Statistics
ADU statistics outside California
39 of 89 figures are HyreADU calculations. National and non-Californian sources only — no Californian figure is scaled, per-capita’d or extrapolated into a statistic about anywhere else on this page.
Written by HyreADU Research Desk Primary-source research and data analysis
Audited by HyreADU Research Desk Statutory citation and data-coverage audit
The short version
Outside California, ADU legalization is common and ADU measurement barely exists. Six state enabling statutes were retrieved directly from state legislatures or official code publishers — Oregon, Washington, Montana, Maine, Colorado and Arizona.
Not one of those six states publishes a statewide ADU permit series. The Mercatus Center counts 18 states with broad ADU legalization as of July 2025, 10 of them strong on its own classification; the measurement gap applies to those states too.
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 does exist, the effect of reform is large and dated. Seattle’s ADU reforms took effect in August 2019.
The post-reform annual mean is 5.16× the pre-reform mean, and 63.2% of every ADU Seattle permitted in twenty years was permitted in the last five.
The reform produced a level shift, not a compounding trend. Permits have sat at roughly 900 a year for three years, and Seattle reports ADU permit applications down 32 per cent year-over-year.
There is no national ADU count to place any of this against. The Census Building Permits Survey has four structure categories and accessory is not one of them; an ADU made by conversion records zero housing units.
The only national estimate with a published method is Freddie Mac’s 1.4 million properties whose MLS listing text matched accessory-unit phrases, which is informative about geography and silent about volume.
States we cannot measure are reported here as unmeasured, never as zero. 39 of 89 figures below are HyreADU calculations (39 hyreadu calculation, 45 agency published, 5 trade / survey estimate).
What this page refuses to do
HyreADU is California-heavy and says so everywhere. That makes a page about the rest of the country the one where the temptations are largest, so the refusals are stated before the figures.
- No Californian figure is scaled into a statistic about anywhere else
The most common way a national ADU number gets manufactured is by taking California’s permit series and adjusting it for population. That assumes the rest of the country has California’s statute, its lot geometry, its rents and its construction market.
It does not. Nothing on this page rests on Californian data, and HyreADU’s own company and permit store — which is a California store — is used nowhere in this file.
- Unmeasured is not zero
Every state below where we could not find a permit series is reported as unmeasured.
Montana, Maine, Colorado and Arizona have enabling statutes and no published count; that means we do not know how many ADUs they are producing, not that they are producing none.
Printing a blank as a zero is the single easiest way to turn a data gap into a false finding.
- Seattle is Seattle
One city with a specific housing market, a specific reform package and, crucially, a specific reporting duty. Its series is presented because it is the best available evidence on the question, not because it stands in for Portland, Bozeman, Denver or Phoenix. Nothing here is scaled from Seattle either.
- A time series with a marker on it is evidence, not identification
Seattle’s permits rose sharply after August 2019 and this page describes the association and the dates.
No difference-in-differences, no synthetic control and no counterfactual was run, and no attempt was made to separate the reform from the pandemic construction cycle, the interest-rate path, or Seattle’s wider middle-housing changes.
There are no comparison jurisdictions with ADU series to build a design from — which is itself the finding.
- Statutory text was read, not summarized from a secondary source
Each of the six statutes below was retrieved from the state’s own legislature or official code publisher on 2026-09-05. States whose text this desk could not retrieve are not listed at all rather than being characterized from a third party. The ADU laws by state logs the retrieval failures by name.
Six states: what the statute requires, and whether anyone counts
Every provision below was read from the state’s own source. The last column is the finding of this page in one word repeated six times. The enabling law has spread; the measurement has not, and the two are not connected by anything except a policy choice.
| 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 owner-occupancy nor additional off-street parking may be imposed. | None located. |
| 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 statewide. Seattle publishes its own under a city ordinance. |
| Montana | MCA 76-2-345 (SB 528, 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. Minimum size a municipality must permit: 190 sq ft. | 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 under an administrative process wherever single-unit detached dwellings are allowed. | 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 too recent for a meaningful series to exist. |
| 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 principal dwelling’s gross floor area or 1,000 sq ft, whichever is smaller. | No required familial, marital or employment relationship; no additional parking. Self-executing where a municipality failed to adopt regulations by 1 January 2025. | None located. Unmeasured. |
Six state ADU statutes retrieved 2026-09-05. A state appears here only if this desk retrieved its statutory text directly. All six are agency published figures — the statutes are the source. The count of six, and the count of 0 publishing a series, are HyreADU calculations.
For a broader statutory landscape, Hamilton and 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 — 44.4% of their own list.
That is their count and their classification, cited as theirs; this desk has not verified the other twelve states and has not tabled them. See also the model-ordinance page.
Seattle: twenty years of ADU permits
The one long, auditable series available outside California. It exists because Seattle Ordinance 125854 requires an annual report — which is why a twenty-year ADU series exists for Seattle and does not exist for any of the six states above.
| Year | ADUs permitted | Attached (AADU) | Detached (DADU) | Detached share | Relative to 2019 | Note |
|---|---|---|---|---|---|---|
| 2015 | 190 | — | — | — | 0.67× | Pre-reform |
| 2016 | 274 | — | — | — | 0.97× | Pre-reform |
| 2017 | 252 | — | — | — | 0.89× | Pre-reform |
| 2018 | 245 | — | — | — | 0.87× | Pre-reform |
| 2019 | 283 | — | — | — | — | Reforms take effect August 2019 |
| 2020 | 476 | 230 | 246 | 51.7% | 1.68× | First full post-reform year |
| 2021 | 762 | 338 | 424 | 55.6% | 2.69× | |
| 2022 | 924 | 407 | 517 | 56.0% | 3.27× | Series peak |
| 2023 | 905 | 447 | 458 | 50.6% | 3.20× | |
| 2024 | 913 | 448 | 465 | 50.9% | 3.23× | Third consecutive year above 900 |
| 2025 (part year) | 507 | 247 | 260 | 51.3% | 1.79× | Through Q2 only. Not annualised. |
Permit counts and the attached/detached split are agency-published (Seattle OPCD, “Accessory Dwelling Units 2024 Annual Report”, October 2025, Exhibit 2, retrieved 2026-09-05). The detached-share and relative-to-2019 columns are HyreADU calculations. The 2005–2014 values, not tabled for length, are 64, 85, 103, 98, 76, 144, 107, 115, 125, 155 and are drawn in the chart.
The reform package took effect in August 2019, so 2019 is a split year and 2020 is the first clean post-reform observation. 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.
The shape of the Seattle change, as arithmetic
Every figure in this table is ours, computed from Seattle’s published annual series and nothing else. Together they say something more specific than "ADU permits went up": the series took a step and then flattened.
| Measure | Figure | What it says | Class |
|---|---|---|---|
| Mean annual permits, 2005–2019 | 154.4 | Fifteen pre-reform years, never above 283. | HyreADU calculation |
| Mean annual permits, 2020–2024 | 796.0 | Five post-reform years, never below 476. | HyreADU calculation |
| Post-reform mean ÷ pre-reform mean | 5.16× | The size of the level shift. | HyreADU calculation |
| Ratio of highest to lowest pre-reform year | 4.42× | The pre-reform series was not flat — it had its own range. | HyreADU calculation |
| Compound annual growth, 2019 to 2024 | 26.4% | Impressive, and dominated by the first two years. | HyreADU calculation |
| Compound annual growth, 2022 to 2024 | -0.6% | The step is over. This is the plateau. | HyreADU calculation |
| Change from the 2022 peak to 2024 | -1.2% | Three years within about two per cent of each other. | HyreADU calculation |
| Share of the 2005–2024 total permitted since 2020 | 63.2% | 3,980 of 6,296 permits in five of twenty years. | HyreADU calculation |
| Part-year 2025 against full-year 2024 | 55.5% | Through Q2 only. Deliberately not annualised. | HyreADU calculation |
HyreADU calculations on Seattle’s published annual series, Seattle OPCD, “Accessory Dwelling Units 2024 Annual Report”, October 2025, Exhibit 2, retrieved 2026-09-05.
Nothing is interpolated and the part year is never scaled to a full year — Seattle reports ADU permit applications down 32 per cent year-over-year, so the second half of 2025 cannot be assumed to look like the first.
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.
What the Seattle mix looks like on the ground
The composition finding is the one that makes Seattle genuinely interesting, and it is the city’s, not ours. In its neighborhood residential zones in 2024, the ADU stopped being an add-on.
| Measure | Figure | Class | Source |
|---|---|---|---|
| Projects combining an ADU with a single-family house | 60% | Agency published | Seattle OPCD, same report, Exhibit 5, retrieved 2026-09-05 |
| ADU-only projects on an existing lot | 34% | Agency published | Seattle OPCD, same report, Exhibit 5, retrieved 2026-09-05 |
| Single-family house alone | 10% | Agency published | Seattle OPCD, same report, Exhibit 5, retrieved 2026-09-05 |
| Detached share of ADUs permitted, 2020 | 51.7% | HyreADU calculation | Seattle OPCD, “Accessory Dwelling Units 2024 Annual Report”, October 2025, Exhibit 2, retrieved 2026-09-05 · growth-outside-California study |
| Detached share of ADUs permitted, 2022 | 56.0% | HyreADU calculation | Seattle OPCD, “Accessory Dwelling Units 2024 Annual Report”, October 2025, Exhibit 2, retrieved 2026-09-05 · growth-outside-California study |
| Detached share of ADUs permitted, 2024 | 50.9% | HyreADU calculation | Seattle OPCD, “Accessory Dwelling Units 2024 Annual Report”, October 2025, Exhibit 2, retrieved 2026-09-05 · growth-outside-California study |
| Pre-approved detached ADU plans permitted since 2020 | 199 | Agency published | Seattle OPCD, “Accessory Dwelling Units 2024 Annual Report”, October 2025, Exhibit 2, retrieved 2026-09-05 |
| Reported year-over-year change in ADU applications | −32% | Agency published | Seattle OPCD, “Accessory Dwelling Units 2024 Annual Report”, October 2025, Exhibit 2, retrieved 2026-09-05 |
Composition shares from Exhibit 5 of the Seattle OPCD report, retrieved 2026-09-05.
Note that the three shares sum to more than 100 per cent as published — they are drawn from the report’s own exhibit and have not been rescaled, because rescaling somebody else’s published shares to make them tidy is a way of quietly inventing data.
Seattle summarizes the same shift in its own words: “In 2019, two single-family homes were built for every ADU. Now, the inverse is true, with ADUs outpacing single-family construction two-to-one.” The pre-approved-plan figure is a good illustration of what a reporting ordinance buys: Seattle can say its pre-approved detached plans have been permitted 199 times since 2020 and publish intake-to-issuance times for each cohort. See the pre-approved-plans study.
Why there is no national number to put Seattle against
The federal instrument does not collect the category
The Census Bureau’s Building Permits Survey classifies residential construction into four structure types — single-family houses, two-unit buildings, three- and four-unit buildings, and buildings with five or more units. Accessory is not among them, and no item on the survey form asks whether a unit is accessory to another.
The signal is then lost twice, in opposite directions. A garage, basement or interior conversion is an alteration under Item 434, which records permits and valuation and carries no housing-unit count — that unit never enters the national units series at all.
A detached new-build ADU does enter it, as a one-unit building, in the same cell as a large single-family house.
The errors do not cancel: conversions are a pure undercount and detached new-builds are an unattributed overcount inside a category used as a proxy for single-family production.
A national ADU figure needs a change to the instrument — an accessory flag on the permit record, and a unit count on alteration permits that create a dwelling — not a cleverer analysis of the published output. The national-permit-data study sets out the mechanics.
The one national estimate is not a count
In July 2020 Freddie Mac text-mined a licensed dataset of roughly 600 million MLS transactions dating to the late 1990s and reported that “a total of 1.4 million distinct single-family properties with accessory dwellings were identified in our final population”.
That is a count of properties whose listing text matched accessory-unit phrases at some point since the 1990s — not a count of accessory units standing today.
Freddie Mac names both understatement bias (owners who never list, transactions outside the MLS, descriptions that do not use a matching phrase) and overstatement bias (an older unit reaching the MLS for the first time), and makes no adjustment for demolition or permanent loss.
Its geography is nonetheless the most useful national signal available. Its top ten states are California, Florida, Texas, Georgia, Arizona, Massachusetts, Washington, New York, North Carolina, Ohio — 9 of the ten outside California, spanning the Sun Belt, the Northeast and the Midwest.
That says ADU stock is not a Californian phenomenon. It says nothing reliable about how many, or about how many were built recently. The how-many-ADUs study treats it as the top of an evidence ladder rather than an answer.
Two different national blindnesses: a permit survey that cannot see the flow, and a listings estimate that gestures at the stock without measuring it.
The policy finding hiding inside the data gap
Read the statute table and the Seattle series together and a specific conclusion falls out, and it is not about ADUs at all. A state that passes an ADU statute without also requiring anybody to count the result has made its own policy unevaluable.
Six states have real, enforceable ADU mandates. Some of them are stronger on paper than California’s: Washington requires two units per lot inside urban growth areas, where California requires one; Montana caps the application fee at $250 outright.
Not one of the six can tell a legislator, a journalist or a housing agency how many units the statute has produced.
Seattle can, because a separate city ordinance requires an annual report. That ordinance is why this page has a twenty-year series for one city and nothing for four states.
It is also why Seattle can say something no state can — that its pre-approved plans have been used 199 times, that applications are down 32 per cent year-over-year, that ADU-only projects are 34 per cent of new residential development in its neighborhood residential zones.
The cost of the reporting duty is trivial next to the cost of the statute.
The value is the difference between a policy that can be improved and one that can only be argued about.
If there is one recommendation this desk would make to a state legislature drafting an ADU bill, it is to add the counting clause — and to make the resulting series machine-readable, dated and public, so that a page like this one does not have to say "unmeasured" four times in a six-row table.
Every figure on this page, with its class
89 figures with geography, date, provenance class and source. 39 HyreADU calculations, 45 agency-published, 5 trade or survey. Every geography in this table is outside California or national — there is no Californian figure in it.
| Statistic | Figure | Geography | Date | Class | Source |
|---|---|---|---|---|---|
| ADUs permitted in Seattle, 2005 | 64 | City of Seattle | 2005 | Agency published | Seattle OPCD, “Accessory Dwelling Units 2024 Annual Report”, October 2025, Exhibit 2, retrieved 2026-09-05 |
| ADUs permitted in Seattle, 2006 | 85 | City of Seattle | 2006 | Agency published | Seattle OPCD, “Accessory Dwelling Units 2024 Annual Report”, October 2025, Exhibit 2, retrieved 2026-09-05 |
| ADUs permitted in Seattle, 2007 | 103 | City of Seattle | 2007 | Agency published | Seattle OPCD, “Accessory Dwelling Units 2024 Annual Report”, October 2025, Exhibit 2, retrieved 2026-09-05 |
| ADUs permitted in Seattle, 2008 | 98 | City of Seattle | 2008 | Agency published | Seattle OPCD, “Accessory Dwelling Units 2024 Annual Report”, October 2025, Exhibit 2, retrieved 2026-09-05 |
| ADUs permitted in Seattle, 2009 | 76 | City of Seattle | 2009 | Agency published | Seattle OPCD, “Accessory Dwelling Units 2024 Annual Report”, October 2025, Exhibit 2, retrieved 2026-09-05 |
| ADUs permitted in Seattle, 2010 | 144 | City of Seattle | 2010 | Agency published | Seattle OPCD, “Accessory Dwelling Units 2024 Annual Report”, October 2025, Exhibit 2, retrieved 2026-09-05 |
| ADUs permitted in Seattle, 2011 | 107 | City of Seattle | 2011 | Agency published | Seattle OPCD, “Accessory Dwelling Units 2024 Annual Report”, October 2025, Exhibit 2, retrieved 2026-09-05 |
| ADUs permitted in Seattle, 2012 | 115 | City of Seattle | 2012 | Agency published | Seattle OPCD, “Accessory Dwelling Units 2024 Annual Report”, October 2025, Exhibit 2, retrieved 2026-09-05 |
| ADUs permitted in Seattle, 2013 | 125 | City of Seattle | 2013 | Agency published | Seattle OPCD, “Accessory Dwelling Units 2024 Annual Report”, October 2025, Exhibit 2, retrieved 2026-09-05 |
| ADUs permitted in Seattle, 2014 | 155 | City of Seattle | 2014 | Agency published | Seattle OPCD, “Accessory Dwelling Units 2024 Annual Report”, October 2025, Exhibit 2, retrieved 2026-09-05 |
| ADUs permitted in Seattle, 2015 | 190 | City of Seattle | 2015 | Agency published | Seattle OPCD, “Accessory Dwelling Units 2024 Annual Report”, October 2025, Exhibit 2, retrieved 2026-09-05 |
| ADUs permitted in Seattle, 2016 | 274 | City of Seattle | 2016 | Agency published | Seattle OPCD, “Accessory Dwelling Units 2024 Annual Report”, October 2025, Exhibit 2, retrieved 2026-09-05 |
| ADUs permitted in Seattle, 2017 | 252 | City of Seattle | 2017 | Agency published | Seattle OPCD, “Accessory Dwelling Units 2024 Annual Report”, October 2025, Exhibit 2, retrieved 2026-09-05 |
| ADUs permitted in Seattle, 2018 | 245 | City of Seattle | 2018 | Agency published | Seattle OPCD, “Accessory Dwelling Units 2024 Annual Report”, October 2025, Exhibit 2, retrieved 2026-09-05 |
| ADUs permitted in Seattle, 2019 | 283 | City of Seattle | 2019 | Agency published | Seattle OPCD, “Accessory Dwelling Units 2024 Annual Report”, October 2025, Exhibit 2, retrieved 2026-09-05 |
| ADUs permitted in Seattle, 2020 | 476 | City of Seattle | 2020 | Agency published | Seattle OPCD, “Accessory Dwelling Units 2024 Annual Report”, October 2025, Exhibit 2, retrieved 2026-09-05 |
| ADUs permitted in Seattle, 2021 | 762 | City of Seattle | 2021 | Agency published | Seattle OPCD, “Accessory Dwelling Units 2024 Annual Report”, October 2025, Exhibit 2, retrieved 2026-09-05 |
| ADUs permitted in Seattle, 2022 | 924 | City of Seattle | 2022 | Agency published | Seattle OPCD, “Accessory Dwelling Units 2024 Annual Report”, October 2025, Exhibit 2, retrieved 2026-09-05 |
| ADUs permitted in Seattle, 2023 | 905 | City of Seattle | 2023 | Agency published | Seattle OPCD, “Accessory Dwelling Units 2024 Annual Report”, October 2025, Exhibit 2, retrieved 2026-09-05 |
| ADUs permitted in Seattle, 2024 | 913 | City of Seattle | 2024 | Agency published | Seattle OPCD, “Accessory Dwelling Units 2024 Annual Report”, October 2025, Exhibit 2, retrieved 2026-09-05 |
| ADUs permitted in Seattle, 2025 (part year, through Q2) | 507 | City of Seattle | 2025 | Agency published | Seattle OPCD, “Accessory Dwelling Units 2024 Annual Report”, October 2025, Exhibit 2, retrieved 2026-09-05 |
| Attached / detached split, 2020 | 230 AADU / 246 DADU | City of Seattle | 2020 | Agency published | Seattle OPCD, “Accessory Dwelling Units 2024 Annual Report”, October 2025, Exhibit 2, retrieved 2026-09-05 |
| Attached / detached split, 2021 | 338 AADU / 424 DADU | City of Seattle | 2021 | Agency published | Seattle OPCD, “Accessory Dwelling Units 2024 Annual Report”, October 2025, Exhibit 2, retrieved 2026-09-05 |
| Attached / detached split, 2022 | 407 AADU / 517 DADU | City of Seattle | 2022 | Agency published | Seattle OPCD, “Accessory Dwelling Units 2024 Annual Report”, October 2025, Exhibit 2, retrieved 2026-09-05 |
| Attached / detached split, 2023 | 447 AADU / 458 DADU | City of Seattle | 2023 | Agency published | Seattle OPCD, “Accessory Dwelling Units 2024 Annual Report”, October 2025, Exhibit 2, retrieved 2026-09-05 |
| Attached / detached split, 2024 | 448 AADU / 465 DADU | City of Seattle | 2024 | Agency published | Seattle OPCD, “Accessory Dwelling Units 2024 Annual Report”, October 2025, Exhibit 2, retrieved 2026-09-05 |
| Attached / detached split, 2025 (part year) | 247 AADU / 260 DADU | City of Seattle | 2025 | Agency published | Seattle OPCD, “Accessory Dwelling Units 2024 Annual Report”, October 2025, Exhibit 2, retrieved 2026-09-05 |
| Projects combining an ADU with a single-family house, neighborhood residential zones | 60% | City of Seattle | 2024 | Agency published | Seattle OPCD, same report, Exhibit 5, retrieved 2026-09-05 |
| ADU-only projects in the same zones | 34% | City of Seattle | 2024 | Agency published | Seattle OPCD, same report, Exhibit 5, retrieved 2026-09-05 |
| Single-family house alone in the same zones | 10% | City of Seattle | 2024 | Agency published | Seattle OPCD, same report, Exhibit 5, retrieved 2026-09-05 |
| Reported year-over-year change in ADU permit applications | −32% | City of Seattle | 2025 report | Agency published | Seattle OPCD, “Accessory Dwelling Units 2024 Annual Report”, October 2025, Exhibit 2, retrieved 2026-09-05 |
| Pre-approved detached ADU plans permitted since 2020 | 199 | City of Seattle | 2020–2025 | Agency published | Seattle OPCD, “Accessory Dwelling Units 2024 Annual Report”, October 2025, Exhibit 2, retrieved 2026-09-05 |
| ADU enabling statute retrieved | ORS 197A.425 (formerly ORS 197.312(5)–(6); SB 1051, 2017) | Oregon | retrieved 2026-09-05 | Agency published | Oregon legislature or official code publisher |
| ADU enabling statute retrieved | RCW 36.70A.681 (2023 c 334, HB 1337) | Washington | retrieved 2026-09-05 | Agency published | Washington legislature or official code publisher |
| ADU enabling statute retrieved | MCA 76-2-345 (SB 528, Ch. 502, L. 2023) | Montana | retrieved 2026-09-05 | Agency published | Montana legislature or official code publisher |
| ADU enabling statute retrieved | 30-A M.R.S. § 4364-B (PL 2021 c.672 [LD 2003], amended through PL 2025 c.385) | Maine | retrieved 2026-09-05 | Agency published | Maine legislature or official code publisher |
| ADU enabling statute retrieved | C.R.S. 29-35-101 to -105 (HB24-1152) | Colorado | retrieved 2026-09-05 | Agency published | Colorado legislature or official code publisher |
| ADU enabling statute retrieved | A.R.S. § 9-461.18 | Arizona | retrieved 2026-09-05 | Agency published | Arizona legislature or official code publisher |
| ADUs a Washington city must allow per lot inside an urban growth area | 2 | Washington | RCW 36.70A.681 | Agency published | RCW 36.70A.681 (2023 c 334, HB 1337) |
| Smallest ADU a Maine municipality must permit | 190 sq ft | Maine | 30-A M.R.S. § 4364-B | Agency published | 30-A M.R.S. § 4364-B, amended through PL 2025 c.385 |
| Arizona municipal population threshold for the ADU duty | 75,000 | Arizona | A.R.S. § 9-461.18 | Agency published | A.R.S. § 9-461.18 |
| Montana cap on the ADU application fee | $250 | Montana | MCA 76-2-345 | Agency published | MCA 76-2-345 (SB 528, 2023) |
| Date the Colorado ADU duty took effect | 30 June 2025 | Colorado | C.R.S. 29-35-101 to -105 | Agency published | C.R.S. 29-35-101 to -105 (HB24-1152) |
| Structure categories in the federal permit survey, none of them accessory | 4 | United States | current methodology | Agency published | U.S. Census Bureau, Building Permits Survey methodology |
| Housing units the federal survey records for a conversion ADU | 0 | United States | current methodology | Agency published | Census BPS Item 434 carries no housing-unit field |
| States with broad ADU legalization, by Mercatus’s count | 18 | United States | July 2025 | Trade / survey estimate | Hamilton and Peterson, “A Taxonomy of State ADU Laws 2025”, Mercatus Center, July 2025 |
| Of those, classified strong by Mercatus | 10 | United States | July 2025 | Trade / survey estimate | Hamilton and Peterson, “A Taxonomy of State ADU Laws 2025”, Mercatus Center, July 2025 |
| Of those, classified weaker by Mercatus | 8 | United States | July 2025 | Trade / survey estimate | Hamilton and Peterson, “A Taxonomy of State ADU Laws 2025”, Mercatus Center, July 2025 |
| Properties whose MLS text mentioned an accessory unit since the late 1990s | 1.4 million | United States | July 2020 | Trade / survey estimate | Freddie Mac, Economic & Housing Research Insight, July 2020 |
| Top ten states by identified ADU properties | California, Florida, Texas, Georgia, Arizona, Massachusetts, Washington, New York, North Carolina, Ohio | United States | July 2020 | Trade / survey estimate | Freddie Mac, Economic & Housing Research Insight, July 2020 |
| Seattle mean annual ADU permits, 2005–2019 (pre-reform) | 154.4 | City of Seattle | 2005–2019 | HyreADU calculation | Seattle OPCD, “Accessory Dwelling Units 2024 Annual Report”, October 2025, Exhibit 2, retrieved 2026-09-05 · growth-outside-California study |
| Seattle mean annual ADU permits, 2020–2024 (post-reform) | 796.0 | City of Seattle | 2020–2024 | HyreADU calculation | Seattle OPCD, “Accessory Dwelling Units 2024 Annual Report”, October 2025, Exhibit 2, retrieved 2026-09-05 · growth-outside-California study |
| Post-reform mean relative to pre-reform mean | 5.16× | City of Seattle | 2020–2024 against 2005–2019 | HyreADU calculation | Seattle OPCD, “Accessory Dwelling Units 2024 Annual Report”, October 2025, Exhibit 2, retrieved 2026-09-05 · growth-outside-California study |
| 2024 permits relative to 2019 | 3.23× | City of Seattle | 2024 against 2019 | HyreADU calculation | Seattle OPCD, “Accessory Dwelling Units 2024 Annual Report”, October 2025, Exhibit 2, retrieved 2026-09-05 · growth-outside-California study |
| 2020 permits relative to 2019 | 1.68× | City of Seattle | 2020 against 2019 | HyreADU calculation | Seattle OPCD, “Accessory Dwelling Units 2024 Annual Report”, October 2025, Exhibit 2, retrieved 2026-09-05 · growth-outside-California study |
| 2021 permits relative to 2019 | 2.69× | City of Seattle | 2021 against 2019 | HyreADU calculation | Seattle OPCD, “Accessory Dwelling Units 2024 Annual Report”, October 2025, Exhibit 2, retrieved 2026-09-05 · growth-outside-California study |
| 2022 permits relative to 2019 | 3.27× | City of Seattle | 2022 against 2019 | HyreADU calculation | Seattle OPCD, “Accessory Dwelling Units 2024 Annual Report”, October 2025, Exhibit 2, retrieved 2026-09-05 · growth-outside-California study |
| 2023 permits relative to 2019 | 3.20× | City of Seattle | 2023 against 2019 | HyreADU calculation | Seattle OPCD, “Accessory Dwelling Units 2024 Annual Report”, October 2025, Exhibit 2, retrieved 2026-09-05 · growth-outside-California study |
| Share of all 2005–2024 Seattle ADU permits issued since 2020 | 63.2% | City of Seattle | 2005–2024 | HyreADU calculation | Seattle OPCD, “Accessory Dwelling Units 2024 Annual Report”, October 2025, Exhibit 2, retrieved 2026-09-05 · growth-outside-California study |
| Total Seattle ADU permits, 2005–2024 | 6,296 | City of Seattle | 2005–2024 | HyreADU calculation | Seattle OPCD, “Accessory Dwelling Units 2024 Annual Report”, October 2025, Exhibit 2, retrieved 2026-09-05 · growth-outside-California study |
| Total Seattle ADU permits, 2020–2024 | 3,980 | City of Seattle | 2020–2024 | HyreADU calculation | Seattle OPCD, “Accessory Dwelling Units 2024 Annual Report”, October 2025, Exhibit 2, retrieved 2026-09-05 · growth-outside-California study |
| Ratio of highest to lowest pre-reform year | 4.42× | City of Seattle | 2005–2019 | HyreADU calculation | Seattle OPCD, “Accessory Dwelling Units 2024 Annual Report”, October 2025, Exhibit 2, retrieved 2026-09-05 · growth-outside-California study |
| Series peak year and value | 2022 (924) | City of Seattle | 2005–2024 | HyreADU calculation | Seattle OPCD, “Accessory Dwelling Units 2024 Annual Report”, October 2025, Exhibit 2, retrieved 2026-09-05 · growth-outside-California study |
| Change from the 2022 peak to 2024 | -1.2% | City of Seattle | 2022→2024 | HyreADU calculation | Seattle OPCD, “Accessory Dwelling Units 2024 Annual Report”, October 2025, Exhibit 2, retrieved 2026-09-05 · growth-outside-California study |
| Consecutive years above 900 permits | 3 | City of Seattle | 2022–2024 | HyreADU calculation | Seattle OPCD, “Accessory Dwelling Units 2024 Annual Report”, October 2025, Exhibit 2, retrieved 2026-09-05 · growth-outside-California study |
| Compound annual growth in permits, 2019 to 2024 | 26.4% | City of Seattle | 2019–2024 | HyreADU calculation | Seattle OPCD, “Accessory Dwelling Units 2024 Annual Report”, October 2025, Exhibit 2, retrieved 2026-09-05 · growth-outside-California study |
| Compound annual growth in permits, 2022 to 2024 | -0.6% | City of Seattle | 2022–2024 | HyreADU calculation | Seattle OPCD, “Accessory Dwelling Units 2024 Annual Report”, October 2025, Exhibit 2, retrieved 2026-09-05 · growth-outside-California study |
| Detached share of ADUs permitted, 2020 | 51.7% | City of Seattle | 2020 | HyreADU calculation | Seattle OPCD, “Accessory Dwelling Units 2024 Annual Report”, October 2025, Exhibit 2, retrieved 2026-09-05 · growth-outside-California study |
| Detached share of ADUs permitted, 2021 | 55.6% | City of Seattle | 2021 | HyreADU calculation | Seattle OPCD, “Accessory Dwelling Units 2024 Annual Report”, October 2025, Exhibit 2, retrieved 2026-09-05 · growth-outside-California study |
| Detached share of ADUs permitted, 2022 | 56.0% | City of Seattle | 2022 | HyreADU calculation | Seattle OPCD, “Accessory Dwelling Units 2024 Annual Report”, October 2025, Exhibit 2, retrieved 2026-09-05 · growth-outside-California study |
| Detached share of ADUs permitted, 2023 | 50.6% | City of Seattle | 2023 | HyreADU calculation | Seattle OPCD, “Accessory Dwelling Units 2024 Annual Report”, October 2025, Exhibit 2, retrieved 2026-09-05 · growth-outside-California study |
| Detached share of ADUs permitted, 2024 | 50.9% | City of Seattle | 2024 | HyreADU calculation | Seattle OPCD, “Accessory Dwelling Units 2024 Annual Report”, October 2025, Exhibit 2, retrieved 2026-09-05 · growth-outside-California study |
| Detached share of ADUs permitted, 2025 (part year) | 51.3% | City of Seattle | 2025 | HyreADU calculation | Seattle OPCD, “Accessory Dwelling Units 2024 Annual Report”, October 2025, Exhibit 2, retrieved 2026-09-05 · growth-outside-California study |
| Part-year 2025 permits as a share of full-year 2024 — not annualised | 55.5% | City of Seattle | H1 2025 against full-year 2024 | HyreADU calculation | Seattle OPCD, “Accessory Dwelling Units 2024 Annual Report”, October 2025, Exhibit 2, retrieved 2026-09-05 · deliberately not scaled to a full year |
| State statutes retrieved directly | 6 | United States | retrieved 2026-09-05 | HyreADU calculation | Six state legislature or code sites · growth-outside-California study |
| Of those, publishing a statewide ADU permit series | 0 | United States | retrieved 2026-09-05 | HyreADU calculation | Searched each state’s housing or planning agency · growth-outside-California study |
| Share of Mercatus’s legalizing states classified weaker by them | 44.4% | United States | July 2025 | HyreADU calculation | Arithmetic on Hamilton and Peterson, “A Taxonomy of State ADU Laws 2025”, Mercatus Center, July 2025 |
| Freddie Mac top-ten ADU states lying outside California | 9 of 10 | United States | July 2020 | HyreADU calculation | Count on Freddie Mac, Economic & Housing Research Insight, July 2020 |
| Jurisdictions outside California publishing a long, auditable ADU permit series | 1 | United States | retrieved 2026-09-05 | HyreADU calculation | growth-outside-California study — and it is a city, not a state |
| Full years in the Seattle series | 20 | City of Seattle | 2005–2024 | HyreADU calculation | Seattle OPCD, “Accessory Dwelling Units 2024 Annual Report”, October 2025, Exhibit 2, retrieved 2026-09-05 · growth-outside-California study |
| Mean annual permits, 2022–2024 (the plateau) | 914.0 | City of Seattle | 2022–2024 | HyreADU calculation | Seattle OPCD, “Accessory Dwelling Units 2024 Annual Report”, October 2025, Exhibit 2, retrieved 2026-09-05 · growth-outside-California study |
| Increase in permits from 2019 to 2020, in units | +193 | City of Seattle | 2019→2020 | HyreADU calculation | Seattle OPCD, “Accessory Dwelling Units 2024 Annual Report”, October 2025, Exhibit 2, retrieved 2026-09-05 · growth-outside-California study |
| Increase in permits from 2019 to 2020, in per cent | +68.2% | City of Seattle | 2019→2020 | HyreADU calculation | Seattle OPCD, “Accessory Dwelling Units 2024 Annual Report”, October 2025, Exhibit 2, retrieved 2026-09-05 · growth-outside-California study |
| Highest pre-reform year and value | 2019 (283) | City of Seattle | 2005–2019 | HyreADU calculation | Seattle OPCD, “Accessory Dwelling Units 2024 Annual Report”, October 2025, Exhibit 2, retrieved 2026-09-05 · growth-outside-California study |
| Lowest post-reform year and value | 2020 (476) | City of Seattle | 2020–2024 | HyreADU calculation | Seattle OPCD, “Accessory Dwelling Units 2024 Annual Report”, October 2025, Exhibit 2, retrieved 2026-09-05 · growth-outside-California study |
| Attached ADUs permitted, 2020–2024 | 1,870 | City of Seattle | 2020–2024 | HyreADU calculation | Seattle OPCD, “Accessory Dwelling Units 2024 Annual Report”, October 2025, Exhibit 2, retrieved 2026-09-05 · growth-outside-California study |
| Detached ADUs permitted, 2020–2024 | 2,110 | City of Seattle | 2020–2024 | HyreADU calculation | Seattle OPCD, “Accessory Dwelling Units 2024 Annual Report”, October 2025, Exhibit 2, retrieved 2026-09-05 · growth-outside-California study |
| Detached share of all ADUs permitted, 2020–2024 | 53.0% | City of Seattle | 2020–2024 | HyreADU calculation | Seattle OPCD, “Accessory Dwelling Units 2024 Annual Report”, October 2025, Exhibit 2, retrieved 2026-09-05 · growth-outside-California study |
| Attached share of ADUs permitted, 2024 | 49.1% | City of Seattle | 2024 | HyreADU calculation | Seattle OPCD, “Accessory Dwelling Units 2024 Annual Report”, October 2025, Exhibit 2, retrieved 2026-09-05 · growth-outside-California study |
HyreADU calculation — arithmetic by HyreADU on somebody else’s published series or count; method on the growth-outside-California study. Agency published — printed by Seattle OPCD, a state legislature or the Census Bureau. Trade / survey estimate — Mercatus’s taxonomy and Freddie Mac’s MLS estimate, cited as theirs. Statutes and the Seattle report retrieved 2026-09-05.
Non-California ADU claims we will not repeat
- A per-state ADU permit count for any state other than California
No state other than California publishes one, as far as this desk has been able to establish, and the federal permit survey has no accessory category to build one from.
Any table of ADU permits by state is either extrapolated, sourced to a single city, or built on a listings estimate that is not a count. We publish "unmeasured" instead, which is less useful and true.
- A national ADU total scaled from California
California’s permit series exists because California requires annual reporting. Scaling it by population assumes forty-nine other states have California’s statute, lot geometry, rents and construction market.
They do not: Colorado’s duty began on 30 June 2025, Arizona’s applies only to municipalities over 75,000, and Washington’s requires two units per lot rather than one. There is no defensible multiplier.
- Seattle’s reform effect presented as what a state will get
Seattle permitted 3.23× as many ADUs in 2024 as in 2019 after an August 2019 reform package.
That is one city, with several policies changing at once, running through the pandemic construction cycle and a large interest-rate move, and with no comparison jurisdiction available.
It is the best evidence there is on the question and it is not a forecast for Montana.
- A part-year figure annualised
Seattle’s 2025 figure of 507 covers Q1 and Q2 only. Doubling it would give a number above the 2024 total, and Seattle reports applications down 32 per cent year-over-year in the same document — so the second half cannot be assumed to resemble the first.
The part-year figure appears here only against the full-year 2024 total (55.5%), explicitly not annualised.
Citing these figures
Cite the growth-outside-California study rather than this digest where you can — the statutory quotations, the retrieval log and the identification caveats live there.
For the Seattle series, cite the City of Seattle Office of Planning and Community Development, "Accessory Dwelling Units 2024 Annual Report", October 2025. For the derived ratios and shares, attribute them as "HyreADU calculation on Seattle OPCD Exhibit 2".
For statutory provisions, cite the statute. For the 18-state count and its strong/weak split, cite Hamilton and Peterson at the Mercatus Center.
For the 1.4 million figure, cite Freddie Mac and keep the sentence "properties identified from MLS listing text since the late 1990s, not a stock count" attached to it — detaching that sentence turns a careful estimate into a false one.
Corrections go on the page with a dated note: hello@hyreadu.com. If you know of a state or city outside California that publishes an auditable ADU permit series, we would genuinely like to hear about it, and we will add it.
Questions
How many ADUs are built outside California each year?
Which states have legalized ADUs?
Which state has the strongest ADU law?
How many ADUs are there in the United States?
How many ADUs does Seattle permit?
Did Seattle’s ADU reform cause the increase?
Will other states see the same growth Seattle did?
Why is there no national ADU permit data?
Are ADUs mostly a California thing?
What would it take to get real ADU statistics for the rest of the country?
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 Seattle OPCD Annual ADU Report (October 2025) and six state statutes retrieved 2026-09-05; Mercatus taxonomy July 2025; Freddie Mac July 2020. 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 , Exhibit 2 carries total ADUs permitted by year back to 2005 and the attached/detached split from 2020. Exhibit 5 carries the composition of new residential development in neighborhood residential zones. Produced under Seattle Ordinance 125854. The 2025 figure is a part year through Q2. Retrieved 2026-09-05.
- Oregon Revised Statutes § 197A.425 — accessory dwelling units , 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. 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; a maximum gross floor area cap may not be set below 1,000 sq ft. Retrieved 2026-09-05.
- Montana Code Annotated § 76-2-345 — accessory dwelling units , 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. 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/Auburn watershed carve-out. Minimum size a municipality must permit: 190 sq ft. Retrieved 2026-09-05.
- Colorado Revised Statutes 29-35-101 to -105 (HB24-1152), enrolled bill text , From 30 June 2025 a “subject jurisdiction” must allow one ADU under an administrative process wherever single-unit detached dwellings are allowed. Retrieved 2026-09-05.
- Arizona Revised Statutes § 9-461.18 — accessory dwelling units , 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 principal dwelling’s gross floor area or 1,000 sq ft, whichever is smaller. Retrieved 2026-09-05.
- Emily Hamilton and Kol Peterson — “A Taxonomy of State Accessory Dwelling Unit Laws 2025”, Mercatus Center , 18 states counted as having broad ADU legalization as of July 2025, 10 classified strong and 8 weaker. Their count and their classification, cited as theirs. Retrieved 2026-09-05.
- Freddie Mac — Economic & Housing Research Insight, July 2020 , “A total of 1.4 million distinct single-family properties with accessory dwellings were identified in our final population,” from roughly 600 million MLS transactions since the late 1990s. Properties, not units; two-sided bias named by Freddie Mac; no adjustment for demolition. Retrieved 2026-09-05.
- U.S. Census Bureau — Building Permits Survey methodology , The four structure categories, and the Item 434 definition that leaves a conversion ADU with no housing-unit count anywhere in the national series. Retrieved 2026-09-05.
Unmeasured is not zero
Four of the six states with ADU mandates on this page publish no count of what those mandates have produced. That is a policy fact, not a data-collection footnote.
HyreADU does not design, permit or build accessory dwelling units. These statistics are informational and are not legal or policy advice.
Nothing on this page rests on Californian data, and no Californian figure is scaled or extrapolated into a statistic about any other place. States for which no permit series could be located are reported as unmeasured, never as zero.
The Seattle series describes the City of Seattle and is not a forecast for anywhere else; the association between its 2019 reforms and its permit volumes is described, not identified.
Statutory summaries reflect text retrieved on 5 September 2026 and should be verified against the current code before being relied on.