Quick answer
An accessory dwelling unit (ADU) is a second, self-contained home on a lot that already has, or will have, a main house or apartment building, with its own space to live, sleep, cook, eat and wash.
It can be attached to the main home, built inside it, or stand on its own in the yard. It shares the lot with the main home and, in most places, cannot be sold off separately.
Quick facts
- Also called
- Granny flat, in-law unit, second unit, backyard cottage, casita, accessory apartment 7
- Must contain
- Permanent provisions for living, sleeping, eating, cooking and sanitation (California definition) 1
- Forms
- Attached, detached, or converted from existing space such as a garage 2
- California law
- Gov. Code Chapter 13, §§ 66310–66342 (formerly § 65852.2) 1
- Sale
- California: may be rented, not sold apart from the main home, except under Article 4 (§ 66340 and following) 2
- Smaller cousin
- Junior ADU: up to 500 sq ft, inside a single-family home (California) 1
ADU meaning in plain English
Think of an ADU as a small, complete home that lives on the same lot as a bigger one. It has its own front door, its own kitchen and its own bathroom.
Someone can live there without walking through the main house to cook a meal or take a shower. That independence is what separates an ADU from a guest room, a pool house or a home office.
The word accessory is a zoning term. It means the unit is secondary to the main use of the lot, which is the primary home. The word dwelling unit means a place built for someone to live in full time.
Put together, an accessory dwelling unit is a full home that the zoning code treats as an add-on to the main one, not as a second house on its own lot.
HUD described ADUs in a 2008 case study as units "also referred to as granny flats, accessory apartments, or second units", built by communities as a low-cost way to add housing on land that is already developed 7. That is still the core idea: more homes without new subdivisions.
Key takeaways
- An ADU is a full second home on a lot with a main home: its own kitchen, bathroom, sleeping space and door.
- Granny flat, in-law unit, casita and backyard cottage are nicknames. The legal term is ADU.
- In California the definition is in Gov. Code § 66313(a). The old § 65852.2 is no longer the current law 1.
- There are five common forms: detached, attached, garage conversion, interior and junior ADU.
- In California an ADU may be rented, but in most cases not sold apart from the main home 2.
- Rules outside California vary by state and city. Check both before you plan.
The legal definition (California)
California writes the definition into statute. Under Government Code § 66313(a), an accessory dwelling unit is "an attached or a detached residential dwelling unit that provides complete independent living facilities for one or more persons and is located on a lot with a proposed or existing primary residence." It must include permanent provisions for living, sleeping, eating, cooking and sanitation on the same parcel as the single-family or multifamily home 1.
The same subdivision says an ADU also includes an efficiency unit (as defined in Health and Safety Code § 17958.1) and a manufactured home (as defined in Health and Safety Code § 18007). So a small studio and a factory-built home both count, provided they meet the rest of the law 1.
The diagram below shows the six parts of the test. If a space is missing any one of them, it is not an ADU under California law.
This definition used to sit in Gov. Code § 65852.2, which SB 477 (Stats. 2024, ch. 7, effective 25 March 2024) repealed and moved into Chapter 13 (§§ 66310–66342).
SB 543 (Stats. 2025, ch. 520) amended § 66313 again, effective 1 January 2026 1. A guide that still cites § 65852.2 as current law is out of date. The full trail is on the ADU regulations hub.
- LivingA permanent space to spend the day in
- SleepingA permanent place to sleep
- EatingRoom to eat inside the unit
- CookingPermanent cooking facilities; a sink alone does not count
- SanitationA toilet and a place to wash
- Same lotOn a parcel with a proposed or existing primary home
California’s definition. Other states write their own, though most use the same idea of a complete home. Source: [1]
Sources: [1]
Granny flat, in-law unit, casita: the other names for an ADU
| Name | Where you hear it | What it usually means |
|---|---|---|
| Accessory dwelling unit (ADU) | Zoning codes, state law, lenders | The legal and planning term for every form below |
| Granny flat | Everyday speech, older city codes | A small unit for a parent or grandparent; no age rule attaches to the name 7 |
| In-law unit / in-law suite | Real-estate listings | A unit for relatives, often attached or inside the house. An "in-law suite" without its own kitchen is not an ADU |
| Second unit / secondary dwelling unit | Older California law and ordinances | California’s earlier statutory term; the current law says accessory dwelling unit |
| Accessory apartment | East Coast and HUD usage | Usually a unit inside or attached to the main house 7 |
| Backyard cottage | Everyday speech | A detached ADU in the yard |
| Casita | Everyday speech (Spanish for "little house") | A small detached guest house; it is an ADU only if it has a kitchen and full facilities |
| Carriage house / garage apartment | Older neighborhoods | A unit over or behind a garage; see above-garage ADU |
The types of ADU
| Type | What it is | Key California rule |
|---|---|---|
| Detached ADU | A separate building on the lot | Up to 1,200 sq ft under the state standard; at least 16 ft of height must be allowed 2 4 |
| Attached ADU | An addition that shares a wall with the main home | Up to 50% of the existing home’s floor area, but a local rule cannot block an 800 sq ft unit 2 4 |
| Garage conversion ADU | An existing garage turned into a home | No new setback for the existing structure and no replacement parking 2 |
| Interior ADU | Space inside the main house (basement, attic, wing) converted to a unit | Approved ministerially when it fits the existing space 5 |
| Junior ADU (JADU) | A unit of up to 500 sq ft carved out of a single-family home | Its own statutory category, with an efficiency kitchen and a deed restriction 1 |
ADU vs junior ADU
| ADU | Junior ADU | |
|---|---|---|
| Where | Attached, detached or inside the home | Entirely within the walls of a single-family home (an attached garage counts) 6 |
| Size | Local limits, with state floors | No more than 500 sq ft of interior livable space 1 |
| Kitchen | Full cooking facilities | Efficiency kitchen: cooking appliance, prep counter, storage 6 |
| Bathroom | Its own | May share with the main home 1 |
| Owner occupancy | Not a state requirement for the ADU itself | Required only when the bathroom is shared 6 |
| Rental term | Set locally | Longer than 30 days 6 |
Important distinction
A junior ADU is not just a small ADU. In California it has its own article of law (Gov. Code § 66333 and after), its own size cap and its own deed restriction. A 450 sq ft detached unit is an ADU, not a JADU 6.
What is not an ADU
Listings and contractors use the word loosely. These are the common cases that fail the definition. The decision guide after the list sorts a space in one pass:
- A guest house or pool house without a kitchen. No cooking facilities means no dwelling unit, whatever the listing says.
- A room addition or bonus room that opens only into the main house and shares its kitchen.
- A tiny house on wheels or an RV parked in the yard. California's definition requires permanent provisions for living, and state law classes a unit on wheels as a recreational vehicle or park trailer; see tiny home ADU.
- A second house on its own lot. If the lot is split, the unit is no longer accessory to anything; that is a lot split, covered on the SB 9 lot-split research page.
- An unpermitted unit. It may function as a home, but it is not a legal ADU until it is permitted. See the legalization research.
- If It has no permanent cooking facilities
- Then Not an ADU. It is a guest room, pool house or office.
- If It opens only into the main house and shares its kitchen
- Then A room addition, not a separate home.
- If It sits on wheels, like an RV or tiny house on a trailer
- Then A vehicle under California law, not an ADU. See the tiny home page.
- If It stands on its own lot after a split
- Then A second house on a new lot, not an accessory unit.
- If It has every part of a home but no permit
- Then Not a legal ADU yet. Ask the city about legalizing it.
- If It passes every check above and is permitted
- Then An ADU, or a junior ADU if it is 500 sq ft or less inside a single-family home.
Based on California’s definition in Gov. Code § 66313. Your city applies the test to your plans. Source: [1]
How an ADU fits the rules around it
- Residential zoning → state ADU law and the local ADU zoning rules → lot, setbacks and height → ADU size → permit → a legal second home on the lot
ADUs as housing
Searches for "ADU housing" usually mean one question: are these real homes? In law, yes.
A permitted ADU is a dwelling unit, which is why cities count it toward housing supply and why California lets it be rented separately from the main home 2.
It is not a hotel room or a vacation add-on by default; short-term rental rules are set locally and summarized on the short-term rental research page.
ADUs house relatives, caregivers, tenants and owners who downsize into the smaller unit and rent the main house. Who actually lives in them is tracked on the who lives in ADUs page, and production counts are on ADU housing supply statistics.
Where ADUs are allowed outside California
California has the most detailed state ADU law, but it is not the only state with one, and most US rules are still set city by city.
Whether a lot can have an ADU, how big it may be, whether the owner must live on site and whether parking is required all vary by state and then by city.
Nothing on this page is a national rule except the core idea: a complete second home on a residential lot. Check the state summary first, then the city’s zoning code.
Common misreadings
- Myth An ADU has to be for family.
- Reality Not in California law. "Granny flat" is a nickname. The statute defines the unit by its facilities, and § 66314(d)(1) lets it be rented 2.
- Myth You can sell the ADU separately.
- Reality Generally no. California says an ADU may be rented but not sold apart from the main home, except under Article 4 (§ 66340 and following), a narrow exception for certain units 2.
- Myth A city can refuse any ADU it likes.
- Reality In California a local ordinance must use ministerial approval only; an ordinance that does not comply is null and void and state standards apply instead 3.
Why the word matters to you
The label decides which rules apply. If the city calls your project an ADU, ADU law applies. If it calls it a room addition or a guest house, other rules apply. Those rules can be stricter on size, parking and approval.
The label also shows up in money talks. A lender, an appraiser and an insurer will each ask whether the unit is a legal ADU. A permitted unit is a dwelling (a home people can legally live in). An unpermitted one is a risk on every one of those forms.
So before you call a contractor, know the test. Does the space have its own kitchen, bathroom, sleeping area and door? Is it on a lot with a main home? If yes, you are planning an ADU.
How getting an ADU works, step by step
The steps are much the same in most places. The rules inside each step change by state and city.
- Check the lot. Is it zoned for homes? Is there a main home, or will there be one? Start with the feasibility check.
- Pick a type. Detached, attached, a garage conversion, or a junior ADU inside the house.
- Find the size you can build. State and city limits set the range. See ADU size.
- Get plans drawn. A designer or architect draws a site plan and floor plans.
- Apply for a permit. In California, the city must review an ADU without a public hearing 3.
- Build and pass inspections. The building department checks the work as it goes.
- Move in or rent it out. In California, a permitted ADU may be rented, but not sold apart from the main home in most cases 2.
Worked example: is this space an ADU?
Four hypothetical spaces on four California lots, tested against the California definition in § 66313(a). The spaces are made up for teaching. The test is the real one.
| Space | Kitchen? | Bathroom? | Own entrance? | An ADU? |
|---|---|---|---|---|
| A backyard studio with a sink, stove and shower | Yes | Yes | Yes | Yes, a detached ADU, once permitted 1 |
| A pool house with a half bath and no stove | No | Half | Yes | No. It lacks cooking facilities |
| A bedroom suite inside the house with a small kitchen and its own door | Efficiency kitchen | Shared | Yes | Possibly a junior ADU, if under 500 sq ft 1 |
| A converted garage with a kitchen and bath, built without a permit | Yes | Yes | Yes | Not a legal ADU until it is permitted |
The rule of thumb: count the five things the law lists. Living, sleeping, eating, cooking and sanitation (a toilet and a place to wash). Miss one, and it is not an ADU.
Where you will see the word ADU
- On a city zoning map or code, as a use allowed in residential zones.
- On a permit application, where you tick the ADU type box.
- On a builder’s bid, where the scope says "new ADU" or "ADU conversion". Compare bids with the bid comparison tool.
- On a loan offer, where the lender asks if the unit is permitted. See the ADU financing hub.
- On a lease, if you rent the unit out. Rental rules are on the ADU rental hub.
- On a real-estate listing, often as "granny flat" or "in-law unit". Ask if it was permitted.
Benefits and limits of an ADU
What it does well
Where it falls short
- It is a real construction project, with design, permits, fees and inspections.
- In most cases you cannot sell it apart from the main home 2.
- It uses yard, garage or house space you may want for other things.
- Rules outside California vary a lot, and some cities still limit ADUs heavily.
What to ask your city and builder
- Does my city allow ADUs in my zone, and which types?
- What is the largest size my city allows for each type?
- Do I have to live on the property? See the owner-occupancy research.
- Is parking required, or does an exemption apply?
- Can my sewer, water and electric service handle a second home?
- What fees will the city and the utilities charge? Try the ADU fee estimator.
- Can I rent it short-term, or only long-term?
A simple checklist for your ADU project
- Confirm your zoning and any overlay (coastal, flood, fire, historic).
- Measure the lot and mark setbacks and easements.
- Choose a type with the ADU type selector.
- Set a budget using cost drivers, not one price.
- Line up how you will pay for it.
- Hire a designer and get plans drawn.
- Apply for the permit and answer plan-check comments.
- Hire a licensed builder and compare bids line by line.
- Pass final inspection before anyone moves in.
Where to go next
- ADU types hubEvery form compared
- ADU type selectorWhich form fits your lot and goal
- Feasibility checkerCan your lot take one?
- What an ADU costsCost drivers and sourced data
- ADU statisticsCounts, sizes and permits
- How many ADUs are there in America?
Questions about accessory dwelling unit
What does ADU stand for?
ADU stands for accessory dwelling unit: a second, complete home on a residential lot, with its own kitchen, bathroom and entrance, that sits alongside a main house or apartment building.
What is the difference between an ADU and a granny flat?
None in law. "Granny flat" is an everyday name for an ADU. HUD lists granny flats, accessory apartments and second units as names for the same thing. No rule limits who can live in one because of the name.
Is a casita an ADU?
Only if it has full living facilities. A casita with a kitchen and bathroom on a residential lot is a detached ADU. A casita used as a guest room with no kitchen is an accessory structure, not a dwelling unit.
Can I rent out an ADU?
In California, yes: state law lets an ADU be rented separately from the main home. Rental terms, short-term rental limits and any owner-occupancy rules are set locally elsewhere, so check your city.
Can I sell an ADU separately from my house?
Usually not. California law says an ADU may be rented but not sold separately from the primary residence, except under a narrow exception in Article 4 of the ADU chapter. Other states set their own rules.
What is the difference between an ADU and a JADU?
A junior ADU is smaller (up to 500 square feet in California), must sit inside a single-family home, needs only an efficiency kitchen and may share the main home’s bathroom. An ADU can be detached or attached and has full facilities.
Is an ADU a good investment?
It can be, but it depends on your costs, your local rents and how you use it. A unit that houses family saves different money than one rented to a tenant. Run your own numbers with the ADU ROI calculator and the rent estimator before you decide.
Does an ADU need its own address?
Rules differ by city. Mail carriers, utilities and emergency crews all need to find the unit, so many owners ask for one. Who assigns it, and when, is set locally. Ask your planning or building department during the permit process how it handles ADU addresses.
Do I need a permit to build an ADU?
Yes. An ADU is a home, so it needs a building permit and inspections. A unit built without one is not a legal ADU. In California, the city must review an ADU application without a public hearing.
Can I build an ADU on any lot?
Not any lot. In California the lot must be zoned for single-family or multifamily homes and have a home on it or planned. The lot must also have room, utilities and access. Other states set their own rules.
Can I live in the ADU and rent out the main house?
In many places, yes. California law treats both as homes on one lot. Some cities outside California require the owner to live in one of the two units. Check your local owner-occupancy rule first.
Sources
- California Legislature, Gov. Code § 66313 (as amended by SB 543, Stats. 2025, ch. 520, eff. 1 Jan 2026), retrieved .
- California Legislature, Gov. Code § 66314, retrieved .
- California Legislature, Gov. Code § 66316, retrieved .
- California Legislature, Gov. Code § 66321 (as amended by SB 543, eff. 1 Jan 2026), retrieved .
- California Legislature, Gov. Code § 66323 (as amended by SB 543, eff. 1 Jan 2026), retrieved .
- California Legislature, Gov. Code § 66333 (as amended by AB 1154, eff. 1 Jan 2026), retrieved .
- HUD User, Accessory Dwelling Units: Case Study (2008), retrieved .
Expert review
Written by the HyreADU Research team. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: Definitions and every California rule were read on leginfo.legislature.ca.gov at the current Chapter 13 section numbers on 7 October 2026; alternative names checked against HUD User. Education, not legal advice: the local planning department decides each application.
Suggest a correction. We fix errors and say what changed.