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

1)  Detached new construction ADUs, also sometimes called backyard cottages, granny flats, laneway houses, or DADUs, depending on the jurisdiction:

2) Garage conversion ADUs

3) ADUs above a garage or workshop, or attached to it. In some areas, these may be called garage apartments or carriage houses:

4) Addition ADUs or “bump-out ADUs”

5) Basement conversion ADUs, also commonly called basement apartments, mother-in-law units, in-law units, secondary suites

6) Internal ADUs, where part of the primary house beside the basement is converted to an ADU.

 

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.

 

 

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 on 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 the 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.