The short answer: ADU laws by state vary because no federal law governs accessory dwelling units (ADUs): each state decides whether to override local zoning. Seven states below have statewide ADU rules we could confirm against official sources: California, Washington, Oregon, Massachusetts, Colorado, Arizona and Maryland. Other states also have statewide ADU laws that we have not covered in detail here: Connecticut has a statewide accessory-apartment law (Public Act 21-29) that towns could opt out of by a supermajority vote, and we have not confirmed what changed in 2025 and 2026, Maine and Montana require municipalities to allow ADUs, Hawaii (Act 39 of 2024) requires counties to allow at least two ADUs by December 31, 2026, Virginia (SB 531 of 2026) takes effect July 1, 2027, and New Hampshire, Vermont, Rhode Island and Utah have narrower mandates. In the remaining states, your city or county sets the rules, and they can be stricter or looser than anything above.
Last verified: October 5, 2026 ยท Written by the ADU Scope editorial team
ADU laws by state: statewide rules at a glance
| State | Who must allow ADUs | What the state requires | Owner occupancy |
|---|---|---|---|
| California | Every city and county | Ministerial approval within 60 days of a complete application; a converted ADU, a JADU and an 800 sq ft detached ADU on a typical single-family lot | Cannot be required for an ADU |
| Washington | All local governments planning under the Growth Management Act | Two ADUs per lot in urban growth areas; impact fees capped at 50% of the main unit’s | Cannot be required |
| Oregon | Cities over 2,500 people and counties over 15,000, inside urban growth boundaries | At least one ADU per detached single-family home | Cannot be required |
| Massachusetts | All cities and towns | First ADU by right in single-family zones, since February 2, 2025 | Cannot be required |
| Colorado | Municipalities of 1,000+ in metropolitan planning areas, and parts of larger counties | One ADU on a single-unit detached lot through administrative approval; compliance deadline June 30, 2025 | Cannot be required under the state Division of Local Government guidance (short-term rental licensing aside) |
| Arizona | Municipalities over 75,000 people | At least one attached and one detached ADU; size cap of 75% of the main house or 1,000 sq ft, whichever is less; no added parking | Not required for long-term rentals; cities may require it for vacation rentals |
| Maryland | All local governments, by October 1, 2026 | Local law must authorize ADUs on lots with a single-family detached home; size up to 75% of the main home; approval need not be by right | Not required |
ADU laws by state: what to watch in each one
California
The strongest statewide framework, and the one with the most detailed rules on fees, parking and timing. See our California ADU Laws 2026 guide.
Washington and Oregon
Both limit their rules to urban growth areas or boundaries, so a rural lot may fall outside them. Washington lets cities charge no more than half the main home’s impact fees. See Washington State ADU Law and Oregon ADU Rules.
Massachusetts
The statewide law took effect February 2, 2025 and applies in single-family zoning districts. Towns keep the power to regulate short-term rental use of ADUs.
Colorado
The law applies only to defined metropolitan-area jurisdictions. A town outside them can still ban or restrict ADUs. See Colorado ADU Law 2026.
Arizona
The law covers only large cities. Cities had until January 1, 2025 to adopt compliant rules. If a city missed the deadline, the state says ADUs are allowed without local restrictions. See Arizona ADU Law.
Maryland
The law took effect October 1, 2025, and local governments had until October 1, 2026 to adopt ADU ordinances. That deadline has just passed, so check whether your county or city has adopted its law yet. Unlike California, the state does not require by-right approval: a county may use a special exception or conditional use process.
ADU laws by state: the states we could not confirm
We did not confirm a statewide, by-right ADU mandate for Texas or Florida as of this date. In both, ADU rules are mostly local, and legislation is active: a Florida bill (SB 48) requiring local governments to allow ADUs in single-family neighborhoods passed the state Senate in February 2026, but the legislature’s bill page shows it did not pass the House, so Florida does not appear to have a statewide ADU mandate. Confirm against the current statutes before relying on this. Check your city’s code and the state legislature’s website before you rely on any summary.
ADU laws by state: other states with statewide rules
Beyond the seven states in the table, several others have statewide ADU laws. They differ a lot in strength, so read each as a different kind of rule rather than a copy of California’s. Where we could only reach an official summary and not the statute itself, we say so.
- Maine. Title 30-A, section 4364-B (from LD 2003, 2022) requires municipalities to allow at least one ADU on a lot with a single-family home as the principal structure, with no added parking and no owner-occupancy requirement. The statute sets a minimum size of 190 sq ft and has been amended since 2022.
- Montana. SB 528 (2023), codified at MCA 76-2-345, requires cities that zone to allow at least one ADU by right, capped at the lesser of 75 percent of the main home or 1,000 sq ft, with no parking, owner-residence or impact-fee requirement. We could not verify the population threshold that decides which cities are covered.
- New Hampshire. RSA 674:72 allows one ADU by right in any zone with a single-family home, and sets size rules built around 750 sq ft and 950 sq ft figures that we have not reconciled against the statute text, so read RSA 674:72 for the exact limits. A town can still require the owner to live in one of the two units. The law was widened in 2025 by HB 577.
- Rhode Island. RIGL 45-24-37 (2022) makes ADUs a permitted use in all residential districts, approved through an administrative building permit rather than a hearing.
Narrower mandates: Utah, Vermont and Connecticut
- Utah. Utah Code 10-9a-530 covers only internal ADUs, requires the owner to live on the property to rent the unit, and lets a city prohibit them on lots of 6,000 sq ft or less. It is one of the weaker mandates.
- Vermont. The 2023 HOME Act (Act 47) addresses ADUs in owner-occupied single-family homes. We did not confirm the effective date of the ADU provisions, so check the Vermont statute before relying on them.
- Connecticut. Public Act 21-29 (2021) required accessory apartments, but with an opt-out that a town’s legislative body could use by a two-thirds vote. We could not confirm what changed in 2025 and 2026, so read the current General Statutes before relying on it.
Texas is worth a note because people expect a law there. The governor’s 2025 housing signings (SB 15 on small lots, SB 840 on residential use in commercial zones, HB 24 on zoning petitions) do not impose a statewide ADU mandate. Nebraska’s LB 1166 in 2024, which would have required ADUs by right, did not leave committee.
ADU laws by state: how big can the unit be?
Size is where states differ most, and the same word (“cap”) can mean different things. This table compares the limits we confirmed:
| State | Size rule | Source note |
|---|---|---|
| California | State-mandated detached ADU of 800 sq ft is guaranteed; local ordinances can allow up to 1,200 sq ft detached or 50% of the main home attached | Gov. Code sections 66314 and 66323 |
| Washington | Cap cannot be lower than 1,000 sq ft | RCW 36.70A.681 |
| Arizona | Lesser of 75% of the main home or 1,000 sq ft | HB 2720 (2024) |
| Maryland | Up to 75% of the main home | HB 1466 (2025) |
| Montana | Lesser of 75% of the main home or 1,000 sq ft | MCA 76-2-345 |
| Massachusetts | Lesser of half the main home or 900 sq ft, per the state’s guidance | mass.gov; 760 CMR 71.00 |
| New Hampshire | Size limits built around 750 and 950 sq ft; read the statute for exact wording | RSA 674:72 |
| Colorado | Covered cities must allow 500 to 750 sq ft; no statewide maximum | HB24-1152 |
The Massachusetts figure comes from the state’s guidance and regulation, not from the text of the statute we reviewed, so confirm it with your town.
ADU laws by state: when a statewide law is not yet in force on your lot
A law being “on the books” is not the same as being usable on your lot. Four patterns come up repeatedly:
- Delayed local compliance. Washington’s HB 1337 (2023) applies in urban growth areas, and cities must conform within six months after their next periodic plan update. Until then the state rule applies directly.
- Population thresholds. Arizona covers only municipalities over 75,000 people, and Oregon only cities over 2,500 and counties over 15,000.
- Deadlines that have passed. Arizona cities had until January 1, 2025. Maryland local governments had until October 1, 2026.
- Wrong bill numbers. Maryland’s 2024 HB 538 is a housing bill that does not regulate ADUs; the ADU law is HB 1466 (2025). In Oregon, the ADU mandate comes from SB 1051 (2017) rather than HB 2001, which deals with middle housing.
How to use this ADU laws by state guide
- Find your state in the table. If it’s listed, the state sets the floor for your city.
- Read your city’s ADU ordinance for the details the state leaves open: size, height and setbacks.
- Confirm with your planning department, because ordinances change faster than guides do.
FAQ
Does a state law mean I can build on any lot?
No. Most state laws apply to single-family zones and leave siting, setbacks and utilities to cities.
What if my city hasn’t updated its rules?
In California, an ordinance that was not sent to the state housing agency on time is void and state law applies. In Arizona, the state says missed deadlines open ADUs statewide. Check your own state’s remedy.
Where are ADUs hardest to build?
Where no statewide law exists and the city or county allows ADUs only by special permit, or not at all.
Next steps
- Can I Build an ADU on My Property? A 5-Step Checker
- Owner-Occupancy Rules for ADUs: Key States Compared
- ADU Impact Fees: When You Pay Them and How to Avoid Them
- Washington State ADU Law: What Homeowners Can Build
- Oregon ADU Rules: Statewide Requirements and Portland Specifics
- Colorado ADU Law 2026: What HB24-1152 Means for Homeowners
General information, not legal advice. Local rules vary and change; confirm with your planning department.
Sources: HCD ADU Handbook (California) ยท MRSC (Washington) ยท ORS 197A.425 (Oregon) ยท MAPC (Massachusetts) ยท Colorado HB24-1152 ยท Arizona HB 2720 fact sheet ยท Maryland Department of Planning, HB 1466 FAQ ยท WCJB, Florida Senate vote ยท Florida Senate: SB 48 (2026) ยท Maine 30-A M.R.S. 4364-B ยท NH RSA 674:72 ยท Mass.gov, ADUs ยท Maryland HB 1466 (Ch. 197)