The Future Of American Cities May Be In Our Backyards

Forget tiny houses–meet the ADU.

We tend to think of housing as fitting into a few generic typologies: Single-family home. Apartment. Condo building. But in reality, the fabric of neighborhoods is made up of plenty of interstitial spaces that are neither here nor there–from the odd backyard shed to an alleyway addition to the hidden carriage house.


Over the past few years, these alternative housing types–often known as Accessory Dwelling Units, or ADUs–have become better known, as demographics and socioeconomic forces reshape our communities. Cities including Los Angeles, Seattle, San Francisco, and Austin, among others, are all moving to ease restrictions on building ADUs.

ADUs are also becoming a mainstay of today’s architectural vernacular: Ben Waechter, a Portland-based architect, even won an AIA Award for an ADU in the backyard of a single-family home last year.

But with the increased attention has come debate–not only over how these buildings are regulated, but how they could potentially reshape cities themselves over coming decades. As that debate evolves, here are the basics.

Kol Peterson

What Are ADUs–And Why Do They Matter?

Accessory Dwelling Unit is a technical name for an incredibly broad array of building types. It might be a shed in a backyard. It could be a carriage house behind an apartment building. Or an apartment above your garage. It could even be a tiny house in your backyard. An ADU is a housing unit on the same lot as a single-family dwelling. They’re also known as “in-law units,” “secondary units,” and a number of other nicknames. It wasn’t until the introduction of formal zoning regulations that they were broadly restricted or prohibited.

So why are you hearing so much about them lately? For one thing, the tiny house movement has moved various forms of alternative housing into the public eye. But ADUs aren’t just the domain of insufferable twenty-somethings who worship at the altar of minimalism. The AARP, for example, has been a prominent advocate for pro-ADU regulations, arguing that they allow older Americans to “age in place” and stay engaged in the fabric of society (they’re often called “granny flats” for a reason). As baby boomers age, and their children become homeowners and parents, the ADU is a natural way for grandparents to stay near their children without having to own their own homes.

Other pro-ADU arguments come from housing advocates, who say that these backyard dwellings could somewhat ease the incredible dearth of reasonable housing in California, where lawmakers are currently crafting bills that would ease restrictions on ADUs. In New York, an analysis by the Pratt Center For Community Development estimated that a whopping 40% of all new housing created in the city between 1990 and 2000 were actually illegal ADUs.


Yet many experts are careful to note that ADUs aren’t a replacement for affordable housing–they may augment it. Martin John Brown, a researcher who blogs extensively about ADUs on, points out that it’s difficult to compare ADUs to traditional housing types–because they’re built by homeowners, not real estate developers. He points to one study that found ADUs cost an average of 19% less to rent than non-ADUs in the Bay Area. In other cities, like Portland, they’ve just been average, though many rent for free or “ultra-low” costs. “Ultimately,” he wrote in a post this spring, “I feel the evidence shows ADUs have tremendous potential for creating reasonably priced housing.”

But Why Were They Prohibited In The First Place?

There are many reasons–just like there are many types of ADU–and every city and region is different, but many restrictions against these units emerged from zoning that tried to keep affluent, low-density neighborhoods that way. Most of the policies that outlaw ADUs around the country date back to the 1950s and ’60s, a time when suburbia was booming, and many Americans prized the notion of a single-family home in carefully orchestrated semi-isolation from the city center. In response, many areas introduced low-density zoning designed to preserve suburbia’s ratio of house-to-yard by restricting homeowners from further developing their land.

Many scholars have demonstrated how these anti-density policies were carefully designed for discrimination–boosting housing prices by decreasing the available housing stock, reducing the number of poor children in public schools, and keeping lower-income residents out of affluent neighborhoods. More recently, low-density zoning has been a tool to restrict the natural evolution of neighborhoods. In a 2013 study published in The Urban Lawyer, two researchers describe how anti-immigration has seemed to drive such policies in California and Virginia in recent years.

Where Are ADUs Flourishing?

Many cities have either eased regulations or created ordinances that allow new ADUs to be built. In Hawaii, there’s a common form of housing called Ohana, which allows family members to inhabit secondary dwellings. Last year, Honolulu–a city with a serious lack of affordable housing–signed into law a new bill that made it easier to build ADUs that anyone can inhabit. Regulations in California have also slowly been eased, though plenty of restrictions remain in the way of many would-be builders.

Portland is a major ADU hub, perhaps unsurprisingly, considering how the tiny house movement has boomed in the city. The city has waived the thousands of dollars in fees that a homeowner would normally pay to develop a new building on their land until 2018. “It’s a steep uphill battle to adopt good ADU codes since NIMBYs fear any sort of change to their single-family residential neighborhoods,” explains Kol Peterson, a Portland-based advocate for ADUs who collaborates with Brown on Peterson built his own backyard ADU, and in the years since, has become an outspoken proponent of the buildings. In Portland, he consults through his company Accessory Dwelling Strategies and leads day-long classes for people interested in building or converting their own ADUs (the next one is in September).

Peterson, who has a Master of Design Studies from Harvard’s Graduate School of Design, says he’s more interested in promoting the ideas behind ADUs than designing them himself. “At this stage, I feel that the impact of this advocacy and education work is more critical to my goal of seeing more ADUs built nationally than focusing my efforts on designing or building another ADU myself,” he says over email. “One of the best things someone can do to understand more about ADUs is to see them in person and talk to the homeowners, builders, and architects about their development process.”


What’s The Future Of The ADU?

The debate over ADUs is complex–like the land use laws and zoning policies that regulate them. Some critics worry the new construction will create overcrowding and burden city infrastructure.

Others argue that the ADU is the calling card of the short-term rental market. That’s been the hesitation of some in cities that range from Portsmouth, New Hampshire, to Asheville, North Carolina, and even in Palo Alto, as Peterson points out. “We want to know if we’re going to accomplish the goals of creating affordable housing or whether we’ll just create a lot of Airbnb opportunities,” a city commissioner, Asher Waldfogel, commented recently in Palo Alto Weekly.

On his blog, Brown points out that there’s simply not enough data on the typology to effectively understand it yet. But as cities around the country debate new measures to allow these alternative housing types, more data is coming–and ADUs (as well as the backyards they often occupy) may soon be a much more common part of the socioeconomic debate on housing.

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About the author

Kelsey Campbell-Dollaghan is Co.Design's deputy editor.