“Up until a few years ago, ‘ADU’ was a term used mostly by architects, as part of their professional jargon. The acronym [accessory dwelling units] has only become part of the lexicon in the last year or so. Now, it seems as if everyone is designing, building, or dreaming about an ADU,” says Mary Maydan, founder and principal of San Francisco Bay area firm Maydan Architects.

“The rise in popularity of ADUs started before the pandemic.… [But] COVID-19 took this interest to a whole new level. After sheltering in place and working from home, we are all trying to fit a lot more functions into our homes. The possibility of having an additional structure in the backyard that can house a home office, a gym, or a hangout place for the family has made ADUs hugely attractive to almost everyone.”

Curious to know what the fuss is all about? Below, we break down everything you need to know about ADUs.

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What is an ADU?

An accessory dwelling unit is a fully functional living space that can fall into one of three categories:

• Interior, within the primary residence and often is converted space in a basement or attic, for example.
• Attached, built as an addition to the main house.
• Detached, a separate standalone structure like a garage, shed, cottage, or carriage house.

“ADU is a very broad term and covers everything from temporary construction to backyard offices to lane houses and converted garages. The intended use has a major design and planning impact because things like plumbing, square footage, and foundation requirements add significant time and expense,” explains Robbie Friedman, cofounder of ootBox, a company that designs, builds, and fully equips 10-foot shipping containers, complete with turnkey furnishings. “If a user just wants a home office, they can probably get away without plumbing while staying under 120 square feet. If they want an in-law suite—or something that is technically habitable—it becomes an entirely different project.”

Friedman says now, 10 months into the pandemic, he’s seeing the greatest need for dedicated environments for remote learning and working from home. “Some have even brought their retail business home—such as hairstylists, tattoo artists, private practitioners, and others,” he notes. “We focus on providing 80-square-foot portable units that users can rent until their needs change.”

How to create an ADU

ADUs may be the conversion of existing spaces or new builds, depending on the space available and the needs of the clients. Maydan says her clients are primarily interested in independent additions that are designed specifically for their needs and offer additional living space, although she does see a fair amount of converting garages, which is less expensive. “However,” she says, “converted garages do require a considerable upgrade. Often a project like this requires significant electrical work, as well as new systems, such as HVAC and plumbing.”

Christina Roughan, founder of Roughan Interiors in Weston, Connecticut, and New York, says: “A new build is typically the easiest to create as it’s our vision, with our clients’ intent, from start to finish. That being said, renovating an existing pool house to an office/kids’ space can be just as fun and turn into a spectacular functional interior.”

Jim Westover, residential practice leader at William Duff Architects in San Francisco, says his firm engages in both types of ADUs. “In dense urban areas like San Francisco, we are more likely to convert existing space—sometimes in a basement or an area behind a garage that lends itself well to being an ADU. In more suburban areas, it could be a conversion, addition, or independent new construction.”

Similarly, Ian Read of Oakland, California, firm Medium Plenty has experience with both conversions and new builds. “Many of the early projects were conversions, typically garages. Because the existing footprint of these structures were grandfathered in, there was some benefit to using them, although they almost always needed to be fully rebuilt,” he says.

For Wendy Yates, creative director and founder of Abigail-Elise Design Studio, conversions are more common. She cites garages and attics as viable options “that have the potential of adding more usable square footage to your property.” But she adds, “If independent additions are what you’re looking for, there are some great build-it-yourself kits that can be added in a backyard…. Shipping containers are quite easy to move onto a property, and they make ideal tiny units if designed well,” referring to structures similar to Friedman’s ootBox.

What are the benefits of an ADU?

ADUs are employed to carve out areas for work, play, learning, or additional living space such as in-law suites, nanny/caretaker’s dwellings, potential quarantine quarters for at-risk or sick family members, or rental apartment units. David Shove-Brown, partner and principal of //3877, notes, “A popular reasoning behind adding an ADU is many find the value of having family members close by, whether it be allowing aging parents to have their own place or a young graduate needing their own space.”

While creating extra living space is certainly welcome—especially this year when all families have been spending more time at home—Shove-Brown sees the main motivation for creating these ADUs as a money-making stream in the form of a rental unit or by increasing property value. “For some families, an ADU providing additional income can help ease the cost of a mortgage,” he says. Roughan adds, “ADUs increase the value of a home immediately and give the owner the opportunity to have a room outside the home that becomes a second dwelling on the property—a place to rest and enjoy peace and quiet. It can serve as a guest house or place for teenage sleepovers.”

What’s the key to a highly functional ADU?

“It’s all about maximizing flexibility,” Westover says about the most critical consideration when designing ADUs. “Life throws curveballs; someone might build an ADU to use as a home office, a gym, or a pool house, but then they may need it as an in-law unit,” he continues. “The ADUs we’ve done have that flexibility. We designed one that opens up to the pool for entertaining but also functions as a private two-bedroom house for our client’s visiting parents.”

Yates agrees about the necessity of multi-functionality, “because we believe needs change over time, and it’s better to be prepared for the future,” she says. “For instance, if you add a fold-down wall table to a home gym, the room can quickly be turned into an office for a Zoom call. Or maximize the area by installing a Murphy bed, minibar, or electric plug-in cooktop.”

Yates also advises designing the space to be timeless rather than trendy, thereby reducing the need to renovate in the future. “Keep the color palette neutral to allow for easy accessorizing,” she says. “Pay attention to lighting by increasing your use of natural light, layering lamps, or including wired fixtures. This will give you options for activities that require more or less luminosity so that the area feels more intentional and becomes a comfortable living environment.”

What are some challenges to consider?

Regardless of where designers are based, local codes and regulations have posed the greatest hardship in successfully creating ADUs. Maydan, Westover, and Read all recall difficulties in the Bay Area well before the COVID era. “In the past, many municipalities made it difficult to create additional dwelling units on most sites by requiring added parking and setbacks in an attempt to guard against illegal rentals. A common solution was to design an ‘accessory building,’ which created the necessary space, but only allowed for a half bathroom or a kitchenette—not both. We would typically build in a utility sink or floor drain in a way that a shower and/or kitchen could be added,” Read explains. “As housing became more scarce (and expensive) there was enough momentum to loosen up the requirements for the creation of units. Within the last five years is when we have seen ADUs become a more prevalent project type.”

Spatial constraints on-site are the main issue. “Unless you have a huge yard, then you are trading outdoor space for building footprint,” he says. “Utilities and other logistics are also a factor, but space, especially in the Bay Area, is the crux.”

In Washington, D.C., where //3877 is based, Shove-Brown had to endure municipality regulation issues similar to California. However, he notes the District has eased restrictions through municipal code changes. Now, he says, “the biggest challenges all depend on the infrastructure of the house—figuring out how to separate the electrical systems from the main house and the ADU, and also determine where the water source should come from.”

Friedman, who’s based in Columbus, Ohio, also acknowledges designing around local permits as the biggest challenge. He says, “You can simplify the process and minimize costs by keeping the structure small and without plumbing. But building a small space that is still inspiring and usable raises a whole different set of challenges.”

The restrictive footprint must be taken into account with every design decision—from the placement of doorways to furniture selection to decor elements. “The criteria for creating a useful, highly functional ADU is the same thought and attention as designing for a full-size house—they’re just smaller,” Westover explains. “If the ADU is going to really serve its purpose, it needs to be something you can actually live in comfortably.”

And one pitfall that should be obvious but is sometimes not considered enough is the budget. “Often people expect building a small space to be inexpensive, but it is actually quite the opposite,” Maydan says. “A small structure costs more per square foot than a big one since there are many budget line-items that don’t change linearly. A small structure still incurs the costs of engineering, permits, grading, and drainage, along with the actual construction of the structure. It often ends up being much more expensive than the homeowners anticipated.”