An explanation of accessory dwelling units (ADUs)

ADUs, or auxiliary dwelling units, are supplementary residences situated on single-family residential lots. Although it may sound formal, the phrase “accessory dwelling unit” is the most often used to refer to this kind of housing in the nation. We use the abbreviation “ADU” instead of the full term since it is more palatable.

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The fact that an ADU is a kind of dwelling unit rather than a predetermined structural form defines it. It is only normal to be curious to see a novel design concept—like an ADU—in person when one learns about it. ADUs differ greatly in terms of their physical appearance, though. Our goal is to mentally relate to the ADU design idea as a tangible entity that is ingrained in our minds. Let’s investigate and comprehend the variety of typical ADU sorts in order to expand that mental model.


ADUs can be built as separate new construction, bump-outs, garage conversions, basement conversions, and more. The most popular structural designs of ADUs are displayed in this collection of pictures, along with a few more words you may hear used to refer to them.

1) A new, detached building Depending on the jurisdiction, ADUs may also be referred to as laneway homes, backyard cottages, granny flats, or DADUs. This kind of ADU is built on a single family home’s land, usually in the backyard or side yard.

2) ADUs for garage conversion: Convert your garage into a house.

3) ADUs that are either connected to or over a garage or workshop. These could be referred to as carriage houses or garage flats in some places:

4) Addition ADUs, also known as “bump-out ADUs,” are directly attached to homes and provide the advantages of shared walls and simpler utility access.

5) ADUs with basement conversions, sometimes referred to by a variety of other names, including English basements, accessory apartments, mother-in-law units, in-law units, supplementary suites, and basement apartments.

6) Internal ADUs, which are created by converting a portion of the main house—apart from the basement—to an ADU.

What characteristics do ADUs all share?

ADUs have different structural shapes, but they have a lot in common, including comparable design and development issues. ADUs are special because, among other things, they are secondary dwelling units on properties zoned for single-family residential use. ADUs can be further defined, distinguished, and set apart from other dwelling types by virtue of a few more unique features.

Adjacent to a principal housing unit, ADUs are an additional dwelling unit.

ADUs are far smaller than the typical American home.

ADUs are often one of two apartments on a single-family dwelling land that are owned by the same person.

Homeowner developers typically build ADUs in parallel to their main residence.

Different municipal land use and zoning restrictions distinguish different types and designs of ADUs and have a significant impact on the uses that are permitted.

Compared to approved ADUs, there are a great deal more unofficial ADUs.

ADUs are a unique kind of housing because of their distinctive features. Up until recently, there was a lack of consensus over the terminology and recommended procedures for developing ADUs.

The dearth of knowledge is corrected by this website and the book Backdoor Revolution, which offer comprehensive information about ADUs and how typical homeowners build them.

After that, we’ll provide a variety of contextualizing details, such the reasons why approved ADUs are so uncommon and the demographics influencing the demand for ADUs.

How big is the ADU population?

Appendix 4 of Backdoor Revolution details the numerous academic studies and professionally financed surveys that have been done on the subject of informal ADUs. All of them have discovered that a staggering 10–20% of all the housing units in their studied region are informal ADUs. While it is true that most of these studies were carried out in densely populated metropolises like Los Angeles, San Francisco, Portland, and Vancouver, British Columbia, more extensive research has also been done in metropolitan areas like the Boston Metropolitan Area and the Bay Area, with comparable outcomes.

Could unofficial ADU-style developments make up 10% of total residential housing stock in the United States? If accurate, this implies that there are over 14 million ADUs in existence.

14,000,000! It seems nearly inconceivable, doesn’t it?

However, when I think back on my own experience, it really begins to appear quite likely. My mother’s childhood home included a makeshift ADU in the attic. There had also been a non-official ADU at my dad’s property across town. My first house in DC had a makeshift ADU in the basement.

When I consider all the informal ADU-style homes I’ve shared over the bulk of my own life, 10% of total housing stock doesn’t seem like such a crazy amount.

The significance of ADU development for cities

There are several reasons why local governments might wish to encourage the construction of ADUs. Here are a few typical causes:

The financial advantages of ADUs

ADUs use the government infrastructure already in place (roads, sewers, schools, etc.), offer flexible housing alternatives in inner city districts, and lessen the need for new infrastructure to be built in the outlying areas of a developed metro region.

Advantages of ADUs for the environment

ADUs offer lodging with a little impact on the environment. Standard new single family rental units offer 44% more rental housing per capita than new, detached ADUs. Compared to normal new single family homes, new ADUs offer accommodation that is 33% smaller per capita overall. Smaller residential buildings need less energy during the construction, demolition, and occupancy phases of a building’s lifetime.

Advantages of ADUs for society

Particularly when compared to other new housing forms, ADUs offer more inexpensive housing alternatives in residential districts without significantly altering a neighborhood’s identity.

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