What Is an Accessory Dwelling Unit (ADU)? Zoning Rules and What You Can Build

An accessory dwelling unit (ADU) is a second, smaller home on the same lot as a main house, such as a backyard cottage, a garage conversion, a basement apartment, or an attached addition. ADU reform is the group of state and city laws that require local governments to allow ADUs by right and limit the conditions a city can attach. For an owner or small builder, an ADU law usually means one or two more rentable homes on a lot with no rezoning.

What counts as an ADU

An ADU has its own entrance, kitchen, bathroom, and sleeping area, and it sits on a lot with a primary dwelling. The common forms:

• Detached: a separate small building, often in the rear yard.
• Attached: an addition to the main house.
• Conversion: a garage, basement, attic, or other existing space turned into a unit.
• Junior ADU: a California term for a unit of no more than 500 square feet inside a single-family home.

How ADU laws work

A typical ADU statute makes the ADU a permitted use wherever a single-family home is allowed, requires staff approval against written standards, and bars specific local restrictions.

• California. A local agency must approve or deny a complete ADU application within 60 days, ministerially and without a hearing, or it is deemed approved (Gov. Code 66317). Under Gov. Code 66323, a single-family lot can have one ADU and one junior ADU within the home, and one detached new ADU of up to 800 square feet with four-foot side and rear setbacks. On a multifamily lot, non-livable space can be converted into ADUs numbering up to 25 percent of the existing units, and up to eight detached ADUs can be added, not more than the number of existing units. Cities may not impose an owner-occupancy requirement on ADUs (Gov. Code 66315).
• Washington. Under RCW 36.70A.681, cities and counties must allow at least two ADUs per lot in single-family zones within urban growth areas. They may not require owner occupancy, set a maximum size below 1,000 square feet, set a roof height limit below 24 feet, or charge impact fees above 50 percent of those for the main house.
• Colorado. HB24-1152 requires covered jurisdictions to allow one ADU as an accessory use to a single-unit detached home, through an administrative approval process, as of June 30, 2025. See our Colorado zoning report.
• Massachusetts. Since February 2, 2025, ADUs under 900 square feet can be built by right in single-family zoning districts statewide, according to the state's announcement. The limit is 900 square feet or half the floor area of the main dwelling, whichever is smaller.
• New York City. City of Yes for Housing Opportunity allows one ADU of up to 800 square feet on a lot with a one- or two-family home, with no added parking required, per the housing agency's ADU FAQ.

What this lets you build

On a single-family lot. One additional home in most states. Washington requires cities to allow two per lot, and California allows a junior ADU inside the house in addition to an ADU.

On a multifamily lot in California. Conversions of non-livable space and detached ADUs, within the counts above.

Which parcels are affected.

• The zone has to allow single-family residential use, or multifamily for California's multifamily provisions.
• Some laws cover only larger jurisdictions. Colorado's applies to municipalities of 1,000 or more people within a metropolitan planning organization and to parts of some counties.
• Location carve-outs apply. In New York City, basement and detached ADUs are not allowed in the coastal flood zone or the 10-year high-risk flood zone, and detached ADUs are not allowed in R1-2A, R2A, and R3A districts outside the Greater Transit Zone.

What approvals are removed. No rezoning, conditional use permit, or public hearing for the ADU itself. Review is a staff check against the written standards, and California adds a 60-day deadline.

What limits remain.

• Size and height caps. New York City limits an ADU to 800 square feet and generally one story or 15 feet.
• Owner occupancy where the law keeps it. New York City requires the homeowner to live on the lot when the ADU is first occupied.
• Rental term. California requires rentals of ADUs approved under these provisions to be longer than 30 days.
• Sale. An ADU is normally part of the same lot and cannot be sold alone. Washington is an exception: cities there may not bar the sale of an ADU as a condominium unit solely because it was built as an ADU.
• Building codes, utility connections, and fees. New York's FAQ notes that adding a third unit to a two-family building can trigger the state Multiple Dwelling Law unless the ADU is detached or separated by a fire wall.

How to check a specific parcel

1. Confirm the parcel's district allows single-family or multifamily residential use. See our R-1 explainer for the typical single-family district.
2. Read the local ADU ordinance for size, height, setback, and parking standards, and compare it to the state statute.
3. Check location carve-outs: flood zones, historic districts, and any excluded districts.
4. Confirm utility capacity and connection fees, and pull title for deed restrictions.

In Plotzy, filter parcels by zoning designation and lot size to find lots with room for a detached unit, then ask Zoning AI "Are accessory dwelling units permitted on this lot, and what is the maximum size?" for an answer with the code section cited.

Frequently asked questions

What is an accessory dwelling unit?

It is a self-contained home, with its own kitchen, bathroom, and entrance, on the same lot as a larger primary dwelling.

Do I need a rezoning or a hearing to build an ADU?

Not where a state or city ADU law applies. California, Washington, Colorado, Massachusetts, and New York City all treat qualifying ADUs as a permitted use that needs no rezoning or special permit.

How big can an ADU be?

New York City caps ADUs at 800 square feet. Massachusetts protects ADUs up to 900 square feet or half the main home's floor area, whichever is smaller. Washington cities cannot set a maximum below 1,000 square feet. California cities must allow at least an 800 square foot detached unit.

Can I rent out or sell an ADU?

Renting is generally allowed, with limits on short-term rentals in some places. Selling an ADU separately from the main house is usually not possible unless the law allows a condominium sale, as Washington's does.

Sources

The bills, statutes, court decisions, and agency documents cited on this page were checked on October 4, 2026. These laws have been amended and litigated since they passed, so confirm the current text and status before relying on any detail. This page is general information, not legal advice.

• California Legislative Information: Government Code Section 66313. Definition of a junior accessory dwelling unit.
• California Legislative Information: Government Code Section 66317. The 60-day ministerial decision and deemed approval.
• California Legislative Information: Government Code Section 66323. Units that must be approved on single-family and multifamily lots, the 800 square foot and setback terms, rental term.
• California Legislative Information: Government Code Section 66315. No owner-occupancy requirement.
• Washington State Legislature: RCW 36.70A.681. Two ADUs per lot, owner occupancy, size, height, impact fee, and condominium rules.
• Colorado General Assembly: HB24-1152. Covered jurisdictions, one ADU, administrative approval, June 30, 2025 date.
• Massachusetts Executive Office of Housing and Livable Communities: Accessory Dwelling Units Officially Allowed Statewide (February 3, 2025). The February 2, 2025 start date and 900 square foot threshold.
• Massachusetts Executive Office of Housing and Livable Communities: Accessory Dwelling Units. The half of the principal dwelling or 900 square feet size limit.
• NYC Department of Housing Preservation and Development: Plus One ADU Program FAQ (September 2025). New York City's ADU size, height, eligibility, location limits, owner occupancy, parking, Multiple Dwelling Law.