Accessory Dwelling Units (ADUs)

Accessory dwelling units (ADUs) — also referred to as accessory apartments, second units, or granny flats — are additional living quarters on single-family lots that are independent of the primary dwelling unit. ADUs offer a variety of benefits to communities. They help increase a community’s housing supply in high-opportunity areas while seamlessly blending into existing neighborhoods. ADUs are affordable by design in that they do not require paying for land, major new infrastructure, structured parking, or elevators. Elderly and/or disabled persons who may want or need to live close to family members or caregivers, empty nesters, and young adults just entering the workforce may find ADUs convenient and affordable. In addition to increasing the supply of affordable housing, ADUs benefit homeowners by providing extra income that can assist in mitigating increases in the cost of living.

Due to the lower per-dwelling unit construction costs, increased affordability, and reduced environmental impacts, the State has enacted many laws aimed at incentivizing ADUs to help combat the worsening housing crisis. The State requires cities and counties to allow all ADUs that comply with specific standards as by-right, not subject to discretionary review, requiring only the issuance of building permits. Local governments may impose specific objective design and development standards, which include, but are not limited to setbacks, parcel size, parking, and height. ADUs and JADUs can be encouraged through creating pre-approved ADU plans, easy-to-navigate ADU zoning code provisions, and public education. ADUs are flexible in use — they are appropriate as a residential unit for both a college student and an elderly retiree. As more people learn about their affordability and practicality, as well as their environmental benefits, ADUs will grow in popularity.

Affordability. ADUs are considered affordable housing by design and can provide housing choices that are more affordable in more costly single-family neighborhoods. Most communities require an ADU to be smaller than the primary residence on the property. The smaller size can reduce the unit’s rental price. Monthly rent would likely be lower than for a house in the same neighborhood, and residents generally do not bear the maintenance and other costs associated with owning a home. 

Depending on how the ADU is constructed, residents may be able to share utility costs with the primary residence. For example, the simple efficiency of shared walls may lower utility costs. For homeowners, an ADU can be an additional source of income, offsetting the cost of home ownership.

Housing Choice. ADUs add variety and housing choice in single-family neighborhoods. They can include detached units, above garage units, garage conversions, backyard cottages, basement units, and upstairs units. In addition to adding different sizes and forms of housing, they can add rental opportunities to largely owner-occupied neighborhoods. ADUs can be a great solution for allowing residents to age in place or live with or near family and caregivers, providing a flexible way to address family needs for additional housing.

Efficient Land Use. ADUs can help achieve housing goals by increasing density in existing neighborhoods. ADUs are a way to create infill housing and add density to low-density, single-family neighborhoods without compromising the existing look or design of the neighborhood. ADUs increase the efficiency of land use by allowing more people to live in the same area, reducing the per capita costs of public facilities and services.

Opportunities to achieve multiple benefits. In developing this type of program, jurisdictions have the opportunity to address other goals in addition to ADU development. For example, the City of Clovis Cottage Home Program was initially designed to target the Old Town Clovis area. While incentivizing ADU development, the program simultaneously worked toward goals of alley revitalization and creating a more walkable environment within Old Town Clovis. 

Pre-approved ADU Plans

Pre-approved ADU plan programs generally fall into one of two broad categories: city-owned pre-approved plans and architect-owned pre-approved plans. In programs where the plans are city-owned, the city procures the services of one or multiple design firms to develop plans, which are then purchased by the city for distribution to interested homeowners. The plans are available for free or for a small fee directly from the city. In many cases, the plans are available for immediate download from the jurisdiction’s website. In programs where the architect maintains ownership of the plans, the city typically provides information on their website about the pre-approved plans, along with a link to the firm’s website (see City of San Jose Preapproved ADUs in the Available Resources section below). Homeowners then purchase the plans directly from the firm. The jurisdiction could still provide multiple designs from multiple firms using this option. 

City-Owned Versus Architect-Owned Pre-Approved Plans. While both city-owned and architect-owned pre-approved plans can result in a successful program, they have different strengths and limitations that should be considered during program design. Programs where plans are city-owned may have a greater up-front cost to the jurisdiction, which must cover the cost of purchasing the plans from the designer(s). However, this allows the jurisdiction to control the level of cost savings for the homeowner by making the plans available for free or for a low cost. 

In programs where the architect maintains plan ownership, the city has less control over the fee for the plans. However, by purchasing plans from the design firm, the homeowner has greater access to the firm’s professional services should changes to the plans be desired or extra support needed through the permitting process. 

Jurisdictions that opt for a city-owned plan program model should anticipate that homeowners may need additional support through the permitting process as they are navigating it without a design professional. It is also important to note that pre-approved plans must be continuously kept up to date with current State law, zoning, and building code requirements.

Design limitations. A key challenge of pre-approved plan programs is ensuring that the city-selected designs are attractive and desirable to homeowners. It is important to bear in mind that by selecting a pre-approved design, the homeowner is trading away the chance for a custom design in order to save what amounts to a small percentage of the total ADU construction cost. Many homeowners may decide that the cost savings is not worth the tradeoff. However, by taking into consideration local architectural styles and typical site configurations, as well as by providing multiple design options, jurisdictions may be able to increase the success of the program.

For example, the City of Clovis offers three separate designs which are intended to address typical lot configurations within the program’s target area. The Town of Danville’s Garden Cottage Program offers a total of five pre-approved plans: one studio, one one-bedroom, and three two-bedroom designs. The two-bedroom units all have the same floor plan but have a different architectural style on the exterior (craftsman, contemporary, and Mediterranean). Other programs include preapproved customization options. Links to both the Clovis and Danville programs are included in the Available Resources section below. 

Technical assistance. Jurisdictions should also take care to ensure that homeowners have a clear understanding of their responsibilities when using a pre-approved plan. Some homeowners may have an over-simplified impression of the permitting process that occurs when using a pre-approved plan. Jurisdictions should be up front about other tasks that will need to be completed, such as a site plan showing the configuration of the pre-approved plan on the homeowner’s lot. The Town of Danville includes a helpful table on their website outlining what is provided through the Garden Cottage Program and what is the responsibility of the homeowner. 

Relevant State Law

Government Code Section 65852.2. Accessory Dwelling Units.

AB 3182, AB 68, AB 881, SB 13, AB 587, SB 13, SB 2, AB 587, AB 670, and AB 671. Changes to existing ADU laws effective January 1, 2021, further reduce barriers, better streamline approval processes, and expand capacity to accommodate ADUs and junior accessory dwelling units (JADUs). ADUs and JADUs may now be constructed in multi-unit dwelling projects as ADUs and JADUs are allowed by-right in residential zones. Concurrently, homes in single-family residential zones are now entitled to one ADU and one JADU by-right. Other recent legislation changes include: a maximum 60-day review period for all ADU and JADU applications, the requirement to install solar panels on newly constructed detached ADUs, impact fee waivers on units less than 750 square feet, and allowances for one ADU and one JADU per parcel. Additionally, the State prohibits local governments from requiring parking if ADUs meet certain criteria, such as being located within a half-mile from public transit or being located within one block of a car share area. Otherwise, parking requirements cannot exceed one space per unit or bedroom.

Survey Results

A survey conducted for the San Joaquin Valley Regional Early Action Planning (REAP) Report asked city and county staff about various housing topics, including accessory dwelling units. From this survey, 12.5 percent of survey respondents stated they offer pre-approved ADU plans for residents to use for projects, while 34.38 percent of respondents have pre-approved ADU plans in progress. Respondent jurisdictions generally offer between two and eight pre-approved ADU plans. These pre-approved plans have initially not been as successful in producing new housing as hoped. Approximately 73 percent of respondents say their pre-approved plans have not been used to produce new housing. Conversely, over 26 percent of respondents say their plans have been used. Most often, homeowners find out about jurisdiction’s pre-approved ADU plans through the jurisdiction’s website, newsletters, brochures, and public meetings. Overall, jurisdiction staff believe that ADUs and pre-approved ADU plans are beneficial for promoting housing production. Around 56 percent of respondents are developing ADU guidance or plans to promote housing production. In addition, 20 percent of respondents recently completed ADU regulatory changes to promote housing production.

Stakeholder Interviews

None of the stakeholders interviewed addressed the issue of ADUs. However, during the MPO Directors interviews, ADUs were described as success stories in the Tulare and Fresno regions, including the award-winning ADU designs that are free to residents in Clovis.


HCD. ADU Handbook

California ADU. Best Practices.


City of Clovis. Cottage Home Program.

Town of Danville. Garden Cottage Program. 

City of Encinitas. Permit Ready ADU Program.

San Diego County. County Standard ADU Building Plans.

City of San Jose. Preapproved ADUs.

City of Del Mar. Accessory Dwelling Units (ADUs) – 3D Models and Floor Plans For Public Use. 

City of Los Angeles. Approved Standard Plans. Design firm-owned plans; total of 42 plans. However, some designs have unusual features that may not appeal to a broad audience and will likely increase construction costs. 

City of Sacramento. Accessory Dwelling Units.


California Department of Housing and Community Development, Accessory Dwelling Unit Handbook (December 2020). 

California Department of Housing and Community Development, Accessory Dwelling Units (ADUs) and Junior Accessory Dwelling Units. 

American Planning Association, Accessory Dwelling Units. 

Maxable Space, Pre-Approved ADU Plans: The Most Common Misconceptions (August 20, 2020)

California Legislative Information, Title 7. Planning and Land Use, Division 1, Article 2. Adoption of Regulations [65850 – 65863.13]

California Department of Housing and Community Development, Understanding Accessory Dwelling Units and Their Importance. 

City of Sacramento, Community Development, Accessory Dwelling Units (ADU)