ADU meaning
ADU stands for Accessory Dwelling Unit: a secondary, independent living space on a property that already has a primary single-family or multifamily home. An ADU has its own entrance, kitchen, bathroom, and sleeping area, and can legally be rented or occupied separately from the main house. It is sometimes called a granny flat, in-law unit, backyard cottage, or secondary unit — all describe the same general category under California law.
The three common ADU types
- Detached ADU: a freestanding structure separate from the main house, built new in the yard. See our detached vs. attached comparison.
- Attached ADU: built as an addition sharing a wall with the primary residence.
- Conversion ADU: created from existing space, most commonly a garage. See our ADU vs. garage conversion guide.
There is also a related but distinct category called a JADU (Junior ADU): a smaller unit, generally created within the walls of the existing home, that may share some systems like a bathroom with the main house. JADUs follow somewhat different size and permit rules than full ADUs. See our glossary for the exact distinction.
Why California homeowners build ADUs
The most common reasons homeowners cite are rental income, housing for aging parents or adult children, and increasing overall property value. California's statewide ADU law (Government Code §65852.2) was specifically designed to make ADUs easier to permit than in the past, which is part of why ADU construction has grown so quickly across the state. Our permit process guide explains how that law actually works.
Is an ADU right for your property?
Lot size, existing structures, local zoning, and your budget all factor in. Rather than guess from general information, the most reliable next step is a conversation with a licensed contractor who can evaluate your specific property — request a match when you are ready.
ADUs vs. a standard home addition
A standard room addition typically expands the primary home's existing living space and is not intended as a separate, independently occupiable unit. An ADU, by contrast, must function as its own legal dwelling — complete kitchen, complete bathroom, and its own entrance — which is what allows it to be rented separately or occupied by someone outside your immediate household under California law.
What an ADU is not
An ADU is not the same as an unpermitted "bootleg" unit sometimes found in older homes — a converted basement or garage space without proper permits does not become a legal ADU just because someone lives in it. Building or converting a space into a legal ADU means going through the permit process in our ADU permit process guide, which is what protects both the homeowner and any future occupant.
Have a question about your specific project?
Call (877) 300-2747 or request a match, and mention you have questions related to “What Is an ADU?” — we will route you to a licensed contractor who can speak to it directly.