ADU laws, explained by state and by bill

TL;DR

ADU laws set how big an accessory dwelling unit can be, where it can go, and how fast a city must approve it. This library explains each state law and bill in plain language, with a city-by-city table and the primary source for every rule, updated for 2026-2027.

Last verified: September 25, 2026 against the California, Massachusetts, Washington, Oregon, Florida, Maryland, Maine, New Hampshire and Colorado legislatures and state housing agencies.

An accessory dwelling unit, or ADU, is a second, smaller home on a lot that already has a house. A junior ADU, or JADU, is a unit of up to 500 square feet created inside the walls of an existing house. Every state writes its own ADU law, and the rules change almost every year, so this library explains each one against the statute text, the state housing agency and the cities that apply it.

California ADU laws

The state pillar, the 2026-2027 legislation tracker, and each bill explained across cities.

Legislation trackerNew California ADU LawsCalifornia enacted four ADU laws in 2025 that take effect for 2026: AB 462, AB 1154, SB 9 and SB 543. Most apply on January 1, 2026. Together they speed permit review to a 15 business day completeness check, ease the junior ADU owner occupancy rule, and add state enforcement of local ADU ordinances.Bill explainerAB 1033 ExplainedAB 1033, signed October 11, 2023 and effective January 1, 2024, lets a California city opt in by local ordinance to allow an ADU to be sold separately from the main house as a condominium. As of September 25, 2026, four cities we track have opted in: San Jose, San Diego, San Francisco and Berkeley.Bill explainerAB 2533 ExplainedCalifornia's AB 2533 took effect January 1, 2025 and amended Government Code Section 66332. It bars a local agency from denying a permit to legalize an unpermitted ADU or JADU built before January 1, 2020, unless correcting a violation is necessary to fix a Health and Safety Code 17920.3 substandard condition.Bill explainerSB 9 and ADUs in CaliforniaSB 9, signed September 16, 2021 and effective January 1, 2022, is a California lot split and two unit law, not an ADU law. It lets an owner ministerially split a single family lot into two parcels, each at least 1,200 square feet, and build two units per parcel.

ADU laws by state

What each state allows, who is exempt, and what local governments may still regulate.

State lawMassachusetts ADU LawUnder the Affordable Homes Act (Chapter 150 of the Acts of 2024), Massachusetts allows one Protected Use accessory dwelling unit by right in single-family zoning districts as of February 2, 2025. The unit may be up to 900 square feet or half the principal home, whichever is smaller, with no owner occupancy required.

ADU law topics

Cross-state questions: size limits, tiny houses, and the rules that follow no single bill.

TopicCan You Put a Tiny House in Your Backyard? State Rules (2026-2027 Updates)A tiny house on a permanent foundation is generally treated as an accessory dwelling unit and is allowed wherever ADUs are allowed. A tiny house on wheels is usually classified as a recreational vehicle under 24 CFR 3282.15 and is rarely a permanent backyard dwelling. Whether your state mandates ADUs varies widely by state.