An accessory dwelling unit, usually just called an ADU, is a secondary housing unit on a single-family residential lot. The term “accessory dwelling unit” is an institutional-sounding name, but it’s the most commonly-used term across the country to describe this type of housing. While the full name is a mouthful, the shorthand “ADU” is better.
The fact that it’s a secondary housing unit—rather than a given structural form—is what defines an ADU. But, when we’re learning about concepts, it’s natural to want to know what that concept looks like in the flesh. We want to visually embed the design concept in our brains as a tangible object that we can mentally reference. However, ADUs vary in their physical form quite a bit, so allow me to broaden that mental model by exposing you to the range of common ADU types, in order to better understand what they are.
Types of ADUs
Here are images of some of the common structural forms of ADUs (as well as some of the other terms you might hear to describe them).
1) Detached new construction ADUs, also sometimes called backyard cottages, granny flats, laneway houses, or DADUs, depending on the jurisdiction:
What ADUs have in common
While their structural forms vary, ADUs share some common traits and face common design and development challenges. For one thing, the fact that they’re secondary housing units on single-family residentially zoned lots places ADUs into a unique category of housing. And ADUs also have some other distinguishing characteristics that help further define, differentiate, and distinguish them from other housing types.
● ADUs are accessory and adjacent to a primary housing unit.
● ADUs are significantly smaller than the average US house.
● ADUs tend to be one of two units owned by one owner on a
single-family residential lot.
● ADUs tend to be primarily developed asynchronously from the primary house by homeowner developers.
● A large range of municipal land use and zoning regulations differentiate ADU types and styles and dramatically affect their allowed uses
● Vast numbers of informal ADUs exist compared to permitted ADUs.
These differentiating characteristics make ADUs a distinct type of housing. Till now, there has been a lack of common understanding around the language and best practices of ADU development.
This site and the book Backdoor Revolution will help change that by providing some clarity about ADUs and how average homeowners develop them.
We’re going to introduce a range of contextualizing information, such as why permitted ADUs are so rare and why so many unpermitted ADUs exist. We’ll tease apart the differences between ADUs and tiny houses on wheels. (Hint: Tiny homes on wheels are on wheels. ADUs aren’t on wheels.)
Let’s spend a minute on that last point.
How many ADUs are there?
In the twelve academic studies and professionally funded surveys that have been conducted in the presence of informal ADUs, they have all found that a whopping 10-20% of all the housing units in their study area are informal ADUs. Granted, these studies were generally conducted in populated areas, such as LA, San Francisco, Portland, and Vancouver, BC, but studies have also been conducted more broadly in metropolitan areas such as the Bay Area and the Boston Metropolitan Area, and the results have been similar.
Could 1/10th of all residential housing stock be informal ADU type development? That means there are more than thirteen million ADUs out there.
13,000,000! It seems almost unfathomable, right?
But as I reflect on my personal experience, it actually starts to seem entirely possible. I grew up in a house with my mom that had an informal ADU in the attic. My dad, who lived across town, had an informal ADU at his house. When I moved to DC, my first home had an informal ADU in the basement. My wife has also lived in and owned an informal ADU.
10% of all housing stock doesn’t seem like such a far-fetched percentage when I start to think about all the informal ADU type dwellings I’ve lived with for the majority of my own life.
Indeed, the presence of informal ADUs may be the single best indicator of the need for ADUs in general. It’s a “gray market” expression of the need for this form of housing.
Cities where ADUs are taking hold
Some cities, by design, have sought to bring this shadow form of housing into the light. Portland, Oregon, is the leader in the US in this particular type of infill housing at the moment. Vancouver BC is the leader in North America. Austin, Texas has also experienced a substantial bump in ADU development since legislative zoning changes in 2015.
There are many signs that legally permitted ADUs will become more mainstream. Recent legislative statewide laws in California marked a tipping point for ADUs in that influential state.
Some cities are jumping on board the ADU train by improving their ordinances and development regulations to make ADUs easier to build. Other cities are watching from the sideline. Frankly, there’s an underlying sentiment of resistance to change, and it’s politically challenging to pass good ADU regulations. Seattle provides the nation with a cautionary tale of the fear that surrounds the idea of ADUs amongst some neighborhood associations. Seattle’s city commissioner’s valiant efforts to improve ADU regulations were hung up in a legal battle and were postponed by a year as a result.
The demographics driving demand for ADUs
Why are some cities so eager to improve their ADU codes?
Well, for starters, most households in the United States are now 1 and 2 person households. Yet, most of our legacy housing stock and even our new residential housing stock is designed for families of 4 or 5 people. That may have made sense 70 years ago. But, things have changed.
Why cities care about ADU development
There’s a lot of reasons that municipalities may want to spur ADU development. Here are a few common reasons:
● ADUs provide flexible dwelling options in central city neighborhoods, utilizes existing governmental infrastructure (eg. roads, sewers, schools), and reduce the demand for expanding infrastructure in far-lying reaches of a developed metropolitan area.
● ADUs provide housing with a relatively small environmental footprint. New, detached ADUs provide rental housing that is 44% smaller per capita than standard, new single-family rental units. And new ADUs overall provide housing that is 33% smaller per capita than standard, new single-family units. In a building lifecycle, smaller residential spaces use less energy in construction, deconstruction, and habitation.
● ADUs provide more affordable housing options in residential neighborhoods without dramatically changing a neighborhood’s character as much as other new housing forms may.
But, ADUs aren’t a policy panacea….yet.