Accessory Dwelling Unit Development Report

Housing Policy and Planning Tools  /  Accessory Dwelling Unit Development Report

Introduction

What are ADUs?

Accessory Dwelling Units (ADUs) are residential dwelling units that provide complete independent living facilities for one or more persons and are located on a lot with a proposed or existing primary residence (also known as in-law units, secondary units, backyard cottages, granny flats, etc.).
Cities and counties are required by state law (Health and Safety Code Section 65583(c)(7)) to develop a plan that incentivizes the creation of ADUs that can be rented affordably to very-low- to moderate-income households. While local ADU ordinances are optional, they are increasingly more common as more cities, counties, and homeowners identify ADUs as an affordable housing solution. 

In This Report

This report contains regional data on ADUs in various categories, including:
  • Development Milestones
  • Geographic Trends
  • Affordability
  • Tenure
  • Permitting Trends
As the San Diego region implements their housing elements, ADUs are expected to play a vital role in future infill development strategies. Understanding key trends can help spur this kind of development to not only meet regional housing objectives but also build more sustainable and equitable communities.
See Gov. Code Section 65852.2 for the full definition of ADUs. More information about ADUs is also available from the California Department of Housing and Community Development (HCD) here: Link to HCD About ADUs Page.

Data Limitations

There are several steps involved before new housing is made available to San Diego residents, and not all units that are planned necessarily end up being constructed. California Department of Housing and Community Development (HCD) tracks units at three key milestones:
  1. “Entitled” units have received all required land use approvals/entitlements.
  2. “Permitted” units have received building permits for new housing construction.
  3. “Completed” units have been issued a certificate of occupancy or other form of readiness.
More definitions used in Annual Progress Reports are provided by HCD here: Link to HCD APR data.
This analysis of ADU development is based on the Annual Progress Report (APR) data collected by HCD, originally provided by each jurisdiction. In general, permitted units tend to make up the largest category because this is the minimum required development status for a project’s units to be counted toward RHNA obligations. As a result, there is a larger degree of data uncertainty (e.g., undercounting) for the entitled and completed unit categories. Given that ADUs are already a smaller subset of APR data, the following charts and maps focus on permitted units to best understand patterns in ADU development using the most robust dataset available.
Note: The following analysis represents a snapshot in time, with data available as of August 2023.

Geographic Trends

Where in the region are ADUs being permitted?

  • While ADUs have been permitted all throughout the region, the City of San Diego represents the most significant share at 43% of total permitted units between 2018 and 2022, followed by unincorporated San Diego County at 15%, and Encinitas at 8%.
  • Overall, coastal cities hold the greatest share of permitted ADUs. This includes Central San Diego and North County Coastal subregions.

Alignment with Regional Planning

Transit Priority Area (TPA) Alignment: In accordance with SB 743, Transit Priority Areas are defined as “an area within one-half mile of a major transit stop that is existing or planned, if the planned stop is scheduled to be completed within the planning horizon included in a Transportation Improvement Program adopted pursuant to Section 450.322 of Title 23 of the Code of Federal Regulations.” 
Mobility Hub Alignment: Mobility Hubs are places where transit and other shared mobility services, amenities, and supporting technology converge to offer a seamless travel experience. The 2021 Regional Plan includes a connected network of “right-sized” Mobility Hubs near major residential, job, and activity centers.
Focusing growth into Mobility Hubs and Transit Priorities Areas will decrease the region’s dependency on single occupancy vehicles and reduce the region’s greenhouse gas emissions. Prioritizing infill and urban development in these areas will also help preserve the region’s natural habitat areas and resources, while curbing greenhouse gas emissions and making the region more resilient to the impacts of climate change, including wildfire, rising sea level, and extreme heat

How does ADU development compare with Transit Priority Areas (2035)?

There are 3,592 ADUs within TPAs, representing a slight majority (54%) of total permitted ADUs. However, with as many as 3,117 ADUs located outside of TPAs, there is little to no spatial relationship. It should be noted that this could be attributed to the location of ADUs predominantly in lower-density communities (e.g., single-family zoned neighborhoods), whereas TPAs are more often characterized by higher density communities, containing multifamily or mixed-use development patterns.

How does ADU development compare with Mobility Hubs?

  • 3,149 out of 6,709 total permitted units (about 47%) are located within SANDAG’s Mobility Hubs.
  • The Mobility Hub with the greatest number of ADUs is the Urban Core, with 880 permitted units, followed by College Area, with 366 permitted units – both in the City of San Diego. These Mobility Hubs are also directly adjacent to one another.

Affordability

The Regional Housing Needs Assessment defines four income categories, based on household income relative to the Area Median Income (AMI) for San Diego County, for determining and allocating funds to jurisdictions:
  • Very Low Income (Less than 50% of AMI)
  • Low Income (50% to 80% of AMI)
  • Moderate Income (80% to 120% of AMI)
  • Above Moderate Income (Greater than 120% of AMI)
The majority (66%) of ADU permits in the San Diego region are issued for the Above Moderate Income category.

What are the patterns in ADU development by income level?

  • The number of permitted units decreases as the affordability level increases. The Very Low Income category has the lowest total permitted units.
  • Nearly all jurisdictions in the region have Above Moderate Income ADUs. The number of jurisdictions that have permitted ADUs decreases with increasing affordability level, with only 11 jurisdictions that have permitted Low Income ADUs and 4 jurisdictions that have permitted Very Low Income ADUs.
  • The City of San Diego holds the overwhelming majority of Above Moderate Income units, while unincorporated San Diego County has the greatest share of Very Low, Low, and Moderate income ADUs.

What does affordability look like by Census Tract?

Census Tracts are important geographies that are commonly used by agencies, organizations, and others to identify areas for research and analysis, funding, outreach, and other focused efforts. While Census Tract boundaries do not necessarily align with jurisdictional limits, they help provide insight into the local context and conditions because many sources of information, including U.S. Census data, is collected at this geographic scale. As a result, many housing elements rely on Census Tract data and geographies, such as to track progress toward affirmatively furthering fair housing (AFFH) requirements.
The maps below show how ADU development by Census Tract vary by affordability level.

Deed Restriction and Financial Assistance

Is deed restriction required to attain affordability?
  • Only a small proportion (54 units or 0.8%) of permitted ADUs are deed restricted, most of which are in the Moderate Income unit category; 2% of total permitted Moderate Income ADUs are deed restricted.
How many ADUs received financial assistance?
  • Most permitted ADUs did not receive financial assistance. (It is noted that more ADUs may have received financial assistance, which may be underreported due to data limitations/uncertainties.) While a greater proportion of deed-restricted units received some form of financial assistance than that of non-deed restricted units, the overall number of units that received financial assistance from the California Debt Limit Allocation Committee (CDLAC) or another source is comparable.

Timeline

How has ADU development changed over time?
  • The number of ADUs permitted in the region has generally increased, with a peak of 2,035 units in 2022 – almost double the amount in 2020 and more than four times the amount in 2018.
Where have ADUs been permitted over time?
  • The City of San Diego, which has the greatest number of permitted ADUs overall, issued the most permits for ADUs in 2021 and 2022. Unincorporated San Diego County has steadily increased their permitting of ADUs, with the most in 2022.
  • There were substantially more ADUs permitted in Chula Vista and Oceanside in 2022 compared to earlier years.
  • Encinitas has continued to permit more than 100 ADUs since 2019. Many other jurisdictions have also maintained similar permitting trends since 2019.
Do ADU permit trends differ by income level?
  • Above Moderate Income ADUs have consistently been the most common affordability level since 2018. (Note: Data for 2023 is incomplete, and many jurisdictions are not yet accounted for that year.)
  • Although Moderate Income ADUs were comparable to Low and Very Low income units between 2018-2019, there have been substantially more Moderate Income ADUs since 2020.
  • There are generally fewer Very Low or Low Income ADUs permitted in the region, though permitting activity for Low Income ADUs briefly increased at the end of 2019 as well as in 2022. Very Low Income units has generally remained the lowest among all affordability levels.

Tenure

How do ADUs differ by tenure?

  • A large majority (80%) of ADUs are renter-occupied units, while just 20% are owner-occupied.
  • Nearly all Very Low Income ADUs are owner-occupied units, while Low and Above Moderate Income units are more likely to be renter-occupied. 
  • Though fewer overall, the number of owner-occupied ADUs permitted has steadily increased since 2018, whereas there was a notable decrease in renter-occupied ADUs permitted in 2020. (Note: Data is incomplete for 2023.)
Data Limitations and Potential Inaccuracies in HCD Published Information
Please note that these data are collected by another agency. As such, SANDAG does not perform data validation procedures on this data set.  Although SANDAG makes efforts to identify and address potential issues, users and analysts should exercise their professional judgment when relying on these data. Please see the Data Terms of Use for additional information.