Roughly 18 states have passed laws telling local governments they must allow accessory dwelling units on single-family lots, and every other state leaves that decision to cities and counties. So the honest answer to what states allow accessory dwelling units is: all of them permit ADUs somewhere, but only in states with a statewide mandate can you count on your own lot being eligible. Everywhere else, a single municipal zoning vote can prohibit them.
States With Statewide ADU Mandates
The states below have enacted laws that override local zoning and require municipalities to allow at least one ADU on qualifying single-family lots. The specifics differ, but each one takes the outright-ban option off the table for local governments.
California
California first authorized ADUs in 1982 and then rewrote the rules in 2016, 2017, and 2019 after local governments buried the original mandate under restrictive requirements.1U.S. Department of Housing and Urban Development. Where Will Accessory Dwelling Units Sprout Up When a State Lets Them Grow? Evidence From California Detached ADUs can be up to 1,200 square feet, and local agencies must allow at least an 800-square-foot unit that is 16 feet tall with four-foot side and rear setbacks regardless of other local standards.2California Legislative Information. California Code GOV Section 65852.2 Detached ADUs near major transit stops can reach 18 feet. AB 976, signed in 2023, permanently banned local owner-occupancy requirements.3California Department of Housing and Community Development. Accessory Dwelling Unit Handbook ADUs under 750 square feet are exempt from impact fees.
Washington
House Bill 1337, effective July 2023, requires cities and counties inside urban growth areas to allow at least two ADUs per single-family lot in any combination of attached and detached structures.4Washington State Legislature. House Bill Report EHB 1337 Owner-occupancy requirements are prohibited.5Washington State Department of Commerce. Guidance for Accessory Dwelling Units in Washington State Cities cannot require off-street parking for ADUs within a half-mile walk of a major transit stop, and parking is capped at one space per ADU on lots under 6,000 square feet and two spaces per ADU on larger lots. Impact fees are capped at 50 percent of the fees for the principal unit.
Oregon
Oregon requires any city over 2,500 residents or county over 15,000 to allow at least one ADU per detached single-family lot inside urban growth boundaries, and local governments cannot impose owner-occupancy requirements or additional off-street parking.6Oregon State Legislature. Oregon Revised Statutes Section 197.312 – Limitation on City and County Authority The statute does not set statewide size caps; state guidance suggests 800 to 900 square feet or 75 to 85 percent of the primary dwelling, but those are recommendations localities can adjust.7Oregon Department of Land Conservation and Development. Guidance on Implementing the Accessory Dwelling Units Requirement The owner-occupancy and parking prohibitions do not apply to ADUs used as short-term vacation rentals.
Massachusetts
The Affordable Homes Act, signed in August 2024, allows ADUs up to 900 square feet by right in single-family zoning districts statewide. Municipalities cannot require a special permit or other discretionary approval to build or rent one.8Mass.gov. Accessory Dwelling Units
Colorado
HB24-1152 requires metropolitan-area jurisdictions to allow at least one ADU on any lot where single-family detached homes are permitted, with compliance required by June 30, 2025. The law also voids HOA provisions that prohibit ADUs or impose overly restrictive design and dimension standards.9Division of Local Government. Accessory Dwelling Units
Maine
Municipalities must allow at least one ADU on any lot where a single-family home is the principal structure, whether built within the home, attached to it, or detached. There is no owner-occupancy requirement, and the minimum size is 190 square feet.10Maine Legislature. Maine Code Title 30-A Section 4364-B – Accessory Dwelling Units
New Hampshire
Municipalities cannot restrict ADU floor area below 750 square feet, and the default maximum is 950 square feet unless the municipality authorizes more. Towns may require owner-occupancy of one unit but cannot dictate which one the owner lives in, and they cannot require a family relationship between the occupants of the two units.11New Hampshire General Court. New Hampshire Code 674:72 – Accessory Dwelling Units
Vermont
State law requires municipalities to allow one ADU on any owner-occupied single-family lot, subject to the same zoning review and dimensional controls as the primary dwelling. Local governments cannot apply stricter standards to a detached ADU conversion than they would to a single-family home.12Vermont General Assembly. Vermont Statutes Title 24 Section 4412
Connecticut
Public Act 21-29 added accessory apartment provisions to the state’s zoning enabling act, though the law focuses more broadly on exclusionary zoning than on building out the kind of detailed ADU framework seen in California or Washington.
What Statewide Laws Typically Require
If your state is on the list above, the ground rules fall into predictable categories. Reading them together helps you anticipate what your city’s implementing ordinance can and cannot do.
Size and Height
Statewide caps generally run from about 750 to 1,200 square feet, sometimes expressed as a percentage of the primary dwelling. Detached-ADU height limits typically land in the 16-to-18-foot range, with 18 feet common near transit. Attached ADUs often match the primary home’s height up to around 25 feet.3California Department of Housing and Community Development. Accessory Dwelling Unit Handbook
Parking
Parking mandates were historically the biggest obstacle to ADU construction on urban lots. California, Oregon, and Washington have all eliminated or sharply limited them, and proximity to transit frequently triggers a full exemption.6Oregon State Legislature. Oregon Revised Statutes Section 197.312 – Limitation on City and County Authority
Owner-Occupancy
California, Washington, Oregon, Maine, and Colorado have all eliminated owner-occupancy requirements, so absentee owners can rent out both the primary home and the ADU. New Hampshire still permits municipalities to require that the owner live in one of the two units, with the owner choosing which.11New Hampshire General Court. New Hampshire Code 674:72 – Accessory Dwelling Units Vermont’s mandate applies specifically to owner-occupied single-family lots.12Vermont General Assembly. Vermont Statutes Title 24 Section 4412
Permitting and Fees
Several states, including California and Massachusetts, require ministerial approval: if your application meets objective, published standards, the local agency must approve it without a hearing or discretionary review.8Mass.gov. Accessory Dwelling Units California waives impact fees for ADUs under 750 square feet and requires that fees for larger units be proportional to the primary dwelling’s size.1U.S. Department of Housing and Urban Development. Where Will Accessory Dwelling Units Sprout Up When a State Lets Them Grow? Evidence From California Washington caps ADU impact fees at 50 percent of what the primary unit would owe.5Washington State Department of Commerce. Guidance for Accessory Dwelling Units in Washington State
Short-Term Rentals Are a Separate Question
A state law allowing you to build an ADU does not automatically let you put it on Airbnb. California permits local agencies to require ADU rentals of at least 30 days, and certain ADUs built under streamlined provisions must meet that minimum by statute.13California Legislative Information. California Code GOV Section 65852.2 Oregon’s statewide protections for ADUs do not apply to vacation rentals, so cities can still impose owner-occupancy rules on short-term use.6Oregon State Legislature. Oregon Revised Statutes Section 197.312 – Limitation on City and County Authority Vermont similarly allows municipalities to regulate short-term ADU rentals separately from long-term ones.12Vermont General Assembly. Vermont Statutes Title 24 Section 4412 If the plan depends on nightly rates, check both state and local short-term rental rules before committing.
If Your State Isn’t On the List
States like Florida, Texas, and North Carolina have no statewide law compelling municipalities to allow ADUs, so whether you can build one depends entirely on your zoning district. Some cities have permissive ordinances and streamlined permitting; neighboring suburbs may ban ADUs outright or impose rules strict enough to make building impractical. Before spending money on design, call your local planning or zoning department and ask four questions:
- Are ADUs permitted in my zoning district, and under what designation (accessory dwelling unit, accessory apartment, secondary suite)?
- What are the maximum size and height, and what setbacks apply?
- Do I need a conditional-use permit, variance, or special permit, or is approval ministerial?
- How much off-street parking must I add, and does an existing driveway count?
The answers determine whether an ADU is realistic on your specific lot. Even in states with strong statewide laws, local implementing ordinances control the details, so the same four questions are worth asking there too.