The Role of Accessory Dwelling Units (ADUs) in a Modern Home

Example of Accessory Dwelling Units

2020 has been a year unlike any other in recent history. The events of this year have bled over into virtually every aspect of life, including architecture and design. One example of this can be seen in the resurgence of interest in accessory dwelling units (ADUs).

Below, we define what an accessory dwelling unit is, why many homeowners consider building them, the laws governing their construction in LA County, and design considerations you should bear in mind if you are considering an ADU of your own.

What is an Accessory Dwelling Unit?

backyard accessory dwelling unit adu

An accessory dwelling unit (ADU) is exactly what it sounds like: A secondary (smaller) dwelling that is built on the same property as the primary dwelling. They are typically detached structures from the main home, though attached or internal ADUs are also possible. An example of an internal ADU might be a basement apartment or an apartment above the garage.

ADUs can go by many different names, depending on who you ask. Some of the most common terms that are applied to them include:

“On the most tactical level, ADUs are meant to act as their own residences,” says Takashi Yanai, FAIA and Partner here at EYRC Architects. 

Why You Might Consider an ADU

adu and home ridge mountain

There are many reasons that a homeowner might consider designing and building an ADU on their property. 

“The recent updates to code for ADUs are meant to increase population density in areas that were zoned for single-family dwellings,” says Yanai. “And that opens up a whole slew of opportunities. If you construct an ADU on your property, you could rent it out, you could allow a relative—such as an in-law or adult child—live in it, or you could use it as flex space for an office or gym.”

While ADUs and guest houses have been around for decades, there has been a recent resurgence in interest around them caused by the COVID-19 pandemic. As more and more families find themselves learning and working from home, they have determined that an ancillary space specifically designed for those needs can be beneficial. 

Additionally, ADUs are particularly helpful in cases where one member of the family finds that they need to quarantine—for example, after traveling, working a high-risk job, or coming into contact with someone who is ill. In this way, adding an ADU to a property has become a means of designing for wellness for many families.

Accessory Dwelling Units in Los Angeles County & Bay Area

California ADU at Blue Sail Residence

The maximum allowable size for a detached ADU in LA County is 1,200 square feet. In San Francisco, the maximum allowable size for an ADU is 850 square feet for a one-bedroom unit and 1,000 square feet for a two-bedroom unit. Under California law, the minimum size for all living spaces is 150 square feet. The building must also abide by certain height and setback requirements. 

By law, an ADU must include a bathroom and a kitchen with a stove, refrigerator, and sink. The structure must abide by all building and safety requirements. Parking spaces may be required depending on where the unit is being built, as well as what the unit is being used for. Other rules apply to attached ADUs or those created by the conversion of other, existing structures (i.e., a garage). 

The specific laws governing the construction of ADUs can vary significantly depending on the municipality you live within. In some locations, it may be illegal to build an ADU altogether; in others, you may need to abide by certain size restrictions or guidelines. Some local governments may also enforce restrictions on short-term rentals, such as AirBnBs. 

With this in mind, be sure to check with your local office or department of planning before you begin the design or construction of your ADU, or work with an architect who is familiar with the requirements in your municipality. 

As just one example, below are some general guidelines about when an ADU is and is not permitted.

In LA County, an ADU is permitted if the following conditions are met:

  • The property is within a zone where single-family or multi-family residence is allowed. This may vary for coastal regions.
  • A legal single-family or multi-family residence exists on the property or is proposed and will be built at the same time as the ADU

An ADU may not be permitted in LA County if:

  • The property is not zoned for single-family or multi-family residency
  • The property is located in a Very High Fire Hazard Severity Zone and does not have two means of access to a highway

Design Considerations for an ADU

view of adu from inside a house

There are different design considerations that will impact the material choices, placement, massing and amenities when planning for an ADU. 

“Some people use an ADU as an opportunity to do something really fun architecturally,” says Yanai. “For example, if they have a more traditional house, they might be more adventurous in designing the ADU. Of course, if you're building the main house and an ADU at the same time, it probably makes sense that the two structures reflect each other in some way, but even then, they don't strictly have to.”

Other design considerations include:

  • Privacy: How much privacy does the ADU require? How might this impact the positioning of windows, doors, access units, etc.?
  • Use: What will the unit be used for? Is it long-term rental, short-term rental, or mixed use? This will impact the building requirements, as well as which amenities are included in the design
  • Connection to nature: How much connection to the outdoor space should the unit have? Can gardening and landscape be used to provide an enticing experience for both the ADU and primary dwelling?

Selecting a Partner for Your Project

If you are considering building an ADU on your property, it’s important to work with a trusted architect who has the experience designing this type of structure, as well as someone who understands both your design goals and the legal requirements of building an ADU in your municipality. Done correctly, building an ADU can increase the value of your home, while simultaneously creating a more enjoyable living experience.


Natalie LaHaie

Written by Natalie LaHaie

Natalie LaHaie is the Business Development and Marketing Director at Ehrlich Yanai Rhee Chaney Architects (EYRC). She is a graphic designer by trade and a natural strategic thinker, employing design to express vision, direction, and values. With over a decade of experience in the field of architecture and design, she admires the role of architecture in bringing people together from all backgrounds and walks of life - specifically through community, education, and civic buildings.