What Is an ADU? Accessory Dwelling Units Explained
Updated June 18, 2026 · Upside ADU
Quick answer
An accessory dwelling unit (ADU) is a small, self-contained home built on the same lot as a primary residence. Also called a granny flat, in-law unit, or backyard cottage, an ADU has its own kitchen, bathroom, and private entrance. In California, ADUs are legal on most residential lots statewide.
What does ADU stand for?
ADU stands for accessory dwelling unit — the legal term for a secondary home that shares a lot with a primary residence. The phrase appears in California Government Code and in every city's zoning code, which is why permit offices, lenders, and appraisers all use it.
You'll also hear informal names for the exact same thing: granny flat, in-law unit, in-law suite, backyard cottage, casita, or secondary suite. They all describe an ADU. What makes a structure an ADU is that it is independently livable — it has its own kitchen, bathroom, sleeping area, and entrance.
What are the types of ADUs?
California recognizes several ADU types. Which one fits depends on your lot size, budget, and goal — rental income, family housing, or home office.
The five ADU types at a glance
| Type | What it is | Typical size | Best for |
|---|---|---|---|
| Detached ADU | A standalone home in the backyard | 400–1,200 sq ft | Highest rent and resale value |
| Garage conversion ADU | An existing garage converted to a home | 200–600 sq ft | Lowest-cost path to an ADU |
| Attached ADU | An addition that shares a wall with the main house | 400–1,000 sq ft | Lots too small for detached |
| Junior ADU (JADU) | A unit up to 500 sq ft inside the existing home | Up to 500 sq ft | Cheapest, fastest to permit |
| Multigenerational ADU | A single-level, aging-in-place design | 500–1,200 sq ft | Housing parents or adult children |
See also:Detached ADU · Garage conversion ADU · Junior ADU (JADU)
What's the difference between an ADU and a JADU?
A JADU (junior accessory dwelling unit) is a smaller, special category of ADU created within the walls of an existing single-family home. A JADU is capped at 500 square feet, must be carved out of the existing house (often a converted bedroom), and can share a bathroom with the main home. It also requires the owner to live on the property.
A standard ADU has no 500 sq ft cap, can be detached or attached, needs its own bathroom, and — for a single ADU on a single-family lot — does not carry a state owner-occupancy requirement. In short: every JADU is an ADU, but most ADUs are not JADUs.
See also:Junior ADU (JADU) guide
Is an ADU the same as a granny flat or a guest house?
A granny flat is an ADU — it's just an older, informal name. The same goes for in-law unit, casita, and backyard cottage. If the structure has a kitchen, bathroom, and its own entrance, it is legally an ADU regardless of what you call it.
A guest house is different. A guest room or pool house without a kitchen is not an ADU — it's an accessory structure or guest quarters, because no one can live in it independently. The kitchen and independent living facilities are the legal line between a guest space and an ADU.
How much does an ADU cost?
A detached ADU in the Sacramento region runs roughly $165,000–$500,000 turnkey in 2026, garage conversions start near $95,000, and Junior ADUs near $85,000. Cost depends on size, finishes, site conditions, and ADU type. For full ranges by city and a personalized estimate, use the resources below.
See also:How much does an ADU cost in Sacramento? — full cost breakdown · ADU Cost Calculator — estimate your build
Are ADUs legal in California?
Yes. California has passed a series of laws that make ADUs legal on most residential lots statewide and limit how much cities can restrict them. Local agencies must review a complete ADU application ministerially — without discretionary hearings — typically within 60 days, and most single-family lots can add both an ADU and a JADU.
Local rules still govern setbacks, height, and design within state limits, so the specifics vary by jurisdiction. See the Sacramento-region rules and the statewide law guides for details.
See also:Sacramento ADU rules, setbacks & permits · California ADU law in 2026
Why build an ADU?
- Rental income — a backyard unit can generate ongoing monthly cash flow.
- Multigenerational living — house aging parents or adult children with privacy.
- Home office or studio — dedicated work space separate from the main home.
- Property value — a permitted, rentable ADU can raise what your property is worth.
- Flexibility — use it as a guest suite now and rent it later.
See also:ADU for rental income · ADU for aging parents · ADU rental income & ROI
This guide is general information, not legal or tax advice. ADU rules change often and vary by city — we confirm the current requirements for your jurisdiction during your free feasibility check.
Sources & references
- Accessory Dwelling Units — official guidance — California Dept. of Housing & Community Development (HCD)
- SB 1211 (2024) — ADUs on multifamily lots — California Legislative Information
- AB 1033 (2023) — separate condo sale of ADUs — California Legislative Information
External links open official government and lender resources. Construction price and rent figures reflect 2026 Sacramento-region market conditions; confirm current rules and fees with your jurisdiction.