What does ADU stand for?

ADU stands for Accessory Dwelling Unit: a smaller, independent home on the same lot as a primary house, with its own kitchen, bathroom and entrance. New York City calls it an ancillary dwelling unit; the idea is the same.

What is the difference between an ADU and a JADU?

A JADU (Junior Accessory Dwelling Unit) is a California category: a unit of 500 square feet or less contained entirely within a single-family home, which may share a bathroom with the house. Since AB 1154, owner occupancy applies to a JADU only when it shares sanitation facilities with the main house. A standard ADU is larger and fully self-contained, and California cities cannot require owner occupancy for it.

Can I rent out an ADU?

In most places, yes, as long-term housing. In California a permitted, finaled ADU can be leased like any other rental, and a city may require long-term rental terms, which rules out nightly short-term rentals. Washington, Oregon, Massachusetts and Colorado also bar owner-occupancy rules for ADUs in most cases. New York City requires the lot to be an owner's primary residence when the ADU is first occupied.

How big can an ADU be in California?

A city cannot use lot coverage, floor area, open space, front setback or minimum lot size rules to prevent an ADU of at least 800 square feet with four-foot side and rear setbacks. Any local maximum size must allow at least 850 square feet, or 1,000 square feet for a unit with more than one bedroom, and many cities, including Los Angeles, allow detached ADUs up to 1,200 square feet.

Do I need a permit to build an ADU?

Yes. An ADU is a dwelling, so it needs a building permit and a final inspection. In California the city must decide within 60 days of a complete application, and the decision is ministerial: no public hearing, no discretionary review, as long as the plans meet the objective standards.

How much does an ADU cost?

It depends mostly on whether you convert existing space or build new. In the City of Los Angeles, the median construction valuation declared on finaled ADU permits is $45,000 for a garage or interior conversion and $110,000 for new construction. Declared valuation leaves out design, permit fees, utility connections and site work, so treat it as a floor, not a quote.

What Is an ADU?

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An ADU (Accessory Dwelling Unit) is a smaller, self-contained home on the same lot as a primary residence, with its own kitchen, bathroom, sleeping area, and entrance.

ADUs are also called granny flats, in-law units, backyard cottages, casitas, carriage houses or secondary units. New York City's zoning calls them ancillary dwelling units.

An ADU sits on a lot that already has a house, shares the address and usually the utility connections, and cannot normally be sold on its own. What makes it a dwelling rather than a guest room is that someone can live in it independently: a place to sleep, cook and bathe, and a door of its own. That independence is why ADUs have become the most common way for homeowners to add a rental, house a parent or an adult child, or live small themselves and rent the main house.

The types of ADU

Detached ADU. A standalone structure in the backyard, separate from the main house. This is the classic backyard cottage, and the type most pre-approved plan catalogs are built around.
Attached ADU. An addition that shares a wall with the primary home but has its own entrance, kitchen and bathroom.
Conversion ADU. Existing space turned into a legal home: a garage, a basement, an attic, or a room inside the house. In Los Angeles, conversions are the most common kind on the permit record, and garages are where most of them start.
Junior ADU (JADU). A California category: up to 500 square feet contained entirely within a single-family home, with an efficiency kitchen, and allowed to share a bathroom with the house.

The type decides most of what follows: the size you are allowed, the fees you pay, and the cost. A side-by-side comparison is in detached vs attached ADU vs JADU.

What makes a unit a legal ADU?

A legal ADU has been permitted and finaled: the local building department approved the plans, inspected the work, and signed off on it. A converted garage with a kitchenette but no permit is not an ADU in the eyes of the city, a lender or an insurer, however livable it is. The final inspection (or, in some cities, a certificate of occupancy) is what lets an owner rent the unit with confidence and what a tenant should ask to see. On A-du, rental listings must be permit finaled, so renters know the home is legal and inspected.

California offers a path for units built without permits before 2020: a city cannot deny a permit to legalize one for failing current building standards unless the unit is a health or safety hazard. See legalizing an unpermitted ADU under AB 2533.

ADU rules by state

ADUs are regulated by cities and counties, but a growing number of states now set a floor that local rules cannot go below. Where A-du works, and in the other states that have written ADUs into law, the rules look like this.

California

California has the most detailed ADU law in the country, now in the Government Code, sections 66310 to 66342 (the main rules are in section 66314) . Approval is ministerial: a city must say within 15 business days whether an application is complete and approve or deny it within 60 days of a complete one, with no public hearing. A city cannot use its lot coverage, floor area, open space, front setback or minimum lot size rules to prevent an ADU of at least 800 square feet with four-foot side and rear setbacks, and any local maximum must allow at least 850 square feet (1,000 for more than one bedroom). A detached unit may be 16 feet tall, or 18 feet near major transit. Owner occupancy cannot be required for a standard ADU, impact fees are waived under 750 square feet, and a city may require long-term rental terms rather than nightly stays. The state housing department keeps an ADU handbook that cities follow. For how this plays out on a real lot, see how to build an ADU in Los Angeles, and the guides for the City of Los Angeles, San Diego and San Francisco.

New York City

The City of Yes for Housing Opportunity amendment, adopted December 5, 2024, allows one ancillary dwelling unit of up to 800 square feet on the zoning lot of a one- or two-family home, subject to district exclusions. Unlike California, New York City requires the lot to be an owner’s primary residence when the unit is first occupied. The full picture, including basements and cellars under Local Laws 126 and 127, is in the New York City ADU guide.

Washington

Under HB 1337, cities and counties planning under the Growth Management Act must allow two ADUs per residential lot in their urban growth areas, attached, detached or one of each, cannot require the owner to live on site except for a short-term rental, and may charge no more than half the impact fees of the main house, according to the state Department of Commerce. More in two ADUs per lot in Washington.

Oregon

Cities over 2,500 people and counties over 15,000 must allow at least one ADU for each detached house in areas zoned for them inside the urban growth boundary, and they may not require owner occupancy or extra off-street parking (ORS 197A.425). From January 1, 2027, HOA covenants that ban ADUs inside the urban growth boundary are void; see Oregon HB 2138.

Massachusetts and Colorado

Since February 2, 2025, Massachusetts has allowed one ADU by right in single-family districts, up to 900 square feet or half the floor area of the house, whichever is smaller, with no owner-occupancy requirement and no parking mandate within half a mile of transit (Mass.gov). Colorado’s HB24-1152 required cities and towns of 1,000 or more people within its metropolitan planning areas to allow one ADU wherever they allow a single detached home by June 30, 2025, and generally bars owner-occupancy requirements (Colorado Division of Local Government).

Everywhere else, the rules are local. The question to ask your city is the same in every state: is an ADU allowed on my lot, how large, how tall, how far from the lot lines, and do I have to live there?

What an ADU costs

The best public record of what ADUs cost is the permit itself. In the City of Los Angeles, across 16,424 finaled ADU permits, the median construction valuation declared on the permit is $45,000 for a garage or interior conversion and $110,000 for new construction, and the median unit is 620 square feet. Declared valuation leaves out design, permit and utility fees, site work and finishes, so treat it as a floor. Real, itemized budgets run from $89,430 for a detached unit whose owner was his own architect to $220,722 for a studio built over a garage. The full breakdown by type, size and neighborhood is in what an ADU costs in Los Angeles, and the permit fees alone can be itemized with the free Los Angeles, San Francisco and New York City permit fee calculators.

Two ways to spend less: design under the fee thresholds (in California, impact fees are waived under 750 square feet and school fees start at 500), and start from a pre-approved plan, which saves design time and shortens plan check. A-du lists pre-approved ADU plans from city and county catalogs, including the LADBS Standard Plans.

Renting an ADU

For renters, an ADU is usually a private, ground-level home with its own entrance and often a bit of yard, in a neighborhood of houses rather than apartment buildings. For owners, it is rental income on land they already own. The tenant rules follow the unit, not the owner: in California, a newly built ADU is generally exempt from the statewide rent cap for its first 15 years, and local rent control depends on the certificate of occupancy date. See California ADU tenant rights and rent control and Los Angeles ADUs.

Next steps: Browse ADUs for rent, learn how to rent out your ADU, read how to build one, or explore pre-approved ADU plans.

Frequently asked questions

What does ADU stand for?: ADU stands for Accessory Dwelling Unit: a smaller, independent home on the same lot as a primary house, with its own kitchen, bathroom and entrance. New York City calls it an ancillary dwelling unit; the idea is the same.
What is the difference between an ADU and a JADU?: A JADU (Junior Accessory Dwelling Unit) is a California category: a unit of 500 square feet or less contained entirely within a single-family home, which may share a bathroom with the house. Since AB 1154, owner occupancy applies to a JADU only when it shares sanitation facilities with the main house. A standard ADU is larger and fully self-contained, and California cities cannot require owner occupancy for it.
Can I rent out an ADU?: In most places, yes, as long-term housing. In California a permitted, finaled ADU can be leased like any other rental, and a city may require long-term rental terms, which rules out nightly short-term rentals. Washington, Oregon, Massachusetts and Colorado also bar owner-occupancy rules for ADUs in most cases. New York City requires the lot to be an owner's primary residence when the ADU is first occupied.
How big can an ADU be in California?: A city cannot use lot coverage, floor area, open space, front setback or minimum lot size rules to prevent an ADU of at least 800 square feet with four-foot side and rear setbacks. Any local maximum size must allow at least 850 square feet, or 1,000 square feet for a unit with more than one bedroom, and many cities, including Los Angeles, allow detached ADUs up to 1,200 square feet.
Do I need a permit to build an ADU?: Yes. An ADU is a dwelling, so it needs a building permit and a final inspection. In California the city must decide within 60 days of a complete application, and the decision is ministerial: no public hearing, no discretionary review, as long as the plans meet the objective standards.
How much does an ADU cost?: It depends mostly on whether you convert existing space or build new. In the City of Los Angeles, the median construction valuation declared on finaled ADU permits is $45,000 for a garage or interior conversion and $110,000 for new construction. Declared valuation leaves out design, permit fees, utility connections and site work, so treat it as a floor, not a quote.

By Chris Koss, AIA, founder of A-du. About the author. State rules checked against the linked statutes and agency pages on September 30, 2026. This guide is general information, not legal advice. Confirm requirements with your city or county before building or renting an ADU.