Guide: An Overview of the Tool
What is it? An accessory dwelling unit (or ADU) is a small, self-contained residential unit built on the same lot of an existing single-family home. (Because they are often used by extended family members, ADUs are also referred to as ‘in-law apartments or ‘granny flats’). As the images below illustrate, ADUs may be built within a primary residence (such as in an attic or basement), attached to the primary residence (like a small duplex unit with a separate entrance), or detached from the primary residence (such as conversion of a detached garage). In all cases, an ADU must be subordinate in size, location, and function to the primary residential unit (which is why ADUs are sometimes referred to as ‘secondary units’).
Why use it? Current unmet housing needs have made ADUs an important component of the housing strategy for many communities. ADUs not only help increase the supply of affordable housing, they provide innumerable other benefits as well. Here’s a few of the ways that ADUs can benefit your community:
| > |
Creating cost-effective affordable housing. ADUs are a particularly cost-effective source of affordable housing that incurs little or no public expense. This is because homeowners themselves, rather than government subsidies, pay for the construction of ADUs.
|
| > |
Encouraging mixed-income, stable neighborhoods. ADUs tend to be better integrated into the community than other forms of affordable housing and over time they can encourage the development of mixed-income neighborhoods (rather than concentrated residential pockets of poverty and wealth). ADUs also serve to improve neighborhood stability by providing homeowners of all ages and all income with the extra money and social support they may need to remain in their homes as their situation changes over time (e.g., during a prolonged period of unemployment or illness that cuts into the household’s income or raises its expenses, the additional income from an ADU could mean the difference between being able to stay in one’s home and being forced to move). ADUs can also help young families who are buying their first home or single parents after a divorce meet their initial mortgage payments or cover unforeseen costs such as paying for major repairs that might otherwise prevent them from staying in the neighborhood.
|
| > |
Developing more efficiently. ADUs use resources very efficiently compared to many other types of residential development. They require no additional land for development and use existing infrastructure (such as water, sewer, roads, and other utilities), so they increase the supply of affordable housing without promoting sprawl or requiring the construction of expensive new infrastructure.
|
| > |
Promoting balanced transportation. ADUs can also increase the rates of carpooling, transit ridership, biking, and walking and reduce traffic congestion on local streets. This is because the higher residential densities created by ADUs make public transit more feasible in neighborhood areas that were not previously well-served by transit (or not served at all). Moreover, ADUs tend to attract persons who own fewer cars (e.g., the elderly, smaller families, and/or those of lesser means), and having fewer cars in neighborhoods where ADUs are encouraged can also help to boost carpooling, transit ridership, biking, and walking in these areas. And since ADUs don’t require additional land, they are more easily incorporated into built-out neighborhoods than other forms of housing and can therefore be located in older, centrally-located neighborhoods that are closer to employment opportunities, making it easier to get to work without needing a car.
|
| > |
Supporting local businesses, expanding the local tax base. Since many residents of ADUs do not drive, they are more likely to shop in their own neighborhoods rather than drive to adjacent communities, thereby providing the patronage necessary to sustain local retailers and increasing local sales tax revenues.
|
| > |
Responding to changing community needs. ADUs can help your community respond to changing future population and demographic trends (such as the growing elderly population and the growing numbers of smaller families). A consequence of these trends is an emerging need for different housing types: just as high-rise towers are no longer seen as the best method for creating affordable housing, the split-level detached suburban house is no longer the an adequate or desirable ideal for many Americans. By creating a greater variety of housing types in your community, ADUs can help meet the housing demands of an increasingly diverse housing market. (For more information on how ADUs can help your community respond to these population and demographic trends, see the sidebar titled
Changing Communities, Changing
Needs).
|
How does it work? The legal authority for simply allowing and/or actively encouraging ADUs in most communities is the local zoning code. (The zoning code essentially defines and describes the kind of development that can occur in different areas of your community.) ADU regulations are developed by city or county planning department and then officially adopted as part of the zoning code via passage of an ordinance by the City Council or County Board of Supervisors. As a result, the zoning code regulations on ADUs vary from community to community. Although some states may require their local governments to adopt an ordinance authorizing the creation of ADUs (as is the case for Washington and California), local governments generally retain the authority to determine the specific details of the ADU ordinance, such as:
| > |
Location. The area in the community where ADUs are permitted (and the areas where they are not)
|
| > |
Development Standards. The standard for the ADU units, including parking, height, setback, lot coverage, architectural review, and size.
|
| > |
Density. The allowable density for the lot upon which the ADU and primary residential unit are located.
|
Some communities’ regulations are more restrictive than others. For example, nearly all communities require homeowner occupancy of the primary residential unit, but the requirement varies widely: Seattle (WA) allows homeowners who can present ‘good cause’ to live elsewhere for up to three years, while in Sunnyvale (CA) the primary residence must be owner occupied for a minimum of 20 years. The stringency of the regulations depends on a community’s objectives and needs, its perceptions of ADUs, and the local political and economic conditions at the time. For example, ADUs would likely face more opposition in areas where rapid population increases and/or neighborhood turnover has led to an anti-growth or anti-development political sentiment; ADU ordinances in such communities would likely be more restrictive than those areas experiencing slower, more manageable growth. (You can get more info on writing zoning regulations that will be effective in encouraging ADUs in your community and see examples of model state and local ordinances that you can use in your community, by jumping to the ‘How to Put this Tool into Action in Your Community’ section.)
 
|
