What you can build

What is an ADU (accessory dwelling unit)?

Checked against state laws and official agency sources · Last reviewed October 9, 2026

What makes a space legally an ADU

An ADU is a full home, not just an extra room. California defines it as an attached or detached unit that "provides complete independent living facilities for one or more persons" on a lot with a proposed or existing main home, and says it "shall include permanent provisions for living, sleeping, eating, cooking, and sanitation." Under California law an ADU also includes an efficiency unit and a manufactured home.

Washington uses almost the same words. Its law defines a dwelling unit as a residential living unit with "complete independent living facilities for one or more persons" and "permanent provisions for living, sleeping, eating, cooking, and sanitation."

Other states word it differently:

• Massachusetts calls an ADU "a self-contained housing unit, inclusive of sleeping, cooking and sanitary facilities" with its own entrance, no bigger than half the main home or 900 square feet, whichever is smaller.
• Arizona says an ADU has "its own sleeping and sanitation facilities" and "may include its own kitchen facilities."

A permit is part of what makes it legal. Seattle's building department says ADUs "are not legal unless they have been established through a permit process." See how the ADU permit process works. [1][5][8][9][10]

Types of ADUs: attached, detached, converted and junior

California's housing agency, HCD, sorts ADUs into these types:

• Attached ADU: new space that structurally connects to the main home. HCD's examples include an ADU on top of an attached garage, a new second story, or a new basement.
• Detached ADU: a new, separate building on the same lot. Washington's law describes it as a unit that is "separate and detached" from the main home "and is on the same property."
• Converted ADU: a unit made from existing space, such as part of the house, a detached garage, a shed or another accessory structure. See our guide to garage conversions.
• Junior ADU (JADU): a California type. It is no more than 500 square feet of interior livable space and sits entirely inside a single-family home. An attached garage counts as part of the home. It may have its own bathroom or share one with the main home. See our junior ADU guide.

Seattle calls a permitted unit inside the house an "attached accessory dwelling unit (AADU)" and a permitted unit elsewhere on the lot a "detached accessory dwelling unit (DADU)."

A backyard ADU can also be a factory-built or manufactured home, or a tiny house that meets the rules. See prefab ADUs and tiny houses in the backyard. [1][8][10]

Other names: granny flat, in-law unit, casita, backyard cottage

ADUs go by many names. California HCD says they "have been known by many names: granny flats, in-law units, backyard cottages, secondary units and more." The City of Phoenix says ADUs are "often referred to as guest houses," casitas or granny flats. In Seattle, a backyard ADU is called a DADU.

The name you use doesn't decide anything. What matters is whether the unit meets your state's and city's legal definition and gets a permit. A guest room or "mother-in-law suite" without its own kitchen generally does not meet California's definition, which requires permanent provisions for cooking. In California, a junior ADU has an "efficiency kitchen," which HCD describes as a cooking facility with appliances, a food preparation counter and storage cabinets. [1][10][11][12]

States that require cities to allow ADUs: examples

Several states have laws that require many of their cities to allow ADUs. The states covered on this site are listed below; other states have ADU laws too, so check your own state's rules. Each law has limits on which places and lots it covers.

• California: state ADU law (Government Code sections 66310 to 66342) applies to every city and county. On a single-family lot, the city must allow at least one ADU made from existing space, one junior ADU, and one new detached ADU.
• Washington: cities and counties that plan under the state's growth management law (chapter 36.70A RCW) must allow at least two ADUs on lots in urban growth areas where single-family homes are allowed.
• Oregon: cities over 2,500 people and counties over 15,000 must allow at least one ADU for each detached single-unit home in areas inside the urban growth boundary that are zoned for those homes.
• Montana: cities and towns must allow at least one ADU "by right" on a lot with a single-family home.
• Arizona: cities and towns over 75,000 people must allow at least one attached and one detached ADU on any lot where a single-family home is allowed.
• Colorado: since June 30, 2025, covered cities and county areas must allow one ADU with a single-unit detached home, through an administrative approval process. Covered means cities of 1,000 or more people inside a metropolitan planning organization, plus county areas in census places of 40,000 or more inside one.
• Massachusetts: zoning may not prohibit, "unreasonably restrict" or require a special permit for a single ADU in a single-family zoning district.

Your city may allow more than the state minimum. California lets local governments be less restrictive than state law, and Montana lets cities adopt rules that are "more permissive." [1][2][3][4][5][6][7][13]

Rules these state laws commonly set

The state ADU laws covered on this site often limit what a city can require:

• Owner occupancy: California, Washington, Oregon, Montana and Massachusetts bar cities from requiring the owner to live on the lot. Colorado bars it too, but a city may ask you to show you live on the parcel when you apply to build or convert the ADU. California still requires owner occupancy for a junior ADU that shares a bathroom with the main home.
• Parking: California caps ADU parking at one space per unit or bedroom, whichever is less, and bans it in many cases, such as near transit. Arizona and Montana bar requiring added parking. Massachusetts allows at most one added space, and none within half a mile of certain transit stations.
• Size: Washington cities may not cap an ADU below 1,000 square feet. California cities must allow at least 850 square feet, or 1,000 for an ADU with more than one bedroom. Arizona cities must allow an ADU of 75% of the main home's floor area or 1,000 square feet, whichever is less. Montana uses the same figure as the most an attached or detached ADU may be.
• Impact fees: In California, an ADU of 750 square feet of interior livable space or less is exempt from impact fees. Washington caps them at 50% of the main home's fees. Montana bars them.

Check your state's page for the full rules and exceptions. [1][2][3][4][5][6][7]

What an ADU costs

Costs vary widely by region, size and type. One statewide source is the first statewide survey of California ADU owners, by a research team at UC Berkeley's Center for Community Innovation, published by the Terner Center in April 2021. These figures are from 2021. It found that "the median statewide construction cost of an ADU is $150,000, or $250/square foot." It also found that 37 percent of new ADUs cost less than $100,000 to build, and 71 percent cost less than $200,000.

See our ADU cost guide and ADU financing guide for more. [14]

Before you start

Your city's rules decide what fits on your lot. In California, state law sets the floor, and cities can add objective local standards on top. In California, a city may limit where ADUs are allowed only based on water and sewer service, traffic flow and public safety.

In California, homeowners association rules can't ban an ADU or junior ADU or unreasonably restrict it on a lot zoned for single-family homes.

Next steps: read the ADU permit process, see who can help you build, and check how an ADU affects your property tax. [1]

Questions people ask