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Whats an ADU: Accessory Dwelling Units?

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An accessory dwelling unit, usually called an ADU, is an additional housing unit on a single-family residential lot. The term accessory dwelling unit is an industrial-sounding name, but its the most commonly used on residential housing.

The fact that the structure is a secondary housing unit, is what defines an ADU. When learning about concepts, its 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).
Detached ADU New construction
Detached new construction ADUs, also sometimes called backyard cottages, granny flats,
laneway houses, or DADUs, depending on the jurisdiction:

Detached ADU
Garage conversion ADUs
Above Garage ADU
ADUs above a garage or workshop, or attached to it. In some areas, these may be called garage apartments or carriage houses:
Bump out ADU
Basement conversion ADUs, also commonly called basement apartments, mother-in-law units, in law units, secondary suites, English basements, accessory apartments, and a host of other names.
Basement ADU
And, heres the inside of a basement ADU:
Basement ADU Interior
Internal ADUs, where part of the primary house besides the basement is converted to an ADU.


What ADUs have in common

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While their structural forms vary, ADUs share some common traits and face common design and development challenges. For one thing, the fact that theyre 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.

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.

Average household size dropped from 3.67 in 1940 to 2.53 in 2016 (US Census data)

3 bedroom and 4 bedroom homes no longer match the demographic realities of the United States:

Percentage of household for 1-2 people
1-2 person households now represent 62% of the countrys households. Only 38% of the the nation's households have more than 3 or more people people in them.

Close to 2/3rds of the population in the US are living in 1-2 person households!

Year by year, 1-2 person households are forced into eating up the single family housing stock (housing pellets, if you will) that was designed for nuclear families, not because they want or need to live in big homes, but because there simply isnt enough houses built in residential areas that were actually designed for 1-2 person households.

Among other demographic factors at play, single person households have become extremely common in major cities, representing more than of the households of many cities.

Why cities care about ADU development

There's a lot of reasons that municipalities may want to spur ADU development.

Heres 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.
ADUs provide more affordable housing options in residential neighborhoods without dramatically changing the character of a neighborhood as much as other new housing forms may.

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