YES IN MY BACKYARD

The purpose of this research project is to evaluate what factors drive ADU interest at the local level. Accessory dwelling units offer an incremental and scaled solution for increasing density in existing single family neighborhoods that are in need of increased housing opportunities. To understand what San Diego ADU development looks like, three objects of data were studied. First, 2017-2019 ADU permit application data showing where people want to build an ADU and trends that have occured in development over the last 3 years. Then, comparing ADU permit application data with 1) socio-demographic, 2) socio-economic, and 3) physical neighborhood characteristics provided an understanding of which neighborhoods could best support future ADU development, as well as the factors that are driving neighborhood ADU interest. The outcome of this research is a detailed spatial analysis of accessory dwelling unit permits within the City of San Diego’s neighborhoods, compared with demographic and economic census information, to conclude whether the City of San Diego’s neighborhoods are experiencing a similar level of interest in building accessory dwelling units. 

While backyard accessory dwelling units (ADUs) are not the only solution to increasing California’s supply of local housing, ADUs offer an incremental and scaled solution for increasing density in existing single family neighborhoods. Incorporating backyard ADUs into the urban housing stock has the ability to decrease urban sprawl by promoting smart growth that emphasizes infill development opportunities, such as underutilized single family yards, where opportunity sites for ADUs benefit from existing infrastructure and community amenities in neighborhoods that can accommodate higher density (Morales 2019).

But, despite the recent legislative support and local interest in accessory dwelling units, the lack of governmental oversight in quantifying cities’ ADU supply must be addressed to evaluate if they pose a solution to increasing local housing stock. This study seeks to examine the factors that drive local interest in accessory dwelling units by using the City of San Diego as a case study. The City of San Diego makes an excellent case study for a variety of reasons. The city’s existing challenge of regional sprawl, combined with the need for increased affordable housing opportunities, justifies the need for exploring accessory dwelling unit implementation. Additionally, the “City of Villages” Strategy established by the City of San Diego’s General Plan guarantees a diversity in neighborhood typology and existing housing supply to evaluate existing ADU development. Finally, the recent publication of the City of San Diego’s 2019 Companion Unit Handbook provides the opportunity to examine if it and other resources to increase ADU production have been effective.

The current failure in quantifying both where and how many ADUs exist on the local level must be addressed. In evaluating accessory dwelling unit permit application data within the City, the following question can be posed: why are some San Diego neighborhoods better (or worse) than others at generating interest in ADUs? By conducting a GIS spatial analysis comparing recent ADU permit application data, we can identify patterns in neighborhoods that are successful in increasing their ADU stock that can perhaps be replicated in other cities looking to increase their housing supply.  

  This research project evaluates what factors drive ADU interest at the local level in San Diego’s neighborhoods. Data is evaluated on both the census-block level as well as by community planning area. A series of maps were produced that showcase three data frames relevant to accessory dwelling unit development. First, a map visualizing ADU permit application data from 2017 through 2019 visualizes which neighborhoods are experiencing increased interest in ADUs. While data is available for more than 3 years, establishing 3 years as the data timespan was a realistic amount of data collection to begin to observe patterns in development. While none of the case studies reviewed used permit application data, permit application data is the easiest to analyze given the time constraints of this study and to gage homeowner interest in ADUs. This data is publicly accessible through the City of San Diego Development Standards’ weekly permit application reports for the last 5 years, and through an Excel sheet can be imported into GIS. Finally, a set of maps were produced that compare permit applications with three categories of independent variables. Using the categories from the Seattle case study, permit applications data are compared with: 1) socio-demographic variables, such as population age and household size, that are public census data layers; 2) socio-economic factors, such as median income, percentage of renters, and percentage of income spent on housing, that are public census data layers and 3) physical neighborhood characteristics, such as alley networks and access to transit, that are publicly available through SanGIS. 

The map below contains all of the data layers used throughout the course of this study. ADU permit applications by year and typology can be compared with other sources of data to evaluate what factors may influence ADU interest locally. All of this data is also publicly available for further research projects under the UC San Diego GIS Lab data sources.

This study was intended to evaluate what factors drive local interest in accessory dwelling units and to be an informational case study about current ADU development trends in the City of San Diego. Are the City’s neighborhoods experiencing a similar level of ADU interest? The answer would be no. Appendix 1 showcases the ADU density of all 47 neighborhoods within the City of San Diego and the average neighborhood ADU density (ADU permits received divided by total housing * 100) is around 0.25%. This is very low. Half of the community plan areas scored under 0.25%, with top neighborhoods such as College Area scoring 2.2%. 

So how can we generate more interest in ADUs? The City of San Diego is on the right track by complying with recent state laws on ADUs, as well as allowing junior ADUs (under 500 square feet), removing owner occupancy requirements, providing available ADU blueprints that are streamlined for approval, as well as publishing a handbook educating homeowners about the process of building an ADU. Additionally, the City’s waiving of development impact fees on ADUs in 2018 proved to make ADUs an accessible housing option for homeowners of all income levels. The City is hoping to continue making ADUs more affordable by launching a ADU homeowner loan program for low income households in Spring 2020. Continued easement of parking restrictions throughout the City, as well as more streamlined permit approval processes for properties within the Coastal Zone may be necessary to increase interest in ADUs. Additionally, it will be important for San Diego to develop an amnesty program to help legalize existing non conforming ADUs and bring them up to code. Most importantly, San Diego will need to experience a cultural shift towards higher density and shared housing to generate interest in ADUs. Single family neighborhoods need to understand that ADUs offer the opportunity for incrementally added density that does not need to disrupt the neighborhood character. The City must continue to incentivize and educate single family homeowners, such as those in La Jolla, Pacific Beach, North County, and South County, on the benefits of ADUs. 

    This study proved several things about neighborhoods that have high ADU interest. First, San Diego residents are not as interested in building ADUs for aging in place opportunities. Homeowners want to build ADUs in areas that are bordering potentially gentrifying neighborhoods to provide more affordable housing. Next, income does not restrict interest in ADUs. Communities with lower income levels that also have high household sizes still have large interest in building ADUs, especially now that construction is becoming more affordable. Neighborhoods that do well in producing interest in ADUs have robust transit networks, lower rates of car ownership, and are not areas with high tourism and vacant short term rentals. Neighborhoods with high ADU interest have alley networks in need of rehabilitation, and alleys offer the opportunity to convert many existing underutilized structures into additional housing units. Neighborhoods that have the highest interest in ADUs in San Diego tend to have more older homes and lower median home values. Areas with close proximity to job centers, colleges, and potentially gentrifying neighborhoods have a higher level of ADU interest in San Diego. Finally, more research is warranted on areas such as political preference of YIMBYs who are willing to build an ADU, as well as the development of ADUs for residents with disabilities.

    ADUs offer an incremental and scaled solution for increasing density in existing single family neighborhoods and the City of San Diego proves that there is huge potential to use ADUs as one of many solutions towards increasing affordable housing opportunities. There are several areas of future research that should be conducted to measure the potential success of ADUs. First, doing a survey that maps existing nonconforming ADUs could provide excellent data on who ADU owners are and what the City needs to do to make it easy for units to be code compliant. Next, an analysis needs to be completed to determine the affordability rate of ADUs. Even if cities try to make ADUs deed restricted to be affordable, it will be important to measure whether homeowners truly offer their unit at below market rates. Additionally, studies need to be done by cities to alleviate the fears of ADU opponents comparing ADUs with traffic impacts, sustainability, and correlation between short term rentals. Finally, data needs to be gathered on how ADUs affect property values to convince homeowners that building an ADU will financially be worth their time.