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Accessory Dwelling Units
Legalizing Backyard Cottages
This bill makes it easier for homeowners to build a small second home — a backyard cottage, garage conversion, or in-law unit — on their property.
Accessory Dwelling Units
About the bill
"These small housing solutions are tremendously powerful. We like to say that this is the big power of small homes.”
— Denise Pinkston, Chair of the Casita Coalition
What is an Accessory Dwelling Unit (ADU)?
An accessory dwelling unit — a backyard cottage, granny flat, or in-law unit — is a small second home on the same lot as a main house; it is considered “accessory” to the main house because it shares all utilities and sewer lines with the primary home, and may not be sold as a separate home (unless specific enabling legislation allows a separate title for ADUs). ADUs can range from a standalone cottage in the backyard, a converted garage, or an apartment carved out of the home itself. ADUs have their own kitchen, bathroom, and entrance from the street, making them complete homes for a parent, an adult child, or a renter.
What this bill does
This bill makes ADUs legal to build on any residential lot in the state and establishes a fast, fair, and predictable approval process. It also eliminates local rules that can make backyard cottages infeasible even when they are technically legal.
Why it's needed
Accessory dwelling units give homeowners options to house a family member, earn rental income, support aging in place, or add a smaller, naturally affordable home to a lot that would otherwise hold just one. Currently, many cities continue to block ADUs through lot size minimums, parking mandates, owner-occupancy rules, oversized setbacks, separate utility requirements, discretionary review, and fees that make ADUs too expensive to build. This bill makes accessory dwelling units an ordinary housing option everywhere, with clear and predictable standards for any homeowner who wants to build them.
Frequently asked questions
What does this bill actually do?
- Legalizes accessory dwelling units (ADUs) statewide. Every residential lot can have one ADU plus one junior ADU (a small unit of up to 500 square feet inside the main home).
- Guarantees an answer in 30 days. Applications are approved or denied ministerially which means no hearings or discretionary review. If a permit meets building and safety codes, it must be approved. If the city misses the 30-day deadline, the permit is automatically approved.
- Addresses common poison pills. Removes the rules that quietly kill projects, such as extra parking requirements, owner-occupancy mandates, minimum lot sizes or street frontage rules. Design review is limited to objective standards.
- Protects a buildable baseline. Homeowners can always build at least an 850-square-foot ADU with four-foot side and rear setbacks.
- Keeps fees fair. ADUs under 750 square feet pay no impact fees, and fees on larger units must be scaled to their size. ADU cannot be charged more than a single-family house.
- Overrides HOA bans. Deed restrictions and HOA rules that prohibit or unreasonably restrict ADUs are void and unenforceable.
- Brings existing units out of the shadows. Homeowners with older, unpermitted ADUs get a simple path to register and legalize them and cities must offer that option before pursuing code enforcement.
Why does this need to be state law instead of a local ordinance?
State law gives homeowners predictability. Under this bill, a homeowner’s ability to build doesn’t depend on which side of a city line they live on, and builders, lenders, and prefab manufacturers can work to one statewide standard, which can bring down costs for builders and homeowners alike. Cities still handle permitting, building codes, and objective design standards; the state sets a floor for guaranteeing every homeowner a fair, fast process.
How many ADUs does reform actually produce?
It depends on how strong the reforms are. Before its 2017 reforms took effect, California permitted about 1,300 ADUs a year. By 2023, that number reached 26,000 and more than one in five new homes were permitted statewide were adus. The same happened in Seattle, after its 2019 reform removed owner-occupancy rules and parking mandates, annual permits quadrupled, and the city now permits more ADUs than single-family houses.
New Hampshire shows why the details matter, in 2016 New Hampshire passed a law requiring towns to allow ADUs but let them keep parking requirements, owner-occupancy rules, and discretionary review, also known as “poison pills” and as a result, the law produced only modest results. The state has since returned to strengthen the law twice, in 2025 and 2026. The more barriers a state removes, the more homes get built, so our model removes them all.
How do ADUs make housing more affordable?
ADUs are inherently lower-cost homes because they are small units built on land the owner already holds. In Seattle, ADUs sold as condos cost 40% less than the house on the same lot, and rent about 25% below the median one-bedroom, and many California ADUs house family members at reduced or no rent.
ADUs add homes where they’re needed most, including in neighborhoods near jobs and schools, where new housing is otherwise most restricted. Additionally, ADUs can help people age in place. Additional rental income can help seniors on fixed incomes keep their homes, and an aging parent or adult child can live close by.
The Legislation
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Annotated
Section 1. Title.
This Act shall be known and may be cited as the “Accessory Dwelling Unit Act”.
Section 2. Purpose and Intent.
(a) To address shortages in housing supply and expand housing affordability, it is the policy of [state] to promote and encourage the creation of accessory dwelling units (ADUs).
(b) It is the intent of [state] that homeowners will be authorized to create and maintain ADUs as either personal residences or rental units in areas zoned for residential and mixed uses; and to do so without arbitrary, excessive, or burdensome standards relating to unit size, parking, fees, utilities connections, or other requirements.
This section establishes the core principle that ADUs expand affordability and help families live closer together, and local rules shouldn’t make them needlessly hard to build. Informed by California’s ADU law and Colorado HB24-1152.
Section 3. Definitions.
For purposes of this Section:
(a) “Accessory dwelling unit” means an attached or detached residential dwelling unit that provides complete independent living facilities for one or more persons and is located on a lot with a proposed or existing primary residence. It shall include permanent provisions for living, sleeping, eating, cooking, and sanitation on the same parcel as an existing or proposed single-family or multifamily dwelling. An accessory dwelling unit also includes the following:
(1) An efficiency unit.
(2) A manufactured home.
(b) “Accessory structure” means a structure that is accessory and incidental to a dwelling located on the same lot.
(c) “Junior accessory dwelling unit” means a unit that is no more than 500 square feet in size and contained entirely within a single-family residence. A junior accessory dwelling unit may include separate sanitation facilities, or may share sanitation facilities with the existing structure.
(d) “Living area” means the interior habitable area of a dwelling unit, including basements and attics, but does not include a garage or any accessory structure.
(e) “Local Authority” means a town, city, county, or city and county.
(f) “Nonconforming zoning condition” means a physical improvement on a property that does not conform to current zoning standards.
(g) “Objective standards” means standards that involve no personal or subjective judgment by a public official and are uniformly verifiable by reference to an external and uniform benchmark or criterion available and knowable by both the development applicant or proponent and the public official prior to submittal.
(h) “Permitting agency” means any entity that is involved in the review of a permit for an accessory dwelling unit or junior accessory dwelling unit and for which there is no substitute, including, but not limited to, applicable planning departments, building departments, utilities, and special districts.
(i) “Underlying zoning” means the height, setback, and other dimensional or design standards that would apply development on a parcel under a Local Authority’s zoning code.
Key terms, drawn from California’s ADU law and HCD guidance. Efficiency units and manufactured homes are defined as ADUs. The “objective standards” definition is the most important in the bill and what makes ministerial approval enforceable.
Section 4. Eligibility.
(a) An ADU or JADU may be built on any lot zoned to permit residential uses.
(b) The use of an ADU and/or JADU unit is a permitted accessory use on any lot where residential uses are permitted.
(c) The construction and use of an ADU or JADU shall comply with all applicable health and safety codes.
ADUs and JADUs are allowed wherever residential use is allowed, subject to ordinary health and safety codes. Tracks California and Colorado HB24-1152.
Section 5. Preemption.
(a) A Local Authority may not establish any restriction or requirement for the construction or use of an ADU or JADU with respect to:
(1) total lot size;
(2) street frontage; or
(3) connectivity between the ADU/JADU and the primary dwelling.
(b) A Local Authority may not require that a primary dwelling, accessory dwelling unit, or junior accessory dwelling unit be occupied by the owner.
(c) A Local Authority’s regulation of architectural elements for ADUs and/or JADUs shall be limited to objective standards that are consistent with the regulation of single-family units, including single-family units located in historic districts.
(d) A Local Authority may not require the installation of a separate utility meter or utility connection for an ADU or JADU.
(e) A Local Authority may not restrict the occupancy of an ADU or JADU based on income, family relationship, age, or any other personal characteristic.
(f) A Local Authority may:
(1) prohibit the installation of a separate utility meter for an ADU and/or JADU;
(2) require the owner of a primary dwelling to abide by local regulations applicable to rentals/landlords for renting an ADU and/or JADU provided that such regulations are consistent with similar regulations for rental property generally;
(3) prohibit the creation of an ADU and/or JADU if the primary dwelling is served by a failing septic tank;
(4) hold a lien against a property that contains an ADU and/or JADU.
(g) Any covenant, restriction, or condition contained in any deed, contract, security instrument, or other instrument affecting the transfer or sale of any interest in a planned development, and any provision of a governing document, that either effectively prohibits or unreasonably restricts the construction or use of an accessory dwelling unit or junior accessory dwelling unit on a lot zoned for residential use that meets the requirements of Section 5 of this bill is void and unenforceable.
(h) This article does not limit the authority of local agencies to adopt less restrictive requirements for the creation of an accessory dwelling unit or junior accessory dwelling unit.
Bans the local rules that quietly block ADUs: minimum lot sizes, street frontage rules, owner-occupancy mandates, and design review beyond objective standards. Also voids HOA rules and private covenants that ban ADUs (California Civil Code 4751) – once cities are reined in, private restrictions become the next chokepoint. Cities can always be less restrictive.
Keep an eye out for owner-occupancy compromises that could impact financing and appraisals. The fear of investor-owned ADUs is unfounded. In California, investors permit just 8% of ADUs despite owning 17% of single-family rentals.
Section 6. Design.
(a) Default design standards for ADUs and JADUs are stated in this section. If not addressed in this section, notwithstanding any local rules or standards, [municipality] must issue a permit if the proposed ADU is in footprint of existing structure; in an existing structure; or is eight hundred fifty square feet or smaller, with four foot side and rear setbacks.
(b) Parking. No additional parking is required for an ADU or JADU. When garage, carport, covered, or uncovered parking is demolished in conjunction with the construction of an accessory dwelling unit or converted to an accessory dwelling unit, the Local Authority shall not require that those off street parking spaces be replaced.
(c) Setbacks. No setback shall be required for an existing living area or accessory structure or a structure constructed in the same location and to the same dimensions as an existing structure that is converted to an accessory dwelling unit or to a portion of an accessory dwelling unit, and a setback of no more than four feet from the side and rear lot lines shall be required for an accessory dwelling unit that is not converted from an existing structure or a new structure constructed in the same location and to the same dimensions as an existing structure. Front setbacks shall be consistent with the underlying zoning.
(d) Height. Height limits shall be set at the greater of:
(1) 25 feet for an attached ADU.
(2) 20 feet for a detached ADU, plus up to 5 additional feet to match the roof pitch of the primary dwelling.
(3) 25 feet for a detached ADU with parking on the ground floor (carriage house), plus up to 5 additional feet to match the roof pitch of the primary dwelling.
(4) The height and bulk limits in the underlying zoning.
(e) Size. A Local Authority may establish minimum and maximum unit size requirements for both attached and detached accessory dwelling units. The Local Authority shall not establish:
(1) A minimum square footage requirement for either an attached or detached accessory dwelling unit that prohibits an efficiency unit.
(2) A maximum square footage requirement for either an attached or detached accessory dwelling unit that is less than either of the following:
(A) Eight hundred fifty square feet.
(B) One thousand square feet for an accessory dwelling unit that provides more than one bedroom.
(3) Any requirement for a zoning clearance or separate zoning review or any other minimum or maximum size for an accessory dwelling unit, size based upon a percentage of the proposed or existing primary dwelling, or limits on lot coverage, floor area ratio, open space, front setbacks, and minimum lot size, for either attached or detached dwellings that does not permit at least an 850 square foot accessory dwelling unit with four-foot side and rear yard setbacks to be constructed in compliance with all other local development standards.
(f) Junior ADUs must meet the following additional requirements:
(1) Size. A JADU shall not expand the footprint of the structure of which it is part by more than 150 square feet for the purposes of ingress and egress.
(g) Nonconformity. An ADU shall not be penalized for pre-existing nonconforming zoning conditions on the lot, nor shall the issuance of an ADU permit trigger a requirement to correct nonconforming zoning conditions elsewhere on the lot.
Any ADU in an existing structure or footprint, or at least 850 square feet with four-foot side and rear setbacks, must be permitted. Old garages and narrow lots are where ADUs are most feasible so legacy nonconforming conditions can’t block a permit. (New Hampshire incorporated this protection in 2025)
Parking mandates are often the biggest ADU killer. Mandating parking made ADUs impossible on 85% of lots in one Washington city.
Section 7. Number.
One ADU and one JADU are permitted per lot.
One ADU and one JADU per lot, by right, meaning no hearing, no rezoning, no discretionary review. Modeled on California Government Code 66323.
Section 8. Creation.
(a) An ADU or JADU may be created through new construction, conversion of an existing structure, addition to an existing structure, or conversion of a qualifying existing house to an ADU while simultaneously constructing a new primary dwelling on the site.
(b) ADUs and JADUs may be prefabricated or otherwise constructed offsite.
ADUs can be new construction, conversions, additions, or built alongside a new home, and can be prefabricated. Prefab ADUs can significantly reduce the cost of a unit. In Vermont, offsite construction costs about $153 per square foot versus $550 to 650 for custom builds.
Section 9. Density and Growth Control.
(a) ADUs and JADUs are exempt from the residential density standards and are not considered to increase or exceed the density on a lot.
(b) ADUs and JADUs are exempt from all local growth-control laws and shall not be counted toward any annual limits on new housing construction under such laws.
ADUs don’t count against density caps or growth-control limits. A house with an ADU is still, functionally, a house.
Section 10. Approval.
(a) A permit application for an ADU and/or JADU that meets the relevant building code and design standards and fire safety codes shall be approved or denied ministerially without discretionary review or a hearing within 30 days after receipt of a completed application.
(b) Denial of an application shall be accompanied by written findings detailing the reason for denial and return in writing a full set of comments to the applicant with a list of items that are defective or deficient and a description of how the application can be remedied by the applicant.
(c) If the Local Authority has not approved or denied the completed application within 30 days, the application shall be deemed approved.
A 30-day shot clock. Denials must list what’s deficient and how to fix it. Silence past 30 days means the permit is automatically approved. Early laws required “timely” review, and they weren’t a strong enough lever.
Section 11. Occupancy and Use.
Occupancy and use standards for an ADU and/or JADU shall be the same as those applicable to a primary dwelling on the same site. [State and Local] Fire and occupancy limits shall apply to the ADU and/or JADU without regard to the number of persons living in other units on the lot.
Same occupancy standards as any home, based on health and safety, not relationship status. Same principle as Colorado’s Golden Girls Act (HB24-1007).
Section 12. Existing Units.
(a) ADUs and JADUs created prior to [date] may be permitted by registering the unit with the (building official) for inclusion into the [Certificate of Occupancy Program]. Application for registration will follow the same ministerial process as an application to build a new ADU and must contain the name of the owner, the address of the unit, the floor area of the two dwelling units, a plot plan of the property, evidence of the date of establishment of the unit, and a signature of the owner. Existing non-conforming ADUs/JADUs shall be permitted irrespective of building code violations unless those violations present a significant threat to occupants’ health and safety. Units permitted before [date 2] shall be exempted from impact fees.
(b) A [Local Authority] may only initiate a code enforcement action on an unpermitted ADU or JADU based on the code governing at the time of construction. If [Local Authority] initiates code enforcement it must notify the owner of the process for legalizing the unit and delay the enforcement action to allow the owner to register the unit for inclusion into the [Certificate of Occupancy Program].
Legalizes existing unpermitted ADUs through registration instead of code enforcement. Older units qualify despite minor code issues absent a real health or safety threat, and cities must offer legalization before enforcement. Brings existing homes out of the shadows and makes them safer.
Section 13. Historic Designation.
(a) ADUs and JADUs are authorized on properties containing structures subject to historic preservation laws, as long as such units do not affect the primary facade as visible from the right-of-way. Homes on corner lots shall not be considered to have two primary facades.
(b) An historic district or designation shall not preclude the permitting and construction of an 850 square foot ADU with four foot side and rear setbacks.
ADUs are allowed in historic districts and on historic properties, as long as they don’t alter the primary, street-facing facade, and historic designation can’t shrink the 850-square-foot guarantee.
Section 14. Impact Fees.
(a) ADU and JADUs of 750 square feet or less are exempt from all impact fees, utility capacity charges, and school fees. Fees applied to larger ADUs and JADUs must be scaled by unit size.
(b) No Local Authority or school district shall set an impact fee or school fee for an ADU or JADU that is larger than the impact fee for a single-family house.
Small ADUs (750 sq ft or less) are exempt from impact fees, utility capacity charges, and school fees. Larger ADUs get fees scaled proportionally – never more than what a single-family house would pay.
Section 15. Enforcement.
(a) All incorporated cities in [state] must pass an ADU ordinance incorporating the provisions of this law and stating any compliant local requirements, processes or procedures for ADU construction or permitting. These ordinances must be filed with [State housing authority or agency].
(b) No additional state-level commission approval shall be required to implement this law and allow the permitting of ADUs or JADUs.
(c) The [State housing authority or agency] shall refer instances of non-compliance to the Attorney General who is empowered to take action to ensure compliance.
Cities must adopt a compliant ADU ordinance and file it with the state housing agency. Noncompliant cities get referred to the Attorney General. Consistent with California’s existing oversight model – and clarifies that no extra state-level approval is needed to start permitting ADUs.
Section 16. Severability.
If any provision of this Act or its application is held invalid, the remainder of the Act shall not be affected and shall continue in full force and effect.
Standard severability clause. If any provision is struck down, the rest stays in effect.
Section 17. Effective Date.
This act is ordered to take effect on January 1st of the subsequent year of when the act is established.
Standard effective date. The law takes effect immediately, so cities can begin permitting ADUs and JADUs without waiting for another legislative or administrative step.
Lessons from other states
More than a dozen states — including Arizona, California, Colorado, Connecticut, Idaho, Massachusetts, Montana, New Hampshire, Oregon, Rhode Island, Utah, Vermont, and Washington — have passed statewide ADU legislation in recent years. The strength of these laws varies widely, and the strongest laws produce more units. California saw a major increase in homebuilding, largely attributable to ADUs, after passing major reforms starting in 2017.
States like Connecticut, Hawaii, and Utah passed weaker reforms that preserve substantial discretion for local jurisdictions or homeowners associations to exclude ADUs and have seen more modest results. New Hampshire’s 2016 law included local barriers and produced few ADUs. The legislature returned to strengthen it in 2025 and again in 2026.
- Reaching California's ADU Potential: Progress to Date and the Need for ADU Finance, Terner Center for Housing Innovation, UC Berkeley.
- How Nonprofits Are Using Accessory Dwelling Units as an Affordable Housing Strategy, Harvard Joint Center for Housing Studies.
- 2024 Home and Community Preferences Survey, AARP.
- To Unleash Housing Supply, Allow and Finance Accessory Dwelling Units, Urban Institute.
- A Taxonomy of State Accessory Dwelling Unit Laws 2025, Mercatus Center.
- Accessory Dwelling Units Reveal Housing Constraints and the Limits of Legalization Alone, Reason.
California — 2016
California has one of the worst housing shortages in America, but ADUs are a California housing success story. California now has the nation's most productive ADU laws, built on decades of trial and error. The state first legalized ADUs in 1982, but the legislation wasn’t strong enough and very few ADUs were built. The breakthrough came in 2016, when SB 1069 and AB 2299 preempted local jurisdictions and streamlined permitting. Binding statewide standards took effect in 2017, and the boom began immediately.
Annual ADU permitting grew more than 150-fold between 2016 and 2022, with nearly 84,000 ADUs permitted statewide. In Los Angeles, permits went from 80 to more than 7,000 a year, and a third of all new homes are now ADUs.
Even then, it took five more years and at least seven cleanup bills to close the loopholes cities found, banning owner-occupancy mandates, eliminating impact fees for smaller units, voiding HOA bans. This model bill consolidates what took California eight bills and forty years into a single act.
New Hampshire —2016, 2025, and 2026
New Hampshire was an early mover on ADU reform. Its 2016 law, passed by a broad coalition of home builders, housing advocates, and AARP, required every town to allow an ADU wherever single-family homes are allowed. But the law let towns choose how: by right, or through discretionary review, and maintained parking requirements, owner-occupancy rules, and design standards, and made detached backyard cottages optional.
The results were real but modest. A Mercatus Center case study found hundreds of ADUs permitted in the years after the 2016 bill passed, but a fraction of what stronger laws produced elsewhere. New Hampshire has since gone back to strengthen the law twice: HB 577 (2025) made detached ADUs a right, protected conversions of nonconforming structures, and barred aesthetic-continuity review, and HB 1540 (2026) made ADUs by-right in most districts.
Issue tags
Related model legislation
Starter Homes • “Golden Girls” Occupancy Reform • Faith and Housing • Parking Mandate Reform
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