SB 9 vs. ADU: What’s the Difference for California Homeowners?

California homeowners now have more ways to add housing to a single-family lot than ever before. Two of the most talked-about options are SB 9 and accessory dwelling units (ADUs). They are often mentioned together, but they are not the same thing, and choosing the wrong path can cost you time and money.

This guide explains SB 9 vs. ADU in plain language, compares the rules side by side, and helps you decide which option fits your property and goals.

What Is an ADU?

An accessory dwelling unit is a secondary home on the same lot as your main house. It has its own kitchen, bathroom, and living space, and it can take several forms:

• Detached ADU: A standalone backyard cottage.
• Attached ADU: An addition connected to the main home.
• Garage or basement conversion: Turning existing space into a legal living unit.
• Junior ADU (JADU): A smaller unit, up to 500 square feet, built within the existing walls of the main home.

ADUs stay on the same parcel as your house. You own one lot, and the ADU is part of it. For a closer look at how these projects work in San Diego, see our homeowner’s guide to ADU construction.

What Is SB 9?

SB 9 is a state law that lets qualifying homeowners do one or both of the following on a single-family lot:

• Build a two-unit development: Up to two primary homes, such as a duplex or a second house.
• Complete an urban lot split: Divide one lot into two separate parcels, each of which can hold up to two units.

The key difference is that SB 9 can change the legal structure of your property. A lot split creates a new parcel that can be owned, financed, and sold on its own. Before you compare options, confirm your lot qualifies by reading our guide to SB 9 eligibility requirements.

SB 9 vs. ADU: Key Differences

Ownership and lot structure

An ADU stays on your existing lot, so you hold one parcel. With an SB 9 lot split, you end up with two parcels. That can unlock separate ownership, which is useful if you want to sell one lot, give it to a family member, or finance it separately.

Number of units

On a typical single-family lot, state ADU law allows one ADU and one JADU. SB 9 can allow up to two primary units on a lot, and cities may allow ADUs on top of that, but total unit limits apply. On a lot created by a split, local rules can cap each parcel at two units total, counting ADUs and JADUs.

Size limits

State law guarantees you can build an ADU of at least 800 square feet, and detached ADUs can generally reach 1,200 square feet. Attached ADUs are usually limited by a percentage of the main home’s size. SB 9 units are full primary homes, so they can be larger, though your setbacks, lot coverage, and local standards still apply.

Setbacks

For both options, cities cannot require setbacks of more than 4 feet from the side and rear property lines in most cases. That gives you real flexibility on tight lots.

Owner occupancy

ADUs do not require the owner to live on the property under current California law. SB 9 lot splits do. If you split your lot, you must sign an affidavit stating you plan to live in one of the homes as your primary residence for at least three years. A two-unit development without a split does not carry this rule.

Eligibility and exclusions

ADU law is generally more flexible. SB 9 is more restrictive and excludes sites in historic districts, certain fire hazard zones, floodways, and earthquake fault zones, among others. It also limits projects that would remove rent-controlled or recently tenant-occupied housing. Our SB 9 eligibility guide covers these in detail.

Rental rules

Units created under SB 9 must be rented for more than 30 days, so short-term rentals are not allowed. ADUs follow similar long-term rental expectations in many cities, so check your local ordinance before planning around vacation rental income.

Cost and timeline

ADUs are usually the faster and more affordable route, especially garage conversions and smaller detached units. SB 9 projects tend to cost more because they involve full homes, and lot splits add surveying, parcel map, utility, and engineering costs. For real numbers, see our breakdown of how much it costs to build an ADU in San Diego.

Quick Comparison

Factor	ADU	SB 9
Lot structure	One lot	Can create two lots
Typical units	1 ADU plus 1 JADU	Up to 2 primary units per lot
Owner occupancy	Not required	Required for lot splits
Separate sale	Limited	Possible after a lot split
Eligibility limits	Fewer	More exclusions
Cost and speed	Generally lower and faster	Generally higher and slower

Which Option Is Right for You?

An ADU is often the better fit if you want to:

• Add rental income or space for family at a lower cost
• Convert an existing garage or unused space
• Keep your property as one parcel
• Move quickly through design and permitting

If a garage is your starting point, read about garage conversion ADUs, including costs and permits.

SB 9 may make more sense if you want to:

• Build two full homes or a duplex
• Sell or finance a separate parcel
• Maximize long-term property value and development potential
• Plan a larger project with a bigger budget

Some homeowners combine both strategies, but unit caps and local rules decide how far you can go. The best answer depends on your lot size, zoning, budget, and long-term goals.

Local Rules Still Matter

State law sets the foundation, but each city handles the details. San Diego has its own ADU standards, fees, and processes that can affect what you build. Read our overview of San Diego ADU rules in 2026 to see what to expect, and always confirm current requirements with your local planning department.

Plan Your Project With XLNC Builder

Deciding between SB 9 and an ADU is easier when you know what your property can support. XLNC Builder offers architectural design and planning and custom ADU construction for San Diego homeowners. Our team is CSLB licensed (#1153455), bonded, and insured, and we will walk you through realistic options, costs, and timelines before you commit.

Ready to explore your options? Request a free quote or call (619) 836-0099 to talk with our team.

Frequently Asked Questions

What is the main difference between SB 9 and an ADU?

An ADU is a secondary unit on the same lot as your main home, so you still own one parcel. SB 9 lets you build up to two primary homes on a lot and, if you choose, split the lot into two separate parcels. SB 9 changes the legal structure of your property, while an ADU does not.

Is an ADU cheaper than an SB 9 project?

In most cases, yes. ADUs, especially garage conversions and smaller detached units, usually cost less and move through permitting faster. SB 9 projects often involve full homes and, with a lot split, added surveying, engineering, and utility costs.

Can I build an ADU and use SB 9 on the same property?

Sometimes, but unit limits apply. On a lot that has not been split, a city may allow a combination of primary units and ADUs up to a total cap, often four units. On a lot created through a split, local rules may limit each parcel to two units total, counting ADUs and JADUs.

Do I have to live on the property for SB 9 or an ADU?

For an ADU, owner occupancy is not required under current California law. For an SB 9 urban lot split, you must sign an affidavit stating you plan to live in one of the units as your primary residence for at least three years after approval.

Can I sell an ADU or SB 9 unit separately?

With an SB 9 lot split, each new parcel can be sold separately because it is its own legal lot. ADUs generally cannot be sold apart from the main home, unless your city has adopted rules that allow it. Check local ordinances before planning a sale.