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What is an ADU?

In construction and real estate, ADU means “accessory dwelling unit.” An ADU is a secondary housing unit that exists on a property that’s zoned for single-family or multifamily residential use. ADUs can be detached from the main house (such as a new stand-alone home in the backyard), attached to the main house (like a garage conversion), or part of the main house (like an upstairs unit). ADUs include a full kitchen, a bathroom, and a separate entrance.

ADUs, commonly referred to as “granny flats,” “in-law units,” or “backyard cottages,” have emerged as a practical response to the housing shortages in many urban areas. These compact, efficient homes provide a versatile option for property owners, enabling them to make the most of their existing space in several ways. Beyond serving as additional living quarters for multigenerational living or a source of rental income, ADUs can fulfill a variety of other needs. They can offer a private and comfortable space for guests, serve as a home office or studio for those working remotely or pursuing creative endeavors, or even function as a dedicated area for hobbies and leisure activities that require more room than the main house can afford. This adaptability not only >enhances the value of one’s property  but also contributes to addressing broader housing challenges by increasing the available housing stock in a community.

ADU terminology FAQs

What is the difference between an ADU and a guest house? Is a granny flat the same as an ADU?

ADU, granny flat, companion unit, casita, guest house, granny cottage, secondary living unit, mother-in-law apartment, second dwelling unit, in law suite, carriage house, au-pair suite… you may hear any of these terms used interchangeably for an accessory dwelling unit (sometimes mistakenly called an “additional dwelling unit,” “auxiliary dwelling unit” or “accessible dwelling unit”… we like to think they’re awesome dwelling units).

What is a Junior Accessory Dwelling Unit (JADU)?

Junior ADUs (JADUs) offer an option to add another unit (in addition to an ADU) within your existing home. JADUs must be contained within an existing or proposed single family home (think garage or spare bedroom), under 500sqft (you can add on up to 150 sqft to get to 500), and have a full bathroom and efficiency kitchen (appliances + cabinets). ADUs also come with owner occupancy requirements, meaning the property owner must live somewhere on the premises.

What are guest living quarters?

ADUs are also sometimes called guest houses, though there is a technical definition of “guest living quarters,” which are different than ADUs. Guest Living Quarters are only for use by those employed on the premises or for temporary use by guests and may not be rented out. Guest Living Quarters may not have kitchens or laundry facilities. In some municipalities, guest living quarters can co-exist with an ADU, but the County of San Diego only allows an ADU or a guest living quarters.

What about tiny homes as ADUs?

Tiny homes have recently been classified as a type of ADU in San Diego – you can have a detached tiny home (150-430 sqft in size, licensed by the DMV) as your ADU in your back yard. Note that this is not in addition to having an ADU; the tiny home is classified as the ADU. Tiny Homes must be connected to all utilities and sit on a concrete pad, which means the all-in costs of using a tiny home for your ADU often end up looking very similar to a permanent ADU.

Read more about  tiny homes vs ADUs.