Understanding SB9 and Its Impact on ADUs

Key Takeaways

Passed in 2021, California’s Senate Bill 9 (or SB9, for short) increased the permitted housing density throughout California in an effort to combat the state’s housing crisis by allowing a second single-family home to be built alongside an existing primary unit. In some cases, SB9 even allows an existing lot to be split, allowing four single-family homes to be built within the space where one previously stood.

While legal across the state, despite some challenges, not all municipalities readily approve SB9 applications. To date, property owners looking to add an independent living space to their lot typically opt to build an accessory dwelling unit, or ADU, rather than another primary dwelling unit under SB9, as the zoning process and city approvals for ADUs are generally easier to navigate.

Compare SB9 and ADU timelines side by side to determine which expansion path aligns with your goals.

What Is SB9 and How Does it Work?

SB9 was one of many state laws passed over the past several years aimed at increasing the availability of housing in the state. Where a lot may have previously been zoned for only a single family home, SB9 now makes it possible to put two primary residences on one residential lot.

In certain cases and under specific conditions, SB9 allows for the splitting of the lot. A split lot would mean that what was formerly a single family property could now house up to four primary residences.

To qualify for SB9:

• The property must be zoned for single-family residential use
• The lot must not be in sensitive areas like high fire risk or historic districts
• The owner must live on-site for at least 3 years if they do a lot split
• The split must result in roughly equal parcel sizes (no smaller than 40% of the original)

Remember that SB9 allows two primary units on one lot, but for most homeowners an ADU remains the simpler, quicker choice.

Is SB9 Still Legal After the LA Lawsuit?

Yes. Despite being challenged by five Southern California cities (Redondo Beach, Carson, Torrence, Whittier, and Del Mar), the state courts have upheld SB9, meaning that SB9 remains a state law throughout California.

Though the state sets the baseline regulations, municipalities can interpret the law as they see fit. This means that some cities may take a more narrow interpretation of the law, which can slow down or even inhibit the building process under SB9.

Even with courts upholding SB9, watch for city-specific interpretations—local variances can make or break your project.

How Many SB9 Projects Have Been Approved in San Diego?

As of October 2024, the City of San Diego is estimated to have approved roughly 7 out of 23 SB9 applications. This is according to the City’s own Permitting Center Dashboard, which provides historical data on:

• Applications Received
• Permits Issued
• Summary for Issuances
• Permits Completed

Permitting Insights for SB9 Projects in San Diego

7 out of 23 total applications is about a 30% approval rating for an SB9 project in San Diego, which is quite low. But the more interesting figure to examine is the 23 total applications over the past three or so years of SB9’s existence.

Given the size and population of San Diego, 23 total applications is surprisingly low. What we know about those who do apply, based on news reports, is that they tend to be professional builders with the means and understanding to navigate a more complicated permit process that can take 8 to 12 months, or they’re homeowners who have hired a professional to navigate the permit process on their behalf.

To put that permit timeline in perspective, at Better Place Home & Design, we can often complete a full ADU build in 8-12 months, which includes both the design and permitting phases.

The reasons for potential SB9 application rejection are many and include:

• Zoning incompatibility, meaning the existing lot is not designated as a single family
• Environmental and hazard constraints, such as wetlands or high fire hazard
• Lot size and configuration issues, meaning the lot is not at least 2400 square feet and/or the lot splits would not be nearly equal in size
• Existing rental or affordable housing units on the property. The existence of these can prohibit SB9 approval.
• Owner-occupancy requirements, meaning the property owner is not able to be on the property for 3 years following the split
• Incomplete or non-compliant applications, meaning those that are missing required documents or do not follow local zoning laws
• Local Coastal Program (LCP) restrictions. SB9 applications in these areas face even stricter requirements than in other areas
• Public safety and health concerns, such as the impacting of emergency routes or exacerbating traffic congestion

Note San Diego’s roughly 30% SB9 approval rate—partnering with ADU experts often yields faster, more reliable results.

Eligibility Criteria for SB9 Projects

What Are the Key Requirements for SB9 Projects?

Eligibility requirements for SB9 projects as dictated by the state of California are that the existing lot must be:

• Zoned single-family (e.g., R1)
• At least 2,400 sq. ft. for a split
• Not in high-risk fire, flood, or historical zones

The City of San Diego expands on that last point a bit, saying that to qualify for SB9, a property must not be located on:

• High or very high fire severity zone
• Wetlands
• 100-year flood zone
• Floodway
• Prime farmland, or farmland of statewide importance
• Historic district or California historical resources inventory, historic property, or landmark
• Hazardous waste, or hazardous list site
• Lands identified for conservation in an adopted natural community conservation plan
• Habitat for protected species
• Lands under a conservation easement
• Delineated earthquake fault zone

Before applying for SB9, confirm zoning and environmental constraints—this pre-check saves time and money.

What Is the SB9 Owner-Occupancy Rule?

The SB9 Owner-Occupancy Rule states that, for lot splits specifically, the owner must agree to live in one of the resulting homes, on either lot, for at least three years.

Does Having an Existing ADU Affect SB9 Eligibility?

Having an existing ADU does not necessarily disqualify you from SB9 eligibility.

Without SB9 approvals, single-family homeowners are allowed to have one single-family home, one accessory dwelling unit, and one junior accessory dwelling unit (JADU) on their property. However, there are size limitations on the ADU and JADU. An ADU can be no larger than 1200 square feet, and a JADU can be no larger than 500 square feet and must be located within the home’s existing footprint.

For homeowners looking to add even more units to their property, and especially another single-family home, SB9 makes that possible, though depending on the size and location of your existing ADU, you may be limited in where or even if you can add another single-family home to your property that meets zoning standards.

What Types of Homes Can You Build After an SB9 Lot Split?

If you split your lot under SB9, you essentially have two single-family lots where there was previously only one.

This opens you up to a variety of building possibilities. You can build:

• Two single family homes on each lot
• Two duplexes on each lot
• A mixture of single family homes and ADUs, depending on the lot size & shape, as well as the  size & shape of the ADU

When planning a lot split, design your layout to accommodate the desired unit types and maximize usable space.

Can You Use SB9 to Split a Lot and Also Build an ADU?

Yes, though it can complicate an already complicated process.

Benefits of doing this include:

• Increased property value
• Increase potential rental income
• Expanded multigenerational living opportunities
• Ability to sell parcels separately (if a lot split is involved)

Challenges of doing this include:

• Even more complex application and approval processes, as SB9 and ADUs have different regulations and requirements
• Longer project timelines 
• Higher construction costs
• Potential community or HOA pushback

For the average homeowner, the ability to build a JADU and an ADU on their existing property, even without SB9 approval, often provides them with the rental income and extra living space they’re looking for without adding unnecessary complications.

When exploring SB9 plus ADU, coordinate your utility plan early to prevent costly reroutes and delays.

Can I Split My Lot Now and Decide What to Build Later?

Yes, but know that state and local laws are always changing, especially when it comes to zoning and building permit requirements, so you may be more limited in your building options later.

Planning ahead and getting building plans approved for both lots will minimize your risk and maximize your potential return on investment.

SB9 ADUs vs. Traditional ADUs: Key Differences

You don’t have to split your lot or pursue an SB9 approval to add more residential units to your property. Many homeowners find that a two-unit development through an ADU addition offers the benefits they’re looking for (increased property value, extra living space, potential rental income) while avoiding some of the biggest pitfalls (long project timelines, a more complicated permitting process, and increased property taxes).

Evaluating the Best Path: SB9 vs. ADU

Aside from the above comparison chart, another way to determine if pursuing an SB9 application or opting for an ADU build is right for you is to ask yourself the following questions:

1. Is your property zoned single-family?

If not, SB9 isn’t an option, though an ADU might still be.

2. Do you want to split the lot?

If so, SB9 approval is your avenue to do that.

3. How many units do you want to build?

If you want to build more than two stand-alone living structures on a single-family lot (including the primary residence), you must pursue an SB9 build.

4. Are you comfortable with a potentially longer and more complex application process?

If the answer is no, avoid SB9. Even cities that have narrow SB9 interpretations often make ADU approval processes relatively streamlined, taking no more than a few months, especially with an experienced ADU builder.

5. Is owner occupancy a concern?

If you anticipate wanting to live off your existing property in the next 3 years, an ADU build is more flexible in that regard, as it does not trigger owner-occupancy rules.

How Better Place Design & Build Can Help

Intimidated by the SB9 process? The good news is you don’t have to pursue SB9 to increase your property value, create a secondary source of income, or house an elderly parent.

A traditional ADU build allows you to do all these things in less time and for less money than pursuing SB9, especially when you use the ADU experts at Better Place.

We tailor-make ADUs to fit a variety of use cases and budgets, and we pride ourselves on a transparent, stress-free construction process for homeowners.

Don’t dive into uncharted territory alone. Protect your investment and see what’s possible on your property by scheduling a free consultation today.

Lean on experienced professionals who manage permitting, design, and construction, streamlining your SB9 vs. ADU decision.