Accessory Dwelling Unit

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Types of ADU

An accessory dwelling unit is a simple and old idea: having a second small dwelling right on the same grounds, or attached to or inside, your regular single-family house, such as:

  • an apartment over the garage
  • a tiny house (on a foundation) in the backyard
  • a basement apartment

Definition source:


Alternative names

Accessory dwelling unit (ADU) is the official name adopted by the State of California as of September 2017.  Other jurisdictions may differ in terminology. Over time ADUs have assumed many names depending on cultural, regional, or communal consensus. Here are just a few terms synonymous with ADU used to describe "second units" on single-family properties:

  • accessory apartment
  • accessory dwelling
  • accessory dwelling unit (the most commonly used term among planners, but weirdly, not used currently on Wikipedia)
  • accessory suite (used here to specify an attached ADU)
  • accessory unit
  • ADU
  • ancillary unit
  • backyard cottage
  • basement apartment
  • carriage house
  • dawdy' house (among Amish)
  • garden cottage (used to specify a detached ADU)
  • garden suite  (used mostly in Canada)
  • Grand Retreat (a commercial name) 
  • granny cottage
  • granny flat
  • granny pod [example]
  • granny unit
  • home within a home (used by Lennar marketing) 
  • in-law
  • in-law suite
  • in-law unit
  • “JADU” or “junior accessory dwelling unit” — a small, internal ADU
  • laneway house
  • mother-in-law flat
  • “mother-daughter” or “mother/daughter” house
  • multigenerational homes
  • Next Gen (a commercial name)
  • Ohana unit (in Hawaii) 
  • SDU
  • secondary suite (more common name in Canada)
  • second unit
  • secondary unit
  • secondary dwelling unit
  • idekick
  • tiny house [sometimes these are ADUs, sometimes not — see this]
  • two-family house

List Source:



Arguments for ADUs

ADUs are most widely used to allow extended family to live together in one location, creating a multi-generational household.  More recently, ADUs are being viewed as investment opportunities for homeowners. For first time home buyers, an ADUs help accelerates mortgage paydown, or homeowners from older generations may establish an ADU from underutilized space to generate additional retirement income.

ADUs are legally permissible in many cities and states, but regulations about size, placement, design, and use vary widely.  For more information about your local land use laws visit your municipal planning department website or call to speak with a planner as rules change frequently.  Recently, many public officials are recognizing the potential for ADUs to satisfy housing needs in their local communities.

For example, California Governor Jerry Brown signed Senate Bill No. 1069 in September 2017 requiring local city governments to adopt ordinances allowing ADUs, effectively legalizing the building of ADUs at the state level.  

There are many reasons and motivations for a homeowner to build an ADU, but one thing they have in common is the provision of housing.  The commodification of housing is very complex and causes many unintended consequences, chief among them is affordability. ADUs present a good opportunity to introduce a social factor alongside the economic benefits of building housing. Plus, the economic benefits for building ADUs goes toward local small property owners. 


Can an ADU be mobile, e.g. a mobile tiny house? 


Should an ADU have to be tied to a main house?

comment in presentation by @JakeWegmann at #BuildSmall2017.

What if homeowners could subdivide their lots by-right?

comment in presentation by @JakeWegmann at #BuildSmall2017.



Detached Accessory Dwelling Unit (DADU) 

Junior Accessory Dwelling Unit (JADU)

In multi-unit building

Affordable Accessory Dwelling Unit (AADU)




Municipal regulations & guides




Grand Rapids








San Francisco

San Jose

November 10, 2016 Amendments to Title 20 (Zoning Code):  (24MB PDF). 

Secondary Unit worksheet: ​​​​​​

see also "Bulletin #250: Accessory Buildings and Structures. (non-dwelling):  6 Feb, 2017.

Scheinin, Richard. “Bank funds ‘granny’ units project in affordable housing experiment for San Jose, L.A.” Mercury News, October 9, 2017.

Giwargis, Ramona. “San Jose eases rules to build granny units to increase housing stock.” Mercury News, November 22, 2016.



City of Boulder "Accessory Dwelling Units (ADUs)" page:


Colorado Springs








Research query for Twitter references (from:YIMBYwiki OR from:houslets OR from:tmccormick) AND (#accessorydwelling OR #accessorydwellings OR ADU OR ADUs): 

Twitter: #accessorydwellings