ADU laws by state

ADU laws by state, in brief

ADU law is made state by state, and among the states this research reached, two patterns appear. In the first, a statute makes local governments allow an accessory dwelling unit (ADU) and limits what they may require, for example: California approves ADUs ministerially under Gov. Code 66310 to 66342 ^{[1, 2]}; Washington requires cities and counties to allow two ADUs per lot in urban growth areas under RCW 36.70A.680 and .681 ^{[5, 6]}; Oregon requires cities over 2,500 and counties over 15,000 to allow at least one ADU per detached single-unit dwelling under ORS 197A.425 ^[7]; Colorado requires subject jurisdictions to allow one ADU, approved administratively, wherever single-unit detached dwellings are allowed, under C.R.S. 29-35-401 to 405 ^[8]; Montana requires at least one ADU by right on a lot with a single-family dwelling under MCA 76-2-345 ^[3]; and Arizona requires municipalities over 75,000 to allow one attached and one detached ADU under ARS 9-461.18 ^[4]. In the second pattern the city decides: Texas has no statewide ADU law after SB 673 died in the House in 2025, and in Dallas an ADU still needs a rezoning or a special exception ^{[9, 10, 11]}. For Florida and North Carolina the state rule was not verified in our research, so their rows show only the county and city codes that were read.

The table below gives each state one row: the statute and its date, who must allow an ADU, the size floor or cap, side and rear setbacks, parking, and owner occupancy.

Method note

Each value in the table comes from a primary text: a statute or session law, on the legislature's own site or in a copy the source list names, a state agency's handbook or guidance, a legislative fiscal note, or a city or county code, bulletin or permit form, cited by number in the cell. The source list gives the date each was read.

Statute numbers are given as they stand now, each with its date. California's ADU law is cited as Gov. Code 66310 to 66342, as recodified, from the HCD handbook update and the ABAG checklist dated 7.1.25 ^{[1, 2]}. Oregon's is ORS 197A.425, as read on October 10, 2026 ^[7]. Arizona's mandate is ARS 9-461.18, enacted by Laws 2024 ch. 196 ^[4], and its pre-approved plan law is ARS 9-461.19, enacted by Laws 2025 ch. 259 and cited from the enacted text pending codification ^[12].

Cells carry one of five marks:

• A value with a citation: the rule in the words or numbers of the cited text, with the section number where the text gives one.
• None in statute: the statute was read and sets no such limit, as with Oregon's size and setbacks ^[7].
• None in state law: the state has no ADU statute, as in Texas ^{[9, 10]}. A city rule after it is a named example of the local layer and holds only in that city.
• Local example: the state rule was not verified, and the cell shows a county or city rule read from that jurisdiction's own code or form. It describes that jurisdiction, not the state.
• not verified in our research: the research did not reach the primary text for that cell, or found the rule only in a secondary summary. The mark records a gap in the research, not an absence of state law.

Figures that rested only on a secondary summary, or on which two sources disagreed, were left out. That is why California's parking and owner-occupancy cells read not verified, and why several city size and setback figures are absent.

ADU law table by state

The table has a row for every state and gives a value only where the research verified it: 11 states have at least one verified cell. Verified cells carry their citations; county and city rules are named as local examples.

Reading the table

Read across the rows that carry values and the statutes sort by how much they fix on the lot itself.

Statutes that put numbers on the lot

Several statutes write a size number into state law, among them California's, Washington's, Montana's and Arizona's, and California and Arizona add a numeric cap on side and rear setbacks, so a site plan can show compliance against a fixed figure ^{[1, 2, 6, 3, 4]}.

California. The HCD handbook sets the floor a local ordinance must meet: at least 850 sq ft, or 1,000 sq ft for an ADU with more than one bedroom, with up to 1,200 sq ft as the maximum for a new detached ADU where the ordinance is compliant ^[2]. Under 66321, lot coverage, floor area ratio, open space and front setback rules cannot block an 800 sq ft unit, and under 66314(d)(7) side and rear setbacks are capped at 4 ft, with no setback at all for a conversion within existing space ^{[1, 2]}. Height runs to 16, 18 or 20 ft depending on conditions in the Government Code ^[2]. The California site question is therefore whether an 800 sq ft footprint fits inside 4 ft side and rear lines, worked through in California ADU site fit; wider state rules are on the California site plan page.

Washington. RCW 36.70A.680, .681 and .696, from HB 1337 (2023), make cities and counties allow two ADUs per lot on every lot in an urban growth area that allows single-family homes. They must allow at least 1,000 sq ft and a height of at least 24 ft, may not set setbacks stricter than for the principal unit, and may not require owner occupancy unless the ADU is a short-term rental ^{[5, 6]}. RCW 36.70A.681(2) bars an off-street parking requirement for ADUs within one-half mile walking distance of a major transit stop, the same distance the Department of Commerce's June 2024 guidance gives ^{[22, 5]}. The state's other site plan rules are on the Washington site plan page.

Montana. MCA 76-2-345 (MCA 2025; in effect since Jan 1, 2024) reaches every zoning municipality. It guarantees at least one ADU by right on a lot with a single-family dwelling, caps it at 75% of the house GFA or 1,000 sq ft, whichever is less, allows no setback stricter than the house's, and bars parking requirements and in-lieu fees ^[3]. The one-time ADU application fee is capped at $250 per unit (76-2-345(4)), impact fees are barred, and a municipality may require municipal water and sewer will-serve letters (76-2-345(7)) ^[3]. The Montana Supreme Court reversed a preliminary injunction against the 2023 housing laws on September 3, 2024 (2024 MT 200) and on March 17, 2026 upheld those laws, this section among them (2026 MT 53) ^[23]. The City of Great Falls cites the MAID v. State decision and enforces the law ^[24]. The state's wider rules are on the Montana site plan page.

Arizona. ARS 9-461.18, enacted by HB 2720 (Laws 2024 ch. 196), gave municipalities over 75,000 until Jan 1, 2025 to allow one attached and one detached ADU as a permitted use, plus a further detached ADU on 1 acre or more where one unit is restricted-affordable ^[4]. The size cap is 75% of the house GFA or 1,000 sq ft, whichever is less, and rear or side setbacks cannot exceed 5 ft ^[4]. Subsection (D) bars fire sprinkler and commercial-code requirements for ADUs, and subsection (E) bars building on a public utility easement without the utility's consent, so the easement lines belong on the site plan ^[4]. A city may require owner residency only where an ADU built after the amendment is used as a short-term rental (ARS 9-500.39(B)(9)) ^[4]. Phoenix, under the state law, allows two ADUs per detached lot and a third in some cases ^[13], and its ADU rules sit in Zoning Ordinance Section 706 ^[25]; see the Arizona site plan page.

Statutes that set the process

Oregon and Colorado set rules of process: who must allow an ADU, how it is approved, and what a city may not demand ^{[7, 8]}. Oregon's ORS 197A.425 applies to cities over 2,500 and counties over 15,000 inside the urban growth boundary; it sets no size or setback and leaves cities to adopt "reasonable local regulations relating to siting and design", but bars an additional off-street parking requirement and an owner-occupancy requirement on either unit, vacation occupancies excepted ^[7]. Colorado's C.R.S. 29-35-401 to 405 (HB24-1152) gave subject jurisdictions until June 30, 2025 to allow one ADU as an accessory use wherever single-unit detached dwellings are allowed, approved administratively ^[8]. The Legislative Council fiscal note of February 21, 2024 names the subject jurisdictions as municipalities of at least 1,000 residents and certain census designated places within metropolitan planning organizations; under the bill, ADUs carry no new parking requirements and owner occupancy cannot be required ^[14]. Colorado's size and setback limits were not verified in our research. DOLA lists Round 3 of its ADU Grant Program as open from September 28 to October 23, 2026 ^[8]. See the Oregon and Colorado site plan pages.

Pre-approved plans in California and Arizona

California and Arizona also require pre-approved ADU plan programs. California's Gov. Code 65852.27 (AB 1332, 2023) requires each local agency to "accept accessory dwelling unit plan submissions for preapproval", with programs due Jan 1, 2025; approved plans are posted with the applicant's contact information, and a detached ADU application that uses a preapproved plan is decided within 30 days ^[26]. Arizona's ARS 9-461.19 (SB 1529, Laws 2025 ch. 259) requires municipalities to post pre-approved ADU plans of 200, 600 and 1,000 sq ft from July 1, 2026, with three elevation options per class, and duplex and triplex plans of 400 to 1,000 sq ft per unit from Jan 1, 2027 ^[12]. An Arizona municipality "may not restrict who may submit", must list the applicant's contact information with the plans, and approves them administratively without discretionary review ^[12].

In Phoenix and Los Angeles a pre-approved plan still needs a plan of the lot. Phoenix's ADU page says: "A plot/site plan will still be required for permitting" ^[27]. The Los Angeles standard plan checklist (dated 7.7.2023) requires, with the approved standard plan, a plot plan drawn to scale showing property lines with dimensions, setbacks, separation from the primary dwelling, site drainage, a north arrow, adjacent streets, and driveways and parking ^[28]. The lot-specific plan is the subject of site plans for pre-approved ADUs, and city checklists are compared in ADU site plan requirements.

Where cities decide

Texas has no statewide ADU law. SB 673 (89th Legislature) passed the Senate on April 10, 2025, was placed on the House General State Calendar on May 26, 2025, and died ^{[9, 10]}. Each city's code governs instead. Dallas offers no by-right path: an ADU needs an ADU Overlay rezoning or a Board of Adjustment special exception ^[11]. San Antonio's Information Bulletin 402 requires owner occupancy with a covenant recorded in Bexar County, side and rear setbacks of 5 ft (3 ft with no overhang), a height of no more than 25 ft or two stories, and one parking space for a detached ADU over 800 sq ft ^[21]. Statewide context is on the Texas site plan page.

For Florida and North Carolina the state ADU statute was not verified in our research, so the rows show local examples only. Unincorporated Miami-Dade County allows one ADU on a lot with a single-family dwelling under County Code Sec. 33-22, at 400 to 800 sq ft in the RU-1 district ^[16]. In North Carolina, Raleigh allows ADUs by right under UDO Sec. 2.6.3, a text change adopted July 7, 2020 ^{[18, 19]}, and Charlotte allows an ADU accessory to any single-family or duplex dwelling under UDO Article 15.6 ^[20]. Local context is on the Miami, Raleigh-Durham and Charlotte metro pages and the Florida and North Carolina state pages.

For most other states, no ADU statute was read at primary-source depth, and the row reads not verified in our research.

Lot splits in the same states

Two of the states above also allow small lot divisions alongside ADUs. California's SB 9 (2021, effective Jan 1, 2022), at Gov. Code 66411.7, allows a one-time ministerial split into two lots with up to two units on each resulting lot, or up to four units on an unsplit lot ^[29]. Oregon's SB 458 (2021), at ORS 92.031, allows a middle housing land division so that each middle housing unit can sit on its own lot ^[30]. The Oregon layout is covered in Oregon middle housing land division.

Where to go from here

For developers and builders placing ADUs on single-family lots, the single-family and townhome site plan hub shows how ADU and lot-split law shapes a lotting plan. A preliminary site plan places the ADU footprint, setbacks, separation from the house, driveway and parking on the lot with a site data table; engineering is by the client's engineer of record. Homeowner permit plans sit in permit site plans, and the background essay is how ADU legislation is reshaping single-family site plans.

ADU guides:

• ADU site plan requirements, site plans for pre-approved ADUs, California ADU site fit, who can draw ADU plans, Oregon middle housing land division

State site plan pages for states with verified rows include:

• California, Washington, Oregon, Colorado, Montana, Arizona, Texas, Florida, North Carolina

Metro pages for the cities named:

• Phoenix, San Antonio, Dallas-Fort Worth, Los Angeles, Miami, Raleigh-Durham, Charlotte

How these sources were chosen and when the page is reviewed: how we research.

By Viola Sauer, Principal, Innowave Studio