Last reviewed: August 11, 2026. Planning ranges are based on contractor experience and are not city service guarantees. Rules and lender requirements vary by property, jurisdiction, and program.
ADU stands for accessory dwelling unit — a complete, self-contained dwelling on the same lot as a primary residence, with permanent provisions for living, sleeping, eating, cooking and sanitation. You may know it as a granny flat, in-law suite, casita or backyard cottage. California law provides ministerial pathways for ADUs on many qualifying residential lots, subject to objective standards, site conditions, and the applicable state and local rules.
“ADU” is the least helpful name in housing — three letters that explain nothing. So here is the plain-English version of what an accessory dwelling unit actually is, what counts as one, and why Orange County homeowners keep building them.
01What “accessory dwelling unit” actually means
Break the term apart and it makes sense:
- Accessory — it’s secondary to the main house on the lot, not a separate property you can sell on its own.
- Dwelling — it’s a real home, not a shed or a bonus room. Legally that means permanent provisions for living, sleeping, eating, cooking and sanitation: a kitchen, a bathroom, and its own entrance.
- Unit — it’s independent. A tenant, a parent or an adult child can live there without walking through your house.
That last part is the legal line that separates an ADU from a “guest room over the garage.” No kitchen, no separate entrance — not a dwelling unit.
02One thing, many names
All of these are the same legal object — an ADU — wearing different outfits:
| What people call it | What it usually means |
|---|---|
| Granny flat | An ADU built with aging parents in mind — see our granny flat guide |
| In-law suite | Same idea, in-laws edition — often attached to the main house |
| Casita | The Spanish-derived name common across Southern California — a permitted casita is an ADU |
| Backyard cottage / backyard home | A detached ADU standing free in the yard |
| Garage conversion | An ADU created inside an existing garage — see the garage conversion guide |
| Second unit / granny unit | Older planning-department vocabulary for the same thing |
The name changes; the permit does not. Whatever you call it, the city reviews it as an accessory dwelling unit.
03The four types (plus one junior)
- Detached ADU
A detached ADU is a separate backyard dwelling. Government Code section 66321 prevents a local ordinance from setting a maximum below 850 square feet, or 1,000 square feet for an ADU with more than one bedroom, and protects a pathway for at least an 800-square-foot unit under specified standards. Local ordinances may allow larger units, so verify the maximum for the property rather than assuming a statewide 1,200-square-foot entitlement.
- Attached ADU
Shares a wall with the main house (including above-garage units) — efficient on tighter lots, and the most reliable path to a two-story unit.
- Garage conversion
The existing garage becomes the home — usually the lowest-cost path because the slab, walls and roof already exist.
- Junior ADU (JADU)
A JADU is no more than 500 square feet of interior livable space and is contained within a proposed or existing single-family residence, which may include an attached garage. Under Government Code section 66333, owner occupancy may be required when the JADU shares sanitation facilities with the existing structure; it is not required when the JADU has separate sanitation facilities, subject to the statute's other exceptions and deed restriction requirements.
04Why California made ADUs easy
Since 2017, a series of state laws has stripped away the barriers that used to make second units nearly impossible: approval is now ministerial (no public hearing, no neighbor veto), side and rear setbacks drop to 4 feet, cities cannot demand replacement parking in most cases, and since 2026 the review itself runs on a legal clock — 15 days to confirm your application is complete, 60 days to decide. Cities can apply objective standards; they cannot say no to a code-compliant unit. The full rulebook lives in our California ADU laws guide.
05What homeowners actually use them for
- Long-term rental use — rent varies by city, unit size, utilities, parking, condition, and current comparable listings. Use current property-specific market evidence rather than a countywide promise.
- Family under a separate roof — aging parents or adult children, close but independent.
- Long-term property value — a legal, permitted second dwelling on the lot.
- Space that flexes — office now, rental later, parents after that.
Project cost depends on the ADU type, scope, site, utilities, access and finishes. Review the current pricing references and project-specific cost drivers in our ADU cost guide, and review the assessment framework in the property-tax explainer.