ADU Laws Coast to Coast: Where You Can Build an Accessory Dwelling Unit in 2026

The accessory dwelling unit is the most-legislated residential building type in America right now. In the last five years, fourteen states have passed laws that override local single-family-only zoning and guarantee homeowners the ability to build an ADU — a backyard cottage, a garage conversion, a basement apartment, or an internal unit carved out of the main house. In the other 36 states, ADU law is a city-by-city patchwork that can flip from permissive to prohibitive across a municipal line. If you are considering an ADU in 2026 as a rental, a family unit, or a path to multi-generational living, the first question is not "what can I build" but "what has my state legislature decided my city is allowed to decide?" This guide walks through the states that have moved, the statutes that matter, and the substantive rules — size, setback, parking, owner-occupancy, HOA preemption — that determine whether an ADU is actually viable on your lot.

The ADU gold standard states

Three states set the national pace: California, Oregon, and Washington. California was first and is still the most aggressive. Government Code §65852.2 is the core ADU statute, amended nearly every legislative session since 2016, and it is now supplemented by §65852.22 (junior ADUs), SB 9 of 2021 (urban lot splits and duplexes on single-family parcels), SB 897 of 2022 (16-foot minimum height, 18 feet near transit, up to 25 feet for two-story attached ADUs), and Civil Code §4751 (HOA preemption). The statute requires every California city to approve ADUs ministerially — no discretionary review, no public hearing — within 60 days of a complete application.

Oregon rewrote its zoning code with SB 1051 in 2017, which required every city over 2,500 population to allow at least one ADU on every lot zoned for a detached single-family dwelling inside the urban growth boundary. HB 2001 in 2019 went further, stripping cities of the ability to impose owner-occupancy requirements or off-street parking mandates as "reasonable" regulations. Portland, Eugene, and Bend had been running permissive ADU programs for years; SB 1051 pushed the same framework into every mid-sized Oregon city.

Washington's HB 1337, effective July 23, 2023, is the most aggressive recent statute. Inside urban growth areas, every city and county must allow two ADUs per residential lot — attached, detached, or a combination. Cities must permit ADUs of at least 1,000 square feet, cannot charge more than 50% of the principal unit's impact fees, and cannot require additional off-street parking for ADUs within a half mile of a major transit stop. HB 1337 also explicitly legalized ADUs built from existing detached structures like garages, even if they violate current setback or lot-coverage rules.

California's framework in detail

California's framework deserves its own section because it is the model other states copy. Under §65852.2, every single-family lot is entitled to one attached or detached ADU of up to 1,200 square feet, plus one junior ADU of up to 500 square feet carved from the existing primary dwelling. Multifamily properties are entitled to an even more generous allowance — up to 25% of existing units as ADUs inside the building, plus two detached ADUs on the property. Cities must review applications ministerially within 60 days, and the state eliminated the owner-occupancy requirement that used to be standard (the prohibition on new owner-occupancy rules sunset in 2025, so the situation is worth rechecking locally, but most cities have not reinstated it).

Parking is the rule California homeowners notice first. A city cannot require any replacement parking for an ADU if the lot is within one-half mile walking distance of public transit, if the ADU is in a historic district, if it is part of a proposed or existing primary residence, if on-street parking permits are required but not offered to the ADU occupant, or if there is a car-share vehicle within one block. In practice, this means most urban California lots owe zero additional parking spaces. Garage conversions are also protected — a city cannot require a replacement covered parking space when a garage is converted into an ADU.

Setbacks follow a similar story. For a converted-garage or other converted-existing-structure ADU, the city may not impose any setback. For new-construction detached ADUs, the maximum side and rear setback a city can require is four feet. San Diego's ADU bonus program layers an additional incentive — homeowners who deed-restrict one ADU as affordable can build additional market-rate ADUs on the same lot, a program that has produced some of the highest per-lot ADU counts in the state.

State-by-state allowance map

Outside the three gold-standard states, a growing group of states now guarantee ADUs by right. Connecticut's HB 6107 (Public Act 21-29), passed in 2021, required every municipality that zones under CGS §8-2 to allow ADUs as of right on single-family lots, subject to an opt-out window that closed January 1, 2023 for most towns. Maine's LD 2003, signed in April 2022, eliminated single-family-exclusive zoning statewide and requires every municipality to permit at least one ADU on any lot where a single-family dwelling is the principal structure, with a floor minimum of 190 square feet and no additional parking requirement beyond what the primary unit already needs.

Utah's HB 82 of 2021 was narrower but decisive on one point: every Utah city must permit an internal accessory dwelling unit — a unit carved out of the existing footprint of a single-family home — as a permitted use in single-family residential zones. The statute also voided any HOA CC&R that banned internal ADUs, preempting private deed restrictions in the same way California's §4751 does.

Montana passed SB 528 in 2023 (not HB 419; HB 419 was a related middle-housing bill), requiring municipalities to allow at least one ADU on any lot with a single-family dwelling, up to 1,000 square feet or 75% of the primary home, whichever is smaller. The law was upheld by the Montana Supreme Court after a city challenge and took effect January 1, 2024. Vermont's S100 "HOME Act" of 2023 likewise required every Vermont municipality to permit ADUs in all residential zones. New York began a more targeted pilot in Queens and other parts of New York City through the City of Yes housing amendments, but has not yet passed a statewide ADU preemption law — ADU legalization in upstate New York remains city-by-city.

HOA preemption

State ADU laws are often hollowed out by private HOA CC&Rs that ban accessory structures. California Civil Code §4751, effective January 1, 2020, solved that for California — any HOA governing document provision that "effectively prohibits or unreasonably restricts" the construction or use of an ADU or JADU on a single-family-zoned lot is void and unenforceable. HOAs may still impose "reasonable" restrictions on design, placement, and safety — matching exterior materials, roof pitch alignment, utility screening — but a blanket ban is no longer permitted.

Utah's HB 82 carries an equivalent preemption for internal ADUs, and Oregon, Washington, and Maine apply their ADU statutes directly to residential lots regardless of HOA status. In the rest of the country, an HOA covenant banning accessory dwelling units is generally enforceable, even if the underlying city code allows them. If you are buying into a planned community in Texas, Florida, or the Carolinas, check the CC&Rs before you assume state or local permissiveness applies.

What local cities still control

Even in the gold-standard states, cities retain meaningful authority. They set objective design standards — setback above the statutory minimum, height up to the state-mandated floor, architectural compatibility, roof pitch, exterior materials. They set unit-count caps beyond the state minimum (a California city can allow three ADUs on a lot, not just the statutory one). They control short-term rental use of ADUs, which is where most of the substantive restriction happens now. And they set the impact-fee schedule, subject to the HB 1337-style caps in Washington and similar caps in California for ADUs under 750 square feet (which are impact-fee exempt).

Los Angeles runs the Accessory Dwelling Services (often shortened by developers to the AAS program) as its ADU permitting pipeline. San Francisco allows ADUs in every residential zone with ministerial approval. Portland runs the Residential Infill Project (RIP), which pairs ADU allowances with middle-housing rights. Seattle's ADU program predates HB 1337 but was expanded in 2019 to allow two ADUs per single-family lot and eliminate owner-occupancy requirements. San Jose, Oakland, and Sacramento all run active ADU programs with pre-approved plan sets designed to compress permitting time.

Restrictive states

The contrast on the other side of the country is stark. Texas, Florida, Georgia, South Carolina, North Carolina, Tennessee, Alabama, Mississippi, Louisiana, Arkansas, and most of the Mountain West have no statewide ADU preemption law. ADU legality is entirely a function of the local zoning code. Austin allows ADUs citywide and has simplified permitting; the rest of Texas ranges from permissive (Dallas, San Antonio) to effectively prohibitive (smaller suburban cities that have not updated zoning for decades). Florida has a strong state preemption framework for short-term rentals but nothing equivalent for ADUs — Miami allows them through the Miami 21 zoning code, while most of the Panhandle and rural Florida counties do not permit them at all in single-family zones.

Atlanta passed an ADU ordinance in 2017 that opened most of the city to ADU construction, but the Georgia suburbs remain largely closed. Nashville allows detached accessory dwelling units in specific zoning categories, but the rest of Tennessee is broadly restrictive. If you are in one of these states, the state legislature is not going to save you — your only path to an ADU is through the local city council and whatever zoning variance or special-use permit process applies.

Financing

ADU financing improved dramatically in 2022 when FHA began accepting ADU rental income in mortgage qualification. Under the October 2023 FHA update, borrowers can count up to 75% of projected ADU rent toward qualifying income on purchase or refinance loans, even for a new-construction ADU that does not yet exist, using a CDA (comparable rental analysis) from the appraiser. Freddie Mac followed with a similar rule for conventional loans. Fannie Mae's HomeStyle Renovation loan can finance ADU construction as part of a purchase, bundling land, primary residence, and ADU construction into a single mortgage.

The tax side is worth understanding before you build. A detached ADU triggers a reassessment in most jurisdictions — the added square footage is assessed, but the main home keeps its existing Proposition 13 basis in California or equivalent assessment cap in other states. A junior ADU or internal ADU carved from existing space generally does not trigger reassessment because no new square footage is created. For long-term financial planning, the JADU/internal-ADU path is often cheaper long-term even though the unit is smaller.

Common pitfalls

Six recurring issues tank ADU projects. First, separate utility meters: most state statutes prohibit cities from requiring a separate water or sewer connection for an ADU under 750 square feet, but not every city has updated its code to match, and some utility districts still charge full new-connection fees. Verify with both the city planning department and the water/sewer utility. Second, fire sprinklers: California exempts ADUs from the fire sprinkler requirement that applies to the primary dwelling under NFPA 13D, but only if the primary dwelling itself is not required to have sprinklers. Third, side-setback encroachment: converting a detached garage that already sits within the setback is allowed under §65852.2, but new construction above that footprint is not. Fourth, septic capacity on rural lots: if you are on septic, the county environmental health department will usually require a percolation test and possibly a septic upgrade before permitting the ADU. Fifth, deed restrictions and easements that predate current zoning — utility easements across the backyard, view easements, and historic preservation overlays can block a project that would otherwise be by-right. Sixth, building code separation: an ADU sharing a wall with the primary dwelling requires a one-hour fire-rated assembly, and the existing wall often fails that requirement without retrofit.

Short-term rental restrictions

A trend worth understanding: many cities that require by-right ADU approval separately prohibit short-term rental use of those units. Portland's short-term rental code bars ADUs from being rented for stays under 30 nights unless the primary dwelling is owner-occupied and the ADU rental is hosted. Los Angeles's Home Sharing Ordinance restricts short-term rental to the primary residence only — an ADU used as a separate STR is not eligible. San Francisco extends its 90-night unhosted cap to ADUs. The policy reasoning is consistent: cities legalize ADUs to expand long-term housing supply, and they do not want those units converted to Airbnb rentals that remove them from the housing market. If your financial model depends on short-term rental income, check both the ADU code and the STR code before you build — the permissive ADU statute does not grant permissive STR use.

How to check your city

State law tells you what your city must allow at minimum. City code tells you what actually applies — the design standards, unit-count caps, impact fees, parking rules, short-term rental restrictions, and HOA interactions that determine whether an ADU on your specific lot is viable. Before you hire an architect, answer six questions: Is my state a preemption state (CA, OR, WA, CT, ME, UT, MT, VT) or a local-control state? What is my city's maximum ADU size and height? Are there parking or owner-occupancy requirements that survive state preemption? What are the impact fees and are they capped for small ADUs? Does my HOA have CC&Rs, and does my state preempt them? Is short-term rental allowed in ADUs? CityRuleLookup maintains an accessory structures page for every city we cover, pulling together ADU allowance, size caps, setbacks, parking, and permit contacts. Start there, then confirm with the city's own code and, if your HOA is involved, a real-estate attorney familiar with your state's ADU preemption statute.