Accessory Dwelling Units (ADUs) are growing in popularity as a senior housing option. An ADU (also known as a “granny flat” or “granny pod”) is a secondary residence that occupies the same land as a primary home. It may be a free-standing building, such as a detached garage that has been converted into living quarters. It may be an addition to an existing structure, such as an apartment constructed above a garage. It may be a basement converted into living space or an addition to a house with a separate entrance.

Most communities require a detached ADU to have a bathroom and a kitchen. An ADU might have a separate bedroom or be set up as a studio apartment that combines a living and sleeping area. An ADU typically ranges in size from 400 to 800 square feet, although some are as small as 150 square feet or as large as 1,200 square feet. Community zoning laws usually set minimum and maximum size limits for detached ADUs.

Advantages of ADUs

Children who own homes are turning to ADUs as a way to provide rent-free housing for their aging parents. Downsizing seniors might use the proceeds of a home sale to fund the construction of an ADU on a child’s property. When parents are no longer living, the family may be able to rent the ADU to long-term tenants (perhaps to another senior couple) or as short-term housing on a site like Airbnb.

An ADU provides aging parents with the opportunity to live independently while still receiving needed help from their children. By occupying their own space, older parents avoid the fear of burdening their children by their constant presence. Parents who live in an ADU can easily drop in on their children for shared meals while enjoying meals in their own residence when they feel the need for privacy.

Many cities are experiencing a shortage of affordable housing, making it difficult for older parents living on a fixed income to find a suitable place to live. Expanding the housing market with ADUs eases some of the pressure caused by too many people looking for too little housing. While ADUs increase housing density, they do so in ways that don’t alter the look or feel of neighborhoods.

Community Responses to ADUs

The New York Times reports that many communities recognize the advantages of ADUs and encourage their construction. Several states and municipalities have eased zoning restrictions, parking regulations, and other barriers that make it difficult for families to add an ADU to their property.

Zoning regulations typically limit the number of dwelling units that can occupy a single residential lot. Amendments to those regulations may be needed to permit the construction of ADUs. While families can seek a zoning variance in communities that are not ADU-friendly, obtaining a variance can be a time-consuming process. According to the Times, communities that have streamlined the process to encourage ADUs have generally been pleased with the results.

About half of all ADUs are in California, Florida, Texas, and Georgia. New Jersey is considering legislation that would encourage ADUs. Communities in other states that have recently eased restrictions on ADUs include Louisville, Denver, Raleigh, and Kansas City.

An ADU can add value to property and enhance its resale value. Whether a property buyer will be allowed to rent an ADU to individuals who are not family members depends on local zoning regulations. Some communities limit ADU occupancy to close relatives.

While the advantage of ADUs is obvious to many, the norm in small communities is to zone certain residential areas for single-family dwellings. City councils may resist change because single-family housing is seen as protecting property values by reducing population density. Historically, single-family housing has also been viewed as a way to keep low-income and minority families out of affluent neighborhoods. Some city governments are averse to changing that historical trend.

Building an ADU

Property owners should investigate zoning regulations before deciding whether to add an ADU to their property. Local regulations might prohibit a new free-standing building but permit a home addition that complies with set-back rules and size limitations. 

Property owners should also consider their parents’ needs. Adding an ADU above a garage might not be practical if aging parents with hip or knee problems will soon be unable to climb stairs. Exterior stairs can also be problematic in colder climates where ice accumulation creates safety issues.

Regulations typically restrain the nature of a detached ADU that homeowners can build. State or local laws may require an ADU to satisfy both a minimum and a maximum size requirement as well as a limitation on height.

The maximum permitted size of a detached ADU may be expressed in square feet, but the maximum size might also be tied to the size of the primary dwelling. For example, an ordinance might allow an ADU that is 1,200 square feet or half the size of the primary residence, whichever is less.

A detached ADU’s maximum size might also depend on its design. Local rules might permit a one-bedroom unit to be larger than a studio unit. An ADU that is located within an existing home is not usually subject to size restrictions, provided that the ADU does not change the home’s existing footprint.

Local ordinances typically require an ADU to have electricity, running water, a kitchen, and a bathroom. A detached ADU will need to comply with building codes that may require windows and doors of a specific size.

A study in California found that the average ADU is a one-bedroom unit of about 600 square feet. With an average construction cost of $250 per square foot, California property owners spend an average of $150,000 to build an ADU. 

Construction costs tend to be higher in California than in most other states, so the expense of building an ADU will vary by location. Even in California, average construction costs per square foot range from $329 in the San Francisco Bay area to $197 in Los Angeles County.

Building permits are almost always required before an ADU can be constructed. Homeowners may learn about unanticipated roadblocks to construction when they apply for the permit. Working with a contractor who has experience building ADUs in a particular community will help property owners avoid surprises.