Research study
ADU growth outside California: do the new laws work?
A statutory right is not a housing unit. This page tests the gap — and finds that the main obstacle to testing it is that hardly any state counts.
Written by HyreADU Research Desk Primary-source research and data analysis
Audited by HyreADU Research Desk Statutory citation and data-coverage audit
The finding
The honest answer is that in most states nobody can tell you, because nobody counts. We retrieved broad ADU enabling statutes for six states beyond California — Oregon, Washington, Montana, Maine, Colorado and Arizona — and then went looking for the permit series that would show whether units followed.
We located a statewide ADU permit series for none of them. That is a measurement failure, not a production failure, and we report those states as unmeasured rather than as zero. The federal permit series cannot help either: the Census Building Permits Survey has no accessory category, so there is no national fallback.
Exactly one jurisdiction outside California publishes a long, auditable ADU permit series, and it is a city. Seattle’s Office of Planning and Community Development has produced an annual ADU report since a 2019 city ordinance required one, and its 2024 edition carries permits by year back to 2005.
Where the data exists, the reform effect is large and dated: annual ADU permits ran between 64 and 283 for the fifteen years to 2019, then reached 476 in 2020, 762 in 2021 and above 900 in each of 2022, 2023 and 2024, and have since flattened.
Seattle is Seattle, and this page does not generalise one city to six states.
Read this first
- One city is not six states, and we do not treat it as one
Seattle is the only place outside California where we could measure production against a dated reform. Everything the Seattle data shows is a finding about Seattle. It is presented here because it is the best available evidence on the question, not because it stands in for anywhere else.
- Unmeasured is not zero
Montana, Maine, Colorado and Arizona all have enabling statutes and no statewide ADU permit series that we could find. That means we cannot say what happened, in either direction. Any table cell reading “unmeasured” on this page means exactly that and must not be read as “little or no production”.
- We did not attempt a causal estimate
Seattle’s permits rose sharply after August 2019 and we describe the association.
We ran no difference-in-differences, no synthetic control and no counterfactual, and we did not attempt to separate the reform from the pandemic-era construction cycle, interest rates or the city’s wider middle-housing changes.
A time series with a marker on it is evidence, not identification.
- Statutes were read as enacted, not as implemented
We retrieved the operative statutory text for each state listed. We did not audit whether individual local jurisdictions have brought their codes into compliance, and in several of these states that gap is known to be live. The statutory map lives on our state law study; this page is about what happened afterwards.
The test, and why it is so hard to run
The policy claim behind every state ADU statute is straightforward: remove the local barriers — discretionary review, parking minimums, owner-occupancy covenants, size caps set below what a useful unit needs — and homeowners will build. It is a claim about behavior, and it is testable in principle by counting permits before and after.
In practice it is barely testable at all, and the reason is administrative. California is unusual in requiring every city and county to report permits and completions by unit category to a state agency every year.
That single reporting duty is why California has an ADU permit series and why our permits versus completions study can exist. No other state we examined has an equivalent.
The federal fallback does not exist either. As our instrument audit sets out, the Census Building Permits Survey has four structure categories and no accessory item; conversion ADUs never enter the units series and detached ADUs are merged into single-family.
So a researcher wanting to know whether Montana’s 2023 statute produced units has no state series and no federal series to fall back on.
HyreADU analysis: this is a policy design point worth stating plainly, and it is ours rather than any source’s. A legislature that legalizes ADUs and does not simultaneously require anyone to count them has passed a law whose effect cannot later be demonstrated — to itself, to a sceptical successor legislature, or to the courts.
The cheapest amendment available to any of these statutes is a reporting clause.
Six states: what the statute requires, and whether anyone counts
Every statutory provision below was retrieved from the state’s own legislature or code site on 2026-09-05. The final column is the finding of this page: the enabling law is common, the measurement is not.
| State | Statute | Core requirement | Owner-occupancy / parking | Statewide ADU permit series? |
|---|---|---|---|---|
| Oregon | ORS 197A.425 (formerly ORS 197.312(5)–(6); SB 1051, 2017) | Cities over 2,500 and counties over 15,000, inside an urban growth boundary and zoned for detached single-family: at least one ADU per detached single-family dwelling. | Neither may be imposed — “reasonable local regulations relating to siting and design” expressly does not include owner-occupancy or additional off-street parking. | None located. The only compilation we found is a July 2021 blog post by an individual with no named data source; we have not tabled it as a measurement. |
| Washington | RCW 36.70A.681 (2023 c 334, HB 1337) | At least two ADUs per lot in all zoning districts inside urban growth areas; a maximum gross floor area cap may not be set below 1,000 sq ft. | No owner-occupancy requirement; no parking within half a mile of a major transit stop; impact fees capped at 50% of the principal unit’s. | None located statewide. Seattle publishes its own, under a city ordinance — see the series below. |
| Montana | MCA 76-2-345 (SB 528; “En. Sec. 1, Ch. 502, L. 2023”) | At least one ADU by right on single-family lots — attached, detached or internal. Size limit 75% of primary floor area or 1,000 sq ft, whichever is smaller. | No occupancy restriction, no additional parking, no design-match mandate, no impact fees; application fee capped at $250. | None located. Unmeasured. |
| Maine | 30-A M.R.S. § 4364-B (PL 2021 c.672 [LD 2003], amended through PL 2025 c.385) | At least one ADU on any lot with a single-family principal structure, in any area where residential uses are permitted, with a Lewiston/Auburn watershed carve-out. | No owner-occupancy; no additional parking. | None located. Unmeasured. |
| Colorado | C.R.S. 29-35-101 to -105 (HB24-1152) | From 30 June 2025, a “subject jurisdiction” must allow one ADU, subject to an administrative approval process, wherever single-unit detached dwellings are allowed. Subject jurisdictions are municipalities of 1,000+ inside an MPO, or the portion of a county inside a census designated place of 40,000+ and inside an MPO. | No new off-street parking space, with stated exceptions; no owner-occupancy requirement beyond an application-time residence demonstration. | None located. Unmeasured — and the duty is recent enough that a meaningful series could not exist yet. |
| Arizona | A.R.S. § 9-461.18 | Municipalities over 75,000: at least one attached and one detached ADU; a second detached unit on lots of one acre or more if one is deed-restricted affordable. Size limit 75% of the single-family dwelling’s gross floor area or 1,000 sq ft, whichever is smaller. | No required familial, marital or employment relationship; no additional parking. Statute self-executes where a municipality failed to adopt regulations by 1 January 2025. | None located. Unmeasured. |
Six state ADU statutes retrieved on 2026-09-05, with the availability of a statewide ADU permit series in the final column. A state appears here only if we retrieved its statutory text directly; a state we could not read is not listed at all.
For a broader statutory landscape, Emily Hamilton and Kol Peterson of the Mercatus Center count 18 states with broad ADU legalization as of July 2025, 10 of which they classify as strong and 8 as weaker, in “A Taxonomy of State Accessory Dwelling Unit Laws 2025”.
That is their count and classification, cited as theirs; we have not verified the other twelve states ourselves and have not tabled them.
Seattle: twenty years of ADU permits
The one long series available outside California, from the city’s fourth annual ADU report. Reporting is mandated by Ordinance 125854, which is itself part of why the data exists.
| Year | ADUs permitted | Attached (AADU) | Detached (DADU) | Note |
|---|---|---|---|---|
| 2015 | 190 | — | — | Pre-reform. Split not published for this year. |
| 2016 | 274 | — | — | Pre-reform |
| 2017 | 252 | — | — | Pre-reform |
| 2018 | 245 | — | — | Pre-reform |
| 2019 | 283 | — | — | Reforms take effect August 2019 |
| 2020 | 476 | 230 | 246 | First full post-reform year |
| 2021 | 762 | 338 | 424 | |
| 2022 | 924 | 407 | 517 | Series peak |
| 2023 | 905 | 447 | 458 | |
| 2024 | 913 | 448 | 465 | Plateau, third year above 900 |
| 2025 (part year) | 507 | 247 | 260 | Through Q2 only. Applications reported down 32% year-over-year. |
Seattle ADU permits by year. The 2005–2014 values, not tabled above for length, are 64, 85, 103, 98, 76, 144, 107, 115, 125 and 155 and are drawn in the chart.
HyreADU note: the reform package took effect in August 2019, so 2019 is a split year and 2020 is the first clean post-reform observation. The package removed off-street parking and owner-occupancy requirements, allowed two ADUs per lot, and raised the maximum height and size.
Seattle also implements Washington’s HB 1337 locally through Ordinance 127211 (May 2025), and HB 1110 middle housing separately — a further reason not to read the whole post-2019 rise as one policy’s effect.
What the Seattle series does and does not show
The Seattle series shows a large, dated, sustained level shift
Fifteen years between 64 and 283 permits a year, then 476, 762 and three consecutive years above 900. That is not a trend continuing; it is a step. And it follows a reform with a known effective date rather than a gradual change in market conditions.
The composition changed too, not just the count. Detached and attached units are now permitted in roughly equal numbers — 448 attached and 465 detached in 2024 — and the median lot size is ordinary: 5,905 sq ft for an attached unit and 6,002 sq ft for a detached one. This is not an estate-lot phenomenon.
It shows a plateau, and the city says so
Permits have sat at roughly 900 a year for three years and applications are reported down 32 per cent year-over-year. Whatever the reform unlocked, it appears to have unlocked a level rather than a growth rate. A reform effect that lands hard and then plateaus is a different policy story from one that compounds.
HyreADU analysis: a level shift that plateaus is what you would expect if a reform removes a binding constraint on a finite pool of interested owners — a stock of latent demand converting over a few years — rather than creating a self-sustaining new construction sector.
We cannot distinguish that from a financing-cost story with this data, and we do not claim to.
It does not show causation
Seattle changed several things at once and continued changing them: the 2019 ADU package, then HB 1110 middle housing and Ordinance 127211 implementing HB 1337 in May 2025. The pandemic construction cycle and interest rate path run straight through the middle of the series.
We have not attempted to separate these. A properly identified estimate would need comparison jurisdictions with credible parallel trends — which is exactly what the absence of ADU series everywhere else makes impossible to assemble.
It comes with a reporting duty attached, which is the transferable lesson
Seattle produces this report because a 2019 city ordinance requires it. That is why a twenty-year ADU series exists for Seattle and does not exist for Portland, Bozeman, Portland Maine, Denver or Phoenix.
The pre-approved plan program in the same report is a good illustration of what the reporting buys: Seattle can say that its pre-approved DADU plans have been permitted 199 times since 2020, and can publish the average intake-to-issuance time for each cohort.
We use those figures in our pre-approved plans study. Without the ordinance, none of it would be public.
For scale: what a state with a reporting duty produces
Not a comparison of policy effectiveness — the jurisdictions, populations and statutes are not comparable, and we are not claiming they are. It is a comparison of what is knowable when a state requires reporting and when it does not.
| Jurisdiction | ADU permits, most recent full year | Series available | Why |
|---|---|---|---|
| California (statewide) | 32,126 (2025) | Annual, by jurisdiction, by unit category, 2018–2025 | Every city and county must file a Housing Element Annual Progress Report with HCD. 511 jurisdictions reported ADU permits over the window. |
| Seattle (city) | 913 (2024) | Annual, 2005–2025, with attached/detached split from 2020 | Reporting mandated by city Ordinance 125854; the 2024 edition is the fourth annual report. |
| Washington (statewide) | Unmeasured | None located | RCW 36.70A.681 requires jurisdictions to allow two ADUs per lot inside urban growth areas. It does not require anyone to count them. |
| Oregon (statewide) | Unmeasured | None located | ORS 197A.425 has been in force since 2017. No statewide ADU permit series was found. |
| Montana, Maine, Colorado, Arizona | Unmeasured | None located | Broad enabling statutes, no reporting duty found in any of the four. |
What is publicly knowable about ADU production, by jurisdiction, on 2026-09-05. “Unmeasured” means we located no series; it is not a statement about the level of production.
HyreADU recommendation, labeled as one: the marginal cost of adding an accessory-unit category to an existing state permit or housing report is very small next to the cost of the enabling legislation itself, and it is the difference between a policy that can be evaluated and one that cannot.
We have no view on whether any particular state should legalize ADUs. We do have a view that a state which does should count them.
What we could not verify
We located no statewide ADU permit series for Oregon, Washington, Montana, Maine, Colorado or Arizona. Those states are reported as unmeasured throughout and never as zero.
For Oregon the only compilation we found was a July 2021 post on an individual’s website giving a cumulative Portland ADU permit figure and an annual run rate. It is five years old and names no data source, so it is not tabled as a measurement anywhere on this page.
We did not verify local implementation in any of the six states — that is, whether individual jurisdictions have amended their codes to comply with the state statute. The statutes are read as enacted.
The Mercatus Center count of 18 broadly legalizing states is Hamilton and Peterson’s, published August 2025. We have not independently verified the twelve states it covers that we did not retrieve ourselves, and we have not tabled them.
Questions
Which states have legalized ADUs statewide?
Did those laws actually produce ADUs?
How many ADUs does Seattle permit a year?
Does Washington require two ADUs per lot?
Why can you measure California but not Oregon?
Can the Census data fill the gap for the unmeasured states?
Does Seattle prove that removing parking and owner-occupancy requirements works?
What is the cheapest thing a state could do to make its ADU law evaluable?
Written and audited by
HyreADU Research Desk
Primary-source research, data analysis and fact checking
We are a research desk, not a builder. We read the permit extract, the statute, the HCD return or the fee schedule ourselves, and publish each figure with its source and retrieval date.
Where a number cannot be traced to a primary source, we leave it out and say what we could not verify. Our store-based claims cover California only.
- CA
- the only state this desk will make store-based claims about
- 5
- jurisdictions with extracted ADU permit evidence
- 735
- CSLB-verified companies in the California store
- 0
- national claims from a one-state store
How this desk works
- Primary sources only. Permit counts come from the city or county that issued the permit. Production counts come from HCD’s Annual Progress Report. Rents come from HUD or the Census. We do not cite an article that cites a source; we download the source and compute the figure ourselves.
- This is a California site. The company store is 734 California firms and one New Mexico firm. Permit evidence exists for five named jurisdictions: Los Angeles, San Francisco, Sacramento, San José and unincorporated Marin. A number from that store is titled to those places, never to the United States.
- A permit is not a completion, and a license is not an ADU grade. California licenses no ADU classification. Being named on an ADU permit is evidence of engagement in that jurisdiction, not of quality, completion, or work anywhere else. Owner-builder permits are excluded from contractor counts.
- Calculation is labeled as calculation. Figures we derive are never presented as something HCD, HUD, the Census or a city published. Terner Center research is cited as Terner’s, never restated as ours.
- We do not design, permit or build ADUs, and we take no payment for placement, ranking or a favorable mention. Pages that look like rankings are not: they publish public-record counts and let the reader decide.
- Nothing here is legal, tax or financial advice. Zoning, underwriting and appraisal practice vary by jurisdiction, lender and appraiser. The useful next step on a specific lot is the planning counter and a licensed professional.
Data as of State statutes retrieved from official legislature or code sites 2026-09-05; Seattle OPCD Accessory Dwelling Units 2024 Annual Report (October 2025) retrieved 2026-09-05; California HCD Annual Progress Report Table A2 extract, HCD file last updated 4 September 2026. Authorship on this site is organizational: the analysis belongs to the desk rather than to a named individual, and we do not publish credentials we do not hold.
Our editorial policy sets out how we source, date and correct what we publish.
Sources & retrieval dates
- City of Seattle Office of Planning and Community Development, “Accessory Dwelling Units 2024 Annual Report” , October 2025, the fourth annual report, produced under a reporting requirement in Ordinance 125854. Exhibit 2 supplies the full 2005–2025 permit series and the attached/detached split from 2020. Exhibit 3 gives the 2024 zone breakdown (NR2 178, NR3 623, RSL 82, LR 5, other 25). Exhibit 5 gives the development composition used in the mix chart. Exhibit 6 gives median lot sizes of 5,905 sq ft (attached) and 6,002 sq ft (detached). Exhibit 8 covers the pre-approved DADU plans permitted 199 times since 2020. Source of the quoted statements on the single-family ratio and the 32 per cent application decline. Retrieved 2026-09-05.
- Oregon Revised Statutes § 197A.425 — accessory dwelling units , Formerly ORS 197.312(5)–(6), enacted by SB 1051 (2017). Requires cities over 2,500 and counties over 15,000, inside an urban growth boundary and in areas zoned for detached single-family dwellings, to allow at least one ADU per detached single-family dwelling, subject to reasonable local regulations relating to siting and design — which the statute states do not include owner-occupancy or additional off-street parking. Retrieved 2026-09-05.
- Revised Code of Washington § 36.70A.681 (2023 c 334, HB 1337) , At least two ADUs per lot in all zoning districts inside urban growth areas; maximum gross floor area cap may not be below 1,000 sq ft; no owner-occupancy requirement; no parking requirement within half a mile of a major transit stop; ADU impact fees capped at 50 per cent of the principal unit’s. Retrieved 2026-09-05.
- Montana Code Annotated § 76-2-345 — accessory dwelling units, regulations, restrictions , Enacted by SB 528, “En. Sec. 1, Ch. 502, L. 2023”. At least one ADU by right on single-family lots, attached, detached or internal; size limited to 75 per cent of primary floor area or 1,000 sq ft, whichever is smaller; no additional parking, no design-match mandate, no occupancy restriction, no impact fees; application fee capped at $250. Retrieved 2026-09-05.
- Maine Revised Statutes, 30-A M.R.S. § 4364-B — accessory dwelling units , At least one ADU on any lot with a single-family principal structure in any area where residential uses are permitted, with a Lewiston and Auburn watershed carve-out; no additional parking; no owner-occupancy. History: PL 2021 c.672 § 6 (LD 2003); PL 2023 c.192 §§ 12–21; PL 2023 c.264 § 3; PL 2025 c.385 §§ 13–18, § 23. Retrieved 2026-09-05.
- Colorado Revised Statutes 29-35-101 to -105 (HB24-1152), enrolled bill text , § 29-35-102(21) defines a “subject jurisdiction” as a municipality of 1,000 or more inside an MPO, or the portion of a county inside a census designated place of 40,000 or more and inside an MPO. § 29-35-103(1): on or after 30 June 2025 a subject jurisdiction shall allow, subject to an administrative approval process, one ADU wherever single-unit detached dwellings are allowed. § 29-35-103(2)(a) and (3): no new off-street parking space, with exceptions. § 29-35-103(2)(b): no owner-occupancy requirement beyond an application-time residence demonstration. Retrieved 2026-09-05.
- Arizona Revised Statutes § 9-461.18 — accessory dwelling units; regulation; applicability; definitions , Applies to a municipality with a population of more than seventy-five thousand persons. At least one attached and one detached ADU, plus a second detached unit on lots of one acre or more where one is deed-restricted affordable. No required familial, marital or employment relationship; no additional parking. Size limited to 75 per cent of the single-family dwelling’s gross floor area or 1,000 sq ft, whichever is smaller. Self-executing default where the municipality failed to adopt regulations by 1 January 2025. Retrieved 2026-09-05.
- Emily Hamilton and Kol Peterson, “A Taxonomy of State Accessory Dwelling Unit Laws 2025”, Mercatus Center , August 2025. Cited as the authors’ own count and classification: 18 states with broad ADU legalization as of July 2025, 10 classified as strong and 8 as weaker. We have not independently verified the states in that count which we did not retrieve ourselves, and none of them is tabled on this page. Retrieved 2026-09-05.
- California Department of Housing and Community Development, Housing Element Annual Progress Report — Table A2 , Full Table A2 extract, HCD file last updated 4 September 2026. Supplies the California benchmark figures: 32,126 ADU permit units in 2025 and 178,495 for 2018–2025, deduplicated by tracking identifier across 511 reporting jurisdictions. Retrieved 2026-09-05.
- U.S. Census Bureau, Building Permits Survey — Methodology , Four structure categories with no accessory item; universe of about 19,900 permit-issuing places. Cited here for the absence of any national fallback series for the unmeasured states. Retrieved 2026-09-05.
A law nobody counts is a law nobody can defend
The measurement gap is the story on this page, and it is fixable with a single reporting clause. Our research index carries the rest of the desk’s work on what the public record does and does not support.
HyreADU does not design, permit or build accessory dwelling units, and does not practice law.
This page is informational and is not legal advice; it describes statutory text as retrieved on 5 September 2026 and does not tell you what is permitted on any particular lot.
Seattle’s findings are the City of Seattle’s and are cited as theirs; the Mercatus Center count is Hamilton and Peterson’s and is cited as theirs. No causal effect of any statute is estimated.
States for which no permit series was located are reported as unmeasured, which is not a statement about their level of ADU production. HyreADU’s own company and permit store covers California only and is used nowhere on this page.