ADU law checker

How this works

An accessory dwelling unit, or ADU, is a second home on a lot that already has a house, attached to it or standing apart. A statewide ADU law sets what every city it covers must allow on that lot. Washington requires at least two ADUs per lot in urban growth areas, attached and detached, two attached or two detached ^[1]. Montana requires all zoning municipalities to allow at least one ADU by right on a lot with a single-family dwelling under MCA 76-2-345 ^[2]. Where the state rule allows the unit, the lot still has to fit it, which is the question a single-family site plan answers.

The checker reads three things for the state you choose. First, the statewide status: a law in force, a law enacted but not yet in force, no statewide mandate, or not verified in our research. Second, the provisions our research verified from the statute, a session law or an official text, each with its citation. Third, what is not verified: a provision our research could not confirm is named, never filled in. The Washington, Montana and Maine state pages set the wider site plan rules around these laws.

The date you enter

The status is read on the date you enter, the day the application would be filed, because a law can be enacted well before it applies. Virginia's SB 531, enacted as 2026 Acts c. 895, takes effect on 1 July 2027 ^[3]. Entered before that day, Virginia reads as enacted but not yet in force, and every provision of the law is listed as taking effect with it. From 1 July 2027 the same provisions read as in force: single-family zoning is deemed to include ADUs as a permitted accessory use, and the locality must issue the permit if the requirements are met ^[4]. The permit fee is capped at $500 ^[5]. The law does not apply to a locality that adopted an ADU ordinance before 1 Jan 2026 or to permits approved before 1 July 2027 ^[4].

Each row holds the law as our research read it. Where a status row carries no start date, the checker reports a date before the research date as not verified for that law instead of assuming it applied then. California's rows carry no date: the checker reads Gov. Code Sections 66310 to 66342 as researched ^[6].

Florida and the states without a mandate

Florida has no statewide ADU mandate: Fla. Stat. 163.31771(3) says a local government may adopt an ordinance allowing ADUs in single-family zones ^[7]. HB 1389 became Ch. 2026-179, approved 26 June 2026, without the ADU mandate ^[8], and SB 48 died on 13 Mar 2026 ^[9]. The statute still requires an affordable-rent affidavit for ADUs permitted under ordinances adopted under the section ^[7]. In Florida the city's own code decides whether and where an ADU is allowed, and the checker says so plainly.

What the verified state laws set

Size rules come in two forms: a cap on the ADU and a floor on 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 ^[10]. 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 ^[11]. Iowa's SF 592 caps an ADU at 1,000 sq ft or 50 percent of the single-family residence, whichever is larger ^[12]. Massachusetts protects ADUs up to the smaller of 900 sq ft or half the principal dwelling under 760 CMR 71.02 ^[13]. California's local agencies must allow at least 850 sq ft, or 1,000 sq ft for more than one bedroom ^[14], and an ADU of at least 800 sq ft with 4 ft side and rear setbacks cannot be precluded by local standards ^[15]. The Iowa and Massachusetts pages cover the rest of the state approval path.

Setbacks follow the same pattern. Arizona's rear or side setbacks may not exceed 5 ft ^[16], and California allows no more than 4 ft side and rear, with no setback for a conversion within existing space ^[14]. Washington allows no setbacks, coverage, design review or aesthetic rules stricter than for principal units ^[1].

Parking is often off the table. Arizona may not require additional parking or in-lieu fees ^[17], and neither may Arkansas ^[18]. Montana cannot require parking or in-lieu fees ^[2], Oregon cannot require additional off-street parking ^[19], and Maine says no additional parking may be required ^[20]. Washington bars off-street parking within one-half mile walking distance of a major transit stop ^[1].

Owner occupancy is barred in several states. Maine must allow the ADU even if the owner does not live on the lot ^[21]. Massachusetts prohibits owner-occupancy requirements ^[22], and Arkansas may not require owner occupancy of the house or the ADU ^[18].

Fees are capped where the statute sets a number. Arkansas caps the ADU application fee at $250 per ADU ^[18], and Montana caps its one-time application fee at $250 per ADU and allows no impact fees ^[23]. Colorado's HB24-1152 bars HOA declarations from restricting ADUs in subject jurisdictions ^[24], and Colorado subject jurisdictions had to comply by June 30, 2025 ^[25].

The city layer

Within the state's limits, the city's own code governs, and the checker adds a city's rules where our research verified them from the city's code or an official city page. San Antonio sets a 5 ft side and rear setback, 3 ft if there is no overhang, and requires one space for a detached ADU over 800 sf ^[26]. Dallas allows an ADU only through ADU Overlay rezoning or a Board of Adjustment special exception ^[27]. Chicago requires no parking for an ADU, but existing parking may not be removed without relief, and the plans must be by an Illinois-licensed architect or structural engineer ^[28]. Who draws a permit set is a local rule, as Chicago shows; who prepares a site plan covers the general case.

Austin's HOME Phase 1 allows up to 3 units on SF-1, SF-2 and SF-3 lots ^[29], and each unit needs its own address ^[30]. Reno allows ADUs wherever single-family homes are allowed ^[31]. Raleigh's accessory dwelling cannot be transferred apart from the principal dwelling ^[32], and Charlotte allows an ADU accessory to any single-family or duplex dwelling ^[33].