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, a detached house. For example: 

  • an apartment over the garage
  • a cottage (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
  • sidekick
  • tiny house [sometimes these are ADUs, sometimes not]
  • 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.

Separate ownership

While it is common to define Accessory Dwellings as being inherently or legally part of the primary home, in various contexts and for various reasons it has been suggested or is possible for an ADU to be separately owned.  You might say that "being part of" a primary house could refer to, occupying part of the lot and using in common services such as utilities -- but not necessarily sharing ownership. 

At #BuildSmall2017, ADU researcher @JakeWegmann in his presentation suggested two ways for separate ownership:

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

What if homeowners could subdivide their lots by-right?



Detached ADU (DADU) 

Junior ADU (JADU)

In multi-unit building (done in San Francisco)

Affordable ADU (AADU)

term suggested by T. McCormick, 2015. 

Movable ADU (MADU)

term suggested by T. McCormick 

Redeployable ADU (RADU) 

same as MADU

term suggested by T. McCormick


Affordable ADUs (subsidized or regulated affordable) 

Proposed 2015

A Place for You - Multnomah County (Portland)

Project page: [now empty - noted 9 Sept 2019].

"The Multnomah Idea Lab (MIL) is a learning lab within Multnomah County’s Department of County Human Services. Enhabit is managing the $350,000 pilot project on behalf of the County. Enhabit (formerly Clean Energy Works), has spent years working with governments, utilities and residential customers on energy-efficiency, seismic and other home-improvement projects. Enhabit’s role in A Place For You includes overseeing design and installation, working on site selection, and serving as a point of contact for participants.

How it Works
"The County has contracted with Enhabit to manage a pilot project to design and install ADUs in four privately owned yards. A homeless family would live there rent free for five years. At the end of that time, homeowners would have unrestricted use of the ADUs. Family tenants would be referred through A Home For Everyone’s (AHFE) Homeless Families’ Coordinated Access system. A Home for Everyone is a community-wide plan for addressing homelessness through a mix of strategies including prevention, housing placement and shelter. During the five years, families would receive full social service support through AHFE.

Benefits to Homeowners

  • We’re working to ensure limited-to-no costs for homeowners
  • We expect to offer a tax abatement for the years the ADU is part of the project
  • Increased social networking, community building, and hope

Benefits for Families

  • Privacy away from a shelter or doubled-up living situation
  • Stability for children who can attend the same school
  • Increased social networking, community building, and hope


LA-Más ADU Pilot Project (City of Los Angeles)


LA County - Second Dwelling Unit Pilot Program (2017-)

"An innovative new Los Angeles County pilot project is underway. As part of the County’s Homeless Initiative, the Second Dwelling Unit (Accessory Dwelling Unit) Pilot Program is exploring ways to make it possible for homeowners to construct housing for homeless families and individuals. An architectural competition is being planned. And we’re on a fast track: The goal is to identify the winning projects and finish construction within 18 months.

"The pilot program is only available for properties in the unincorporated areas of Los Angeles County (properties that are not located within any city). Additional details of the program, approved on Aug. 15, 2017, by the Board of Supervisors, can be found here. The application period for the pilot program has closed."

"The County will provide a maximum subsidy of $75,000 per unit to build 2-3 new ADUs. The County will provide a maximum subsidy of $50,000 per unit to preserve 2-3 existing unpermitted ADUs. The subsidy will be provided in the form of a soft loan or forgivable loan tied to a commitment to rent the ADU to a homeless family/individual or participant in the housing choice voucher program.

What is the purpose of the design competition?:
The design competition will help raise awareness of the importance of accessory dwelling units as an affordable housing strategy, explore the role of design in creating cost savings, and help get community buy-in around accessory dwelling units.

What is the potential to permit new and existing accessory dwelling units?
There are tens of thousands of potential sites in the unincorporated areas.


San Mateo County (San Francisco peninsula)

Subsidized loan program for homeowners creating ADUs who agree to rent them to low-income households. 

"For a limited time, the County of San Mateo will provide eligible homeowners in unincorporated areas with the opportunity to explore the feasibility of improving the condition of an unpermitted secondary unit without incurring fines, penalties, or code enforcement action that
typically result from unpermitted construction.
Am I eligible to participate in the Second Unit Amnesty Program?
Any second unit created without some or all necessary approvals will potentially be eligible for the program, subject to the following conditions: the property must be located in the unincorporated area of San Mateo County; only one unit per property will be eligible; units must be in “R” (residential zones); and units must have been both built prior to January 2017 and occupied prior to February 2018."


Additional Dwelling Unit Loan (Boston, 2017-) 

Pilot ran Oct 2017 - May 2019, was extended into permanent program in May 2019.  
Boston's program is so far limited to internal ADUs, i.e. within the envelope of an existing house. 

Owners of single-family homes, duplexes and triplexes could qualify to receive a loan up to $30,000 for an ADU remodeling project. Loan amount is based on estimated cost of the project. The loan has no monthly payments and does not become due until the owner sells, transfers ownership, or undertakes a cash-out refinance of the home. The loans are interest-free and offer gap funding to eligible Boston homeowners who wish to create an ADU in their homes in the pilot areas of East Boston, Jamaica Plain, and Mattapan.

"On May 8, Boston Zoning Commission approved a citywide zoning text amendment that scaled the pilot program citywide and made it permanent. In addition, Mayor Walsh announced funding for the expansion of the ADU program as part of his Fiscal Year (FY20) budget recommendations. This $650,000 investment will provide zero-interest loans for income-eligible homeowners. For more information see the overview presentation below."

Mayor’s FY20 budget includes $650,000 to create ADU loans program: 

  • No-interest loans up to $30K
  • for income-eligible homeowners
  • Create approximately 21 ADUs


City of San Jose - ADU Forgivable Loan Program (2019)

Accessory Dwelling Unit Forgivable Loan Program. 

Considered at City Council meeting Tuesday September 10, 2019: 
Meeting info:

Details from: City of San José, Housing Department: Memorandum on Agreement with Housing Trust Silicon Valley for the  Accessory Dwelling Unit Forgivable Loan Program. 29 August 2019:

Tenants household income must be no more than 120% of Area Median Income (AMI) or below at the time of initial occupancy and for any subsequent new tenant. Rents will be set at 30% of 90% of the AMI based on the size of the ADU per the City of San Jose Income and Rent Limit Table (Attachment A).  [YIMBYwiki note: no attachment was included. We think this refers to city HCD Income and Rent Limit Tables].  Owners will be required to provide a one-year written lease and annual rent increases will be capped at five percent per year or the annual increase in the area median income, whichever is less. Exceptions will be made in the case where a tenant holds a Housing Choice Voucher. The loan will be considered in default if the owner fails to rent the ADU or lists the ADU in any short-term rental services such as AirBnB. As a result, staff have consolidated all ADU information for all development service partner departments onto a single shared website (



Santa Cruz - Habitat for Humanity ADU program

project discussed in Kol Peters, Backdoor Revolution, 2018.



Movable ADUs

as "caregiver dwellings," in some California counties

In the counties of Alameda, Contra Costa, Lake, Mendocino, Napa, Sacramento, and Sonoma, tiny houses on wheels are allowed as "caregiver dwellings" in the backyard of a person who needs assistance.

in Fresno, California


in Ojai, California 



City of Los Angeles proposed ADU ordinance (2018-)

City Clerk tracking page for updates:
Note you can subscribe to email updates about any new activity, by clicking on the envelope icon at center.  

City of Los Angeles, Department of City Planning.  July 12 2018 "Recommendation Report" on proposed Accessory Dwelling Units ordinance. [Los Angeles 2018a]. 

Movable Tiny Houses

"The City Council amended ordinance includes a movable tiny house (MTH) as a type of allowable ADU. As defined in the ordinance, a MTH is a 150-430 square foot independent living quarters for year-round residence that meets the “park model” construction standard for tiny homes on wheels. MTHs are built to resemble a typical cottage or bungalow and usually use conventional residential building materials. They are a transportable recreation vehicle but are primarily
designed for long-term placement at a destination. When stationary, MTHs are connected to the utilities necessary to operate fixtures and appliances, the same as applicable to ADUs built on a permanent foundation.
"A number of California jurisdictions have taken the lead in recent years to allow MTHs as a new housing typology. MTHs make particular sense in coastal California due to the habitable climate and need for creative housing solutions. Recently, the city of Fresno approved tiny houses on wheels as backyard cottages. The City of Ojai has recently followed suit and allowed movable tiny houses as ADUs. In the counties of Alameda, Contra Costa, Lake, Mendocino, Napa, Sacramento, and Sonoma, tiny houses on wheels are allowed as "caregiver dwellings" in the backyard of a person who needs assistance.
"MTHs are built using conventional materials and standard methods, including a set of building certification standards called the American National Standards Institute (ANSI). ANSI is a nonprofit association that oversees the creation of thousands of standards and guidelines. The ANSI code requires the structure meet or exceed more than 500 building and safety standards including electrical, plumbing, structural, heating & AC, fire safety, and egress. The proposed ordinance
requires adherence to the ANSI 119.5 code, which is the “park model” RV standard used for most MTHs. Some other cities have also included the use of ANSI 119.2, which is the RV standard, but the design standards are intended, in part, to prevent RV-like appearance so the inclusion of the ANSI 119.2 standard was not included.
"Professional tiny house companies build movable tiny houses in factories to ANSI 119.5 requirements and have their builds certified by a third party inspection body such as the Recreational Vehicle Industry Association (RVIA) or Pacific West Associates. The proposed language also allows for self-builds provided they are certified by a third party. "The Department has created a set of design standards to ensure MTHs resemble traditional homes and not park trailers or RVs. These standards address exterior cladding, roofing, windows and doors and are intended to be ministerial in nature, in that they do not require any subjective judgement. MTHs will also meet zoning siting criteria applicable to all ADUs."

Los Angeles, City of. "Proposed Accessory Dwelling Unit Ordinance: CPC-2016-4345-CA Exhibit A."  November 29, 2018.

MOVABLE TINY HOUSE. A structure intended for the separate, independent living
quarters of one household for year-round residence that meets all of the following:
(a) Is licensed and registered with the California Department of Motor Vehicles;
(b) Meets the American National Standards Institute (ANSI) 119.5 requirements, and
certified by a qualified third party inspector for ANSI compliance;
(c) Cannot move under its own power;
(d) Is no larger than allowed by California State Law for movement on public highways;
(e) Has not less than 150 and no more than 430 square feet of habitable living space,
including bathrooms and fixed counters.
32. Accessory Dwelling Units (ADU):  [....]
(f) Requirements for Movable Tiny Houses as Accessory Dwelling Units.
Movable Tiny Houses must comply with all requirements for Detached ADUs and all of the
following provisions:
(1) Only one Movable Tiny House is allowed to be located on a parcel and no parcel may be approved for more than one moveable tiny house in a twelve month period.
(2) Movable Tiny Houses shall be located behind the primary dwelling unit and shall not be located in any required front yard.
(3) When sited on a parcel, the undercarriage (wheels, axles, tongue and hitch) shall be hidden from view.
(4) If the wheels are removed so the unit may sit on a foundation, the foundation requirements for a Movable Tiny House shall follow the State approved requirements for foundation systems for manufactured housing. If the wheels are not removed, the wheels and leveling or support jacks must sit on a paving surface compliant with LAMC 12.21 A.6(c), and the wheels and undercarriage must be hidden.
(5) Mechanical equipment shall be incorporated into the structure and not located on the roof.
(6) Movable Tiny Houses shall be connected to water, sewer and electric utilities.
(7) Moveable Tiny Houses are not required to have separate street addresses from the primary unit.
(8) Movable Tiny Houses are not required to have sprinklers, but shall follow the ANSI 119.5 standards relating to health, fire and life-safety.
(9) Movable Tiny Houses shall have the following design elements:
     (i) Cladding and Trim - Materials used on the exterior of a moveable tiny house shall exclude single piece composite, laminates, or interlocked metal sheathing;
     (ii) Windows - Windows shall be at least double pane glass and labelled for building use, shall include exterior trim, and excludes windows and doors that have radius corners for windows and doors;
     (iii) Roofing - Roofs shall have a minimum of a 12:2 pitch for greater than 50% of the roof area, and shall be in compliance with building code roofing material; and
     (iv) Living Area Extensions – all exterior walls and roof of a moveable tiny houses used as ADUs shall be fixed with no slide-outs, tip-outs, nor other forms of mechanically articulating room area extensions.


See updates at for some amendments relating to roof pitch, etc. 

New Starter Homes proposal (Portland, 2018-)

see: McCormick, Tim. "New Starter Homes: creating a network of highly affordable, detachable, ownable, 'starter,' smart, tiny homes in Portland." Proposal, initial version August 2018, ongoing updates.


State / local regulations & guides








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






Grand Rapids








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

Twitter: #accessorydwellings