An accessory dwelling unit (ADU) is the term used to describe an additional home on a large lot but is not deemed the primary home or residence. This building can be attached to the primary home or serve as a stand-alone building. ADUs are becoming more popular to build for additional housing and sources of income.
How Big is an ADU
The size of an ADU is dependent on the size of the primary home and the property. These regulations are dictated by state law, more specifically county and city laws, with some municipalities having stricter regulations and size requirements than others.
Laws are being instituted to increase ADU sizes to account for housing shortages and affordability.
We will look more specifically at the size of ADUs and the regulations that are in place to ensure that safety is being upheld. Because these sizing laws are quite specific to states and local governments, we will provide general guidelines for sizing as well as a sample of limits across the country.
Sizing of an ADU
The regulations imposed on the size of an ADU vary based on state or local government law. California has established itself as one of the leaders in ADU legislation and protections, especially to combat the growing need for affordable and accessible housing in the state.
The size of an ADU is now solely dependent on the size of the primary unit on a property.
In Los Angeles County, for example, the size of an ADU varies based on if it is built on a single-family or multi-family residence:
- For single-family residences: An ADU cannot exceed 1,200 sq. ft. and can be 50% of the existing unit (if smaller than 1,200 sq. ft.). This applies to both attached and detached ADUs.
- Junior accessory dwelling unit (JADU) cannot exceed 500 sq. ft.
- For existing multi-family homes: An attached ADU must be less or equal to 50% of the largest unit’s sq. ft. Detached ADUs for multi-family homes cannot exceed 1,200 sq. ft.
Height Regulations of an ADU
Height requirements will vary based on the zone the home is in, the number of stories on the ADU, and environmental factors. Adaptations may need to be considered in areas with powerlines.
Heights typically cannot exceed 24 feet for two-story dwellings (multi-family) and range between 16-18 feet for one stories. Local municipalities will have specific limitations. If cities do not have specific requirements for ADUs or JADUs, state law applies.
Guidelines for an ADU in Some U.S. States
Many of the previously mentioned guidelines are applied throughout the state of California and are an example for other states to follow; however, California is generally more liberal in its regulations when compared to other states.
Here are some restrictions on other states to get a good idea of what your general size guidelines will be (these will vary by local government, so check in with your area’s rules):
| State | ADU Size Limits | Other Restrictions |
| Oregon | 800 sq. ft. | |
| Washington | 800-1,000 sq. ft. | Many counties based on lot size and size of primary home |
| Arizona | 900 sq. ft. | Cannot exceed 50% of primary house floor size. Height cannot be taller than primary dwelling. |
| New Hampshire | Typically 800 sq. ft. | |
| Florida | Cannot exceed 35% of backyard (Orlando) | Some cities (including Jacksonville) do not allow for ADUs |
| Vermont | Cannot exceed 30% of the floor area of single-family unit | |
| Texas | 1,100 sq. ft. maximum or 15% of lot size (smaller) | Varies by city, but includes lot requirements in many cases |
While this is a small sample size of states, it shows that the regulations regarding ADUs are broad in their definitions and vary according to local governments.
Generally, ADUs will range from 700-1,200 sq. ft. across the United States. You should look at the local ordinances for specific dimensions and other restrictions in your state or city.
What are the General Guidelines for ADUs?
Also known as a granny flat, accessory unit, or secondary suit, an ADU can be an internal, attached or separate unit for the purpose of providing larger living spaces. The loosening of restrictions on ADUs aims to make housing more affordable and alleviate the shortage of affordable housing in many communities across the United States.
The zoning and regulatory restrictions in place in many states have made it difficult to build ADUs in some areas of the United States; however, these laws and regulations are beginning to change as more people need affordable housing options.
There are general guidelines as to how states can and should set up ordinances and laws to allow for the building of accessory dwelling units.
- Definition: An ADU is defined as being an independent residential unit designed for one or more persons to live in. This means it is equipped with necessities for cooking and sanitation.
- Zoning: Many cities may restrict the locations of ADUs in a community based on need as well as feasibility in construction. ADUs can typically be built on properties of single and multi-family homes.
- Rights: Some states have determined that building an “ADU” is a right of citizens, while others require more extensive permit processes.
- Owner occupancy: Most properties with ADUs must be owner-occupied, whether that be in the primary unit or ADU.
- Density and city planning: ADU building will often adhere to zoning and city planning regulations that are already in place
- Parking: There should not be more than one parking spot per unit or number of bedrooms (whichever is higher). Additional parking will be dependent on the lot and local parking regulations that exist.
- Fees: Standard fees that align with the cost of building a single-family home are in place as not to discourage building.
- Building restrictions: Any codes applied to the general building of residential properties will be applied to ADUs
- Safety: Building the unit must not interfere with community safety or be in areas that pose risks to public safety.
Many of these standards are based on California law as they have been some of the most progressive and detailed in ADU regulations. These are standards that local governments can model their ADU regulations after and modify to meet local needs.
What is a JADU?
A junior accessory dwelling unit (JADU) is a smaller unit that must be attached to an existing primary residence. In the state of California, this is a smaller unit that must not exceed 500 sq. ft. Unlike an ADU, the JADU makes use of existing space within a home and converts it into an independent living space. The home is not increased in size; rather, it is repurposed.
The JADU can make use of the primary unit’s utilities and does not need a separate electrical, sanitation, or heating system to remain in compliance. This places less stress on resources and is less invasive to overall building and planning.
An ADU and JADU may both be built on one property, according to California law. The same size restrictions apply for the building of both units.
How Big Should an ADU Be?
Now that you have the data regarding how big an ADU can be, you will have to determine the appropriate sizing. The size should be dependent on needs, economic feasibility, and the value it will bring to your property. A smaller ADU may serve your needs without needing to maximize the allowed dimensions.
Especially if you are looking at attached ADUs or JADUs, the sizing will be largely dependent on the space and capabilities of the existing primary unit. In addition to restrictions and zoning, careful planning should be done to optimize the building process and take advantage of these additional living opportunities.
Conclusion
In conclusion, the size of an ADU will vary from state, county to county and city to city. However, even with laws that vary, the size of an ADU will never be greater than that of the primary residence.
As ADUs continue to grow in size and in popularity, you should expect to see laws and ordinances to adapt and change to help meet the growing demand for these accessory dwelling units.