ADU state law map

How the map reads a state

An accessory dwelling unit, or ADU, is a second home on a lot that already holds a house, attached to it or standing apart. A statewide ADU law sets what every city it covers must allow, and the map shows, state by state, whether our research verified that status from the statute, a session law or an official text. The title of each shaded state names the law it rests on. Washington's title reads RCW 36.70A.680 and 36.70A.681, the statute as amended in 2025 c 148 ^[1]. Montana's reads MCA 76-2-345, which requires all zoning municipalities to allow at least one ADU by right on a lot with a single-family dwelling ^[2]. Point at a state, or move to it with the keyboard, and the panel beside the map lists every provision our research verified there, each with the section it comes from. The ADU law checker reads the same rows against the date an application would be filed.

The shading carries one fact: whether the statewide status is verified. A state whose provisions our research verified without its statewide law is hatched, and its title says the law is not verified in our research. A state with no verified row reads not verified in our research, whatever a secondary list says about it. The filter sorts the verified states by their status: a statewide ADU law, a law enacted with an effective date after our research, or no statewide mandate. The ADU laws by state guide sets these laws beside the history behind them.

What the verified laws require

Most of the shaded states require a city to allow at least one ADU by right. Arizona's A.R.S. 9-461.18 requires at least one attached and one detached ADU as a permitted use ^[3]. Iowa's SF 592 requires at least one attached or detached ADU on a single-family lot, and the permit must be approved if the application meets requirements ^[4]. Maine requires at least one ADU on any lot where a single-family dwelling is the principal structure under 30-A M.R.S. 4364-B ^[5]. Oregon requires cities over 2,500 and counties over 15,000 to allow at least one ADU per detached single-family dwelling inside the urban growth boundary under ORS 197A.425 ^[6]. Washington goes further: at least two ADUs per lot in urban growth areas, attached and detached, two attached or two detached ^[7]. The Oregon and Washington state pages cover the site plan rules around these laws.

Size rules come in two forms, a cap on the ADU and a floor under the cap a city may set. Washington may not set a maximum gross floor area below 1,000 sq ft, and height may not be capped below 24 ft ^[8]. Montana caps an attached or detached ADU at no more than 75 percent of the primary gross floor area or 1,000 sq ft, whichever is less ^[9]. Iowa caps it at 1,000 sq ft or 50 percent of the single-family residence, whichever is larger ^[10]. Massachusetts protects ADUs up to the smaller of 900 sq ft or half the principal dwelling under 760 CMR 71.02 ^[11]. California's local agencies must allow at least 850 sq ft, or 1,000 sq ft for more than one bedroom ^[12]. How such a unit fits a real lot is the subject of California ADU site fit.

Setback caps are written in feet where a statute names one. Arizona's rear or side setbacks may not exceed 5 ft ^[13], and California allows no more than 4 ft side and rear, with no setback for a conversion within existing space ^[12]. Washington allows no setbacks, coverage, design review or aesthetic rules stricter than for principal units ^[7]. A permit set shows these distances on the drawing, which is what the ADU site plan requirements page walks through.

Parking is often off the table. Arizona and Arkansas may not require additional parking or in-lieu fees ^{[3, 14]}, Montana cannot require parking or in-lieu fees ^[2], Oregon cannot require additional off-street parking ^[6], and Maine says no additional parking may be required ^[15]. Washington bars off-street parking within one-half mile walking distance of a major transit stop ^[7]. Owner occupancy is barred in several states: Maine must allow the ADU even if the owner does not live on the lot ^[5], Massachusetts prohibits owner-occupancy requirements ^[16], and Oregon cannot require owner occupancy of either unit, vacation occupancies excepted ^[6].

Fee caps appear where a statute sets a number. Arkansas caps the ADU application fee at $250 per ADU under Act 313 of 2025 ^[14]. Montana caps its one-time application fee at $250 per ADU and allows no impact fees ^[17]. Virginia caps the ADU permit fee at $500 from 1 July 2027 ^[18]. Colorado's HB24-1152 bars HOA declarations from restricting ADUs in subject jurisdictions ^[19].

Status, dates and the states without a mandate

Florida is shaded for a verified status that is not a mandate. Fla. Stat. 163.31771(3) says a local government may adopt an ordinance allowing ADUs in single-family zones ^[20]. HB 1389 became Ch. 2026-179, approved 26 June 2026, without the ADU mandate ^[21], and SB 48 died on 13 Mar 2026 ^[22]. In Florida the city's own code decides whether and where an ADU is allowed, and the filter lists the state under no statewide ADU mandate.

Virginia's law is enacted with an effective date after our research. SB 531, enacted as 2026 Acts c. 895, takes effect on 1 July 2027 ^[23], and it does not apply to a locality that adopted an ADU ordinance before 1 Jan 2026 or to permits approved before 1 July 2027 ^[24]. The map shows each of its provisions with that date beside it. Dates sit in other rows too: Colorado's subject jurisdictions had to comply by June 30, 2025 ^[25], and Arizona's municipal ADU regulations were due by 1 Jan 2025 ^[13].

Citations move as states recodify. California renumbered its ADU law into Gov. Code Sections 66310 to 66342 ^[26], Oregon's rule now sits in ORS 197A.425 ^[6], and New Hampshire's RSA 674:71 to 674:73 carries HB 577, Laws 2025 ch. 197 ^[27]. The title of each state on the map uses the citation as our research verified it, and the Montana and Maine state pages carry the same statutes into their site plan rules.