Understanding ADUs and SB9 Splits in California: A Guide written by an ADU Builder

California faces a housing shortage that affects millions of residents. To address this, the state has introduced policies encouraging homeowners to add more housing units on their properties. Two key tools in this effort are Accessory Dwelling Units (ADUs) and Senate Bill 9 (SB9) splits. Understanding how these work together can help homeowners, developers, and communities make the most of available land while increasing housing options.

This guide explains what ADUs and SB9 splits are, how they interact, and what you need to know if you want to use these options in California.

What Are Accessory Dwelling Units (ADUs)?

ADUs are small, secondary housing units built on the same lot as a single-family home. They are sometimes called granny flats, in-law units, or backyard cottages. ADUs can be attached to the main house or stand alone as separate buildings.

Key Features of ADUs

• Size: Typically range from 400 to 1,200 square feet.

• Purpose: Provide additional housing for family members, renters, or guests.

• Permitting: California law has simplified the approval process to encourage ADU construction.

• Types: Detached units, attached units, and garage conversions.

  
  

Benefits of ADUs

• Increase housing supply without drastically changing neighborhood character.

• Provide affordable rental options.

• Help homeowners generate rental income.

• Support multi-generational living arrangements.

  
  

California’s ADU Laws

Since 2017, California has passed several laws to reduce barriers for ADUs. These include:

• Eliminating minimum lot size requirements.

• Reducing parking requirements.

• Streamlining the approval process.

• Allowing ADUs in all single-family zones.

  
  

These laws make it easier and faster to build ADUs, encouraging more homeowners to add them.

What Is Senate Bill 9 (SB9)?

SB9, signed into law in 2021, allows homeowners in many California cities to split their single-family lots into two separate parcels and build up to two homes on each parcel. This effectively allows up to four units on what was previously a single-family lot.

How SB9 Works

• Lot Splitting: Homeowners can divide their lot into two smaller lots.

• Housing Units: Each new lot can have one primary home and one additional unit (like an ADU).

• Owner Occupancy: The owner must live in one of the units for at least three years.

• Restrictions: Some local rules still apply, such as setbacks and height limits, but many traditional single-family zoning restrictions are waived.

  
  

Purpose of SB9

SB9 aims to increase housing density in single-family neighborhoods without requiring large-scale zoning changes. It encourages more homes on existing lots to address the housing shortage.

How ADUs and SB9 Splits Work Together

Combining ADUs with SB9 lot splits can multiply housing opportunities on a single property. Here’s how:

• After splitting a lot under SB9, each new lot can have a primary home plus an ADU.

• This means a single original lot could have up to four housing units: two primary homes and two ADUs.

• Homeowners can build new homes and add ADUs to maximize the use of their land.

  
  

Example Scenario

Imagine a homeowner with a 6,000-square-foot lot:

1. They split the lot into two 3,000-square-foot parcels under SB9.

2. On each parcel, they build a new single-family home.

3. They add an ADU to each home, such as a detached backyard unit or a garage conversion.

   
   

This results in four separate housing units on what was once a single-family lot.

Backyard ADU next to the main house near Sacramento California

Backyard ADU next to main house near Sacramento California

Important Considerations When Using ADUs and SB9

Local Regulations and Exceptions

While state laws encourage ADUs and SB9 splits, local governments can impose some rules:

• Historic districts may have restrictions.

• Environmental or safety concerns can limit development.

• Some cities may require design review.

  
  

Always check with your city’s planning department before starting.

Financing and Costs

Building ADUs or splitting lots requires investment:

• Construction costs vary widely but expect $150,000 to $300,000 for a typical ADU.

• Lot splits may involve survey, legal, and permit fees.

• Financing options include home equity loans, construction loans, or specialized ADU loans.

  
  

Impact on Neighborhoods

Adding units can increase density and change neighborhood dynamics:

• More residents may mean more traffic and parking demand.

• Proper design can maintain neighborhood character.

• Community input is important to balance growth and quality of life.

  
  

Owner Occupancy Requirement

SB9 requires the owner to live in one of the units for at least three years after the split. This rule aims to prevent speculative flipping and maintain community stability.

Steps to Build ADUs and Use SB9 Splits

1. Research Local Rules: Confirm your property qualifies for SB9 and ADU development.

2. Consult Professionals: Talk to architects, planners, or contractors familiar with California housing laws.

3. Prepare Plans: Design your ADU and lot split layout considering setbacks, height limits, and parking.

4. Apply for Permits: Submit applications to your city or county planning department.

5. Build and Inspect: Complete construction following approved plans and pass inspections.

6. Occupy and Manage: Move in or rent out units according to local regulations.

   
   

What does it cost to do an SB9 Split?

$15,000-25,000 for the surveyors' labor to work on the parcel map and miscellaneous administrative work

$5,000-$12,000 for city/county permits and engineering fees.

$5,000-$30,000+ for new utility tap fees

Typical range $26,000 - $67,000