An Accessory Dwelling Unit (ADU) is a small, self-contained residential structure sharing a lot with an existing house. The development of ADUs can be traced back to the early twentieth century, when they were a common feature in single-family housing. The idea was to provide a home for elderly members of the family in a familiar and accessible location. However, their construction was often illegal and unsafe, so it became an issue for local administrations to encourage ADUs but to bring them within planning and building regulations at the same time.
In the late 1970s to the 1990s, some municipalities adopted ADU programmes to permit the use and construction of accessory units. The main objectives of the programme were to:
- Increase the affordable housing stock.
- Regulate formerly illegal and unsafe subdivisions.
- Allow the increasing population of elderly people to remain in secure, comfortable and familiar surroundings.
In Seattle, Santa Cruz, and the Canadian city of Vancouver, legislation has been enacted to allow ADUs to be constructed on sufficiently sized lots in one- and two-family zones to design standards pre-approved by the Department of Planning. 
“Seattle’s experiment with ADUs offers an intriguing solution. One- and two-family homes are permitted to build a self-contained residential structure on their lot.” The Seattle Department of Construction & Inspections has set out the allowable dimensions and standards in its Code. “An ADU is limited to 1,000 square feet in a single-family structure and up to 650 square feet in a rowhouse or townhouse. The ADU must meet current standards of the Seattle residential, building, mechanical, electrical, energy, land use, environmentally critical areas, and shorelines codes.” 
Design competitions, targeted primarily at the specific needs of the elderly have stimulated the creation of cost-effective plans for ADUs. The city prepared training workshops and manuals, such as the “Guide to Building a Backyard Cottage”, to help homeowners take advantage of a one-off amnesty period to make their illegal units comply with planning laws without penalty. “In Vancouver, efforts to expand the affordable housing stock have been even more aggressive. In 2007, the building code was relaxed to allow detached ADUs (not exceeding 500 square feet, a size more appropriate for New York) and to legalise basement conversions (‘secondary suites’).”
The public impact
In Vancouver, from 2010 to 2012, the city issued permits for 778 ADUs and 932 new homes with “secondary suites’”. while legalising 608 existing secondary suites.
Since the start of the Barnstable, Massachusetts’ amnesty programme, the city has approved 160 affordable ADUs under its specific legislation: “Chapter 9, Article II - Accessory Apartments and Apartment Units”.
Public Confidence Strong
Public opinion on ADUs has not been sought in all cities. However, those cities that have surveyed citizen’s opinions seem to have received a positive response:
- Of the nearly 200 people surveyed about ADUs in Minneapolis, 92 percent of survey respondents would support allowing ADUs, and 72 percent would be interested in adding an ADU to their property if this were allowed.
- In Portland Oregon, ADU applications have increased every year since legislation changes paved the way for the construction of ADUs.
- The ‘Yes In My Back Yard’ report indicates that overall public confidence in the legalisation of ADUs is high.
Stakeholder Engagement Good
The main stakeholders in the regulation and construction of ADUs are the local planning departments, any architects, surveyors and builders involved in their construction, and the residents and their families.
In Vancouver, the city provides loans and incentives to homeowners as well as incentives for keeping the units affordable.
Political Commitment Good
There is evidence for state governments amending their planning legislation for ADUs.” “California has passed several laws to lower local regulatory barriers to construction, most recently Assembly Bill 1866 of 2003, which requires that each city in the state have a ministerial process for approving secondary units.” Santa Cruz in California has not only supported accessory dwellings with legislation, but “provides loans to homeowners as well as incentives for keeping the units affordable”. 
There is also significant political commitment in cities further north, especially on the western seaboard, that have implemented ADU legislation.”Seattle first piloted an ADU programme in 1994 and fifteen years later its zoning code was officially amended to allow detached ADUs throughout the city. The application and development process was streamlined by the Department of Planning.” 
Clear Objectives Strong
Although priorities vary from city to city and state to state, as does the name (from ‘granny flat’ to ‘mother-in-law apartment’), there are certain constants:
- Increasing the range of choice of housing accommodations.
- Encouraging greater diversity of population with particular attention to young adults and senior citizens.
- Enabling senior citizens to retain links with their homes and their families.
- Increase the number of small dwelling units available in urban areas.
- Bringing the high number of existing illegal ADUs into compliance with current requirements.
- Encouraging a more economical and energy-efficient use of urban areas’ housing supply while maintaining the appearance and character of its single-family neighbourhoods.
The existing and longstanding ADU programmes in cities such as Santa Cruz, Seattle and Vancouver and states such as California demonstrate that the principle has been firmly established.
ADUs have great potential to address the housing needs of the increasing ageing population and their needs for small and accessible dwellings.
The legal feasibility has been addressed by a number of states and cities by making changes in their planning codes, as set about above. However, those undertaking ADU construction may still require legal advice and support to keep with the planning laws.
There remain a number of other problems related to feasibility.
ADUs are generally built by a small number of people, an individual homeowner, couple or a family unit, so there may be a lack of financial and human resources to undertake the project. One issue is that, if they are being built as a source of rental income, “financing is currently a major barrier to ADU development, largely because ADU rental income is not recognized by the lending system”.  Equally, those seeking to build ADUs are likely to lack sufficient technical knowledge about architectural design and construction techniques, and lack the financial resources to hire them.
Each city and state implements its own housing legislation, so generalising about the management of ADUs is difficult. In Seattle, one of the first American cities to encourage the construction of ADUs, ADUs are managed by homeowners and overseen by the Department of Planning & Development. The oversight includes the following steps:
“Homeowners make a preliminary application to the council, and must meet minimum land area criteria to be accepted. A DPD representative will then visit the site and check that the conditions are suitable for construction. Once the ADU has been approved, the homeowner works with architects and construction firms to plan the ADU. These plans then go back to the DPD before a permit will be issued. Once work is complete on the ADU, it will be inspected by the DPD to ensure compliance with building codes.” 
The aspects of ADU programmes that can be measured are the increase in legal ADU developments as a result of changes to legislation, such as zoning changes for car parking spaces (Seattle) and allowing a maximum of three affordable accessory dwelling units per lot (Wellfleet, Massachusetts), to facilitate and encourage ADUs.
This includes measuring the number of units transitioning from being illegal constructions to legally compliant ADUs as a result of amnesty programmes, e.g. Barnstable approving 160 such ADUs (see Public impact above).
The development of ADUs depends on the coordination and settlement between the local planning authorities, the professionals engaged in the design and build process, and the homeowners.
There are examples of good alignment throughout North America, for example in California (the State generally and in Santa Cruz and other cities), Massachusetts (Barnstable and Wellfleet), Washington State (Seattle) and British Columbia (Vancouver). In these cases, changes to planning and financial legislation are key.