you to build two residential units on a single-family lot without splitting it. This can take several forms: constructing a new duplex, converting an existing single-family home into two units, or adding a second detached unit to the property.

Combining Both Options

The maximum development potential under SB 9 comes from combining both options. Split your lot into two parcels, then build two units on each parcel. The result is four housing units where one previously stood. Each unit is a primary dwelling, not an accessory unit, which affects how they are valued, financed, and taxed.

Eligibility Requirements

Not every property qualifies for SB 9. The law sets clear criteria that your property must meet.

Zoning and Location

Your property must be zoned for single-family residential use and located within an incorporated city or an urbanized area as defined by the U.S. Census Bureau. Properties in unincorporated county areas generally do not qualify unless they fall within a census-designated urban cluster.

Restricted Areas

SB 9 does not apply to properties located in:

• Very high fire hazard severity zones (unless the property is within a city that has adopted a fire hazard mitigation plan)
• Flood zones and floodways as designated by FEMA
• Earthquake fault zones mapped under the Alquist-Priolo Act
• Environmentally sensitive habitat areas or coastal zones with restrictions
• Conservation lands, wetlands, or prime farmland
• Properties with existing deed restrictions for affordable housing

Historic Districts

Prior to 2026, SB 9 excluded properties within designated historic districts. However, AB 1061, which took effect on January 1, 2026, changed this rule. Properties in historic districts are now eligible for SB 9 lot splits and duplex development, provided the project does not alter or demolish any existing historic structure.

Tenant Protections

SB 9 includes strict tenant protections. You cannot use the law on a property where any unit has been occupied by a rental tenant within the past three years. Properties with a record of tenant eviction within the last 15 years are also excluded. These provisions prevent SB 9 from being used to displace existing renters.

Owner-Occupancy: When It Applies and When It Does Not

The owner-occupancy requirement is one of the most commonly misunderstood aspects of SB 9. Here is how it works.

For lot splits: If you split your lot under SB 9, you must sign an affidavit declaring that you intend to occupy one of the housing units on either resulting lot as your primary residence for a minimum of three years from the date of lot split approval. This requirement is enforced through the recorded affidavit.

For duplex development without a lot split: If you simply add a second unit or convert your existing home into a duplex on the same lot (no lot split), SB 9 does not impose an owner-occupancy requirement at the state level. Some cities may have local requirements, so check with your planning department.

Short-term rental prohibition: Regardless of whether you split the lot, all SB 9 units are prohibited from being used as short-term rentals (stays of fewer than 30 days). This restriction is permanent and applies to all future owners.

The Ministerial Approval Process

One of the strongest features of SB 9 is its ministerial approval requirement. If your application meets all objective, non-discretionary standards, your city must approve it. There are no public hearings, no design review board meetings, and no neighbor notification processes. The decision is based solely on whether your plans comply with the applicable rules.

SB 450, which took effect on January 1, 2025, strengthened this process further by applying SB 9 to charter cities, limiting additional local standards cities can impose, and creating a firm 60-day approval deadline. If the city does not act within 60 days, the application is deemed approved.

What Cities Can and Cannot Require

Cities retain limited authority to impose objective standards on SB 9 projects:

• Setbacks: Cities can require up to four-foot side and rear setbacks for new construction
• Unit size: Cities cannot impose a minimum unit size below 800 square feet per unit
• Lot access: Each lot must have access to a public right-of-way, either directly or through an easement
• Parking: Cities can require a maximum of one parking space per unit, but cannot require any parking if the lot is within half a mile of a major transit stop

Cities cannot deny SB 9 applications based on subjective criteria like “neighborhood character,” aesthetic preferences, or cumulative impact concerns.

How SB 9 Differs from ADU Laws

Bay Area homeowners often wonder whether SB 9 or ADU construction is the better path. The two laws serve different purposes and offer distinct advantages.

Feature	SB 9	ADU Laws
Unit type	Primary dwelling units	Accessory (secondary) units
Zoning	Single-family zones only	Single-family and multifamily zones
Lot split	Yes, creates separate parcels	No lot split allowed
Sellable separately	Yes, each parcel can be sold	Generally no (some cities now allow ADU sales)
Owner-occupancy	Required for lot splits (3 years)	Not required for standard ADUs
Impact fees	Full fees apply	Waived for units under 750 sqft
Maximum units	Up to 4 (2 per lot after split)	1 ADU + 1 JADU per primary unit
Short-term rentals	Prohibited	Varies by city

The most important distinction is that SB 9 creates primary units with separate titles, while ADUs are secondary units tied to the main property. This makes SB 9 more attractive for homeowners who want to sell a portion of their land or build units that function as fully independent properties.

You can also combine both pathways. After an SB 9 lot split, each resulting lot can have ADUs added under standard ADU law. This layered approach maximizes the total number of units you can develop on a single original parcel.

SB 9 in the Bay Area: Local Implementation

While SB 9 is a state law, Bay Area cities have implemented it with varying degrees of enthusiasm and local overlay. A Terner Center study at UC Berkeley found that no two city ordinances were alike.

What This Means for Bay Area Homeowners

Some cities, like San Jose and San Francisco, have published clear SB 9 guidelines and streamlined their application processes. Others have adopted the minimum state requirements without adding local incentives or clarifications. A few cities initially attempted to impose additional restrictions, though state oversight has pushed most toward compliance.

Before starting an SB 9 project, contact your city’s planning department to understand:

• Local application forms and submission requirements
• Any objective design standards that apply (setbacks, height, lot coverage)
• Utility connection requirements and associated fees
• Timeline expectations for review and approval

The Association of Bay Area Governments (ABAG) provides resources and templates to help local jurisdictions implement SB 9, including draft staff reports, architectural drawings, and guidance on “missing middle” housing development.

Investment Potential: Why Bay Area Homeowners Should Pay Attention

The Bay Area’s combination of high land values, strong rental demand, and limited housing supply makes SB 9 particularly valuable for local homeowners.

Building Equity Through Lot Splits

A lot split effectively doubles the number of titled parcels you own. In markets where land alone is valued at $500,000 or more per parcel, the equity creation from a single lot split can be substantial. Even after accounting for surveying, legal, and permitting costs, the net value added by creating a second buildable parcel is significant.

Rental Income from New Units

Building rental units under SB 9 generates income comparable to ADU rentals. Bay Area two-bedroom units command $3,000 to $4,500 per month depending on location and finishes. With up to four units possible under a full SB 9 build-out, the income potential is significantly higher than a single ADU.

Long-Term Flexibility

SB 9 gives you options that ADU construction alone does not. You can build and hold all units for rental income. You can sell one parcel to fund construction on the other. You can house family members on one lot and rent units on the other. The separate titles created by a lot split give you maximum flexibility to adapt your strategy over time.

Steps to Get Started with SB 9

If SB 9 sounds like the right fit for your property, here is how to move forward.

1. Confirm eligibility. Verify that your property is in a single-family zone within an urban area and is not located in a restricted hazard zone or other excluded category.
2. Research local requirements. Contact your city’s planning department to understand their specific SB 9 application process and any local objective standards.
3. Hire a surveyor. For lot splits, you will need a licensed surveyor to prepare a tentative parcel map showing the proposed division.
4. Work with an experienced builder. SB 9 projects involve navigating state law, local ordinances, and construction requirements simultaneously. A builder with SB 9 experience can streamline the process and avoid costly mistakes.
5. Submit your application. File with your city and track the 60-day review timeline. Remember that your city must approve the application ministerially if it meets all objective standards.

Ready to Explore SB 9 for Your Property?

Custom Home Design and Build helps Bay Area homeowners navigate SB 9 lot splits and duplex projects from initial feasibility through construction completion. Our team understands the local permitting landscape across Silicon Valley cities and can help you determine whether SB 9, ADU construction, or a combination of both is the best strategy for your property.

Contact us today for a consultation to discuss your SB 9 project and discover how to unlock the full potential of your property.