Research study
Washington ADU permits data: what Seattle’s records show
Seattle is the only Washington city that tags ADUs on every building permit in open data. We counted them, year by year.
What to do next: use this research to check your own lot and bids, then tell us about your project. Matching is not live yet, so we reply with information, not a quote.
Get ADU optionsWritten by HyreADU Research Desk Primary-source research and data analysis
The finding
Washington ADU permits data is best measured in Seattle, where the city issued 871 ADU building permits in 2024, six times the 145 it issued in 2010. We counted every permit in Seattle’s open permit data whose dwelling unit type includes an attached or detached ADU: 5,545 permits from 2010 through 2024.
Growth jumped after 2019: permits went from 289 in 2019 to 814 in 2022. Detached units now lead. Since 2020, 58% of ADU permits included a detached unit, up from 44% in 2010–2019.
The median permit spent 132 days in city plan review in 2024. Washington has no statewide ADU permit count, so these are Seattle numbers, not state totals.
Read this first
- These are Seattle numbers
Washington publishes no statewide count of ADU permits, and the Census Building Permits Survey has no ADU category. Seattle tags ADUs on each permit, so it is the one Washington city we can measure this way. Do not read these as state totals.
- A permit is not a finished unit
We count building permits issued. Some are never built. One permit can cover an attached and a detached ADU together; those are shown as their own group.
- 2025 is half a year
The city filled the dwelling unit type field through June 30, 2025. The 493 permits shown for 2025 cover January to June only and are shaded in the chart.
- No cost figures
The dataset carries a declared project value. We do not publish it: a permit valuation is not a build price.
Seattle ADU permits issued, by year
Seattle ADU permits rose slowly through the 2010s, jumped in 2020–2022, and have held near 850 a year since.
| Year | Permits issued | Attached only | Detached only | Both | Pre-approved DADU plan | Median plan review (days) |
|---|---|---|---|---|---|---|
| 2010 | 145 | 90 | 55 | 0 | 0 | not recorded |
| 2011 | 109 | 66 | 43 | 0 | 0 | not recorded |
| 2012 | 115 | 69 | 45 | 1 | 0 | not recorded |
| 2013 | 125 | 75 | 50 | 0 | 0 | not recorded |
| 2014 | 155 | 92 | 63 | 0 | 0 | not recorded |
| 2015 | 192 | 106 | 85 | 1 | 0 | not recorded |
| 2016 | 274 | 154 | 118 | 2 | 0 | not recorded |
| 2017 | 254 | 139 | 115 | 0 | 0 | not recorded |
| 2018 | 244 | 125 | 119 | 0 | 1 | 126 |
| 2019 | 289 | 157 | 132 | 0 | 9 | 92 |
| 2020 | 440 | 191 | 220 | 29 | 3 | 88 |
| 2021 | 661 | 234 | 332 | 95 | 39 | 135 |
| 2022 | 814 | 301 | 443 | 70 | 76 | 155 |
| 2023 | 857 | 399 | 445 | 13 | 59 | 148 |
| 2024 | 871 | 406 | 452 | 13 | 72 | 132 |
| 2025 (Jan–Jun) | 493 | 234 | 258 | 1 | 32 | 147 |
Plan review days are recorded in the dataset from 2018 onward. “Pre-approved DADU plan” is the city’s housing category for permits using its pre-approved detached ADU plans.
What changed after 2019
Seattle issued 289 ADU permits in 2019 and 440 in 2020. By 2022 it was 814. The permit data shows when the jump happened, not why. This page does not attribute the rise to any one cause; the city’s own code history is the place to test that.
The mix also shifted. Before 2020, attached ADUs, such as basement units, made up most permits. Since 2020, 58% of permits included a detached unit. The “both” group, where one permit adds an attached and a detached ADU, peaked at 95 in 2021.
Since 2023, Washington state law (RCW 36.70A.681) has required cities in urban growth areas to allow two ADUs per lot. Our Washington ADU laws page explains what that law requires. Seattle permits in 2023 and 2024 stayed near their 2022 level rather than rising again.
How long city review takes
The median Seattle ADU permit spent 92 days in plan review in 2019 and 155 days in 2022, when volume peaked. In 2024 the median was 132 days.
This field counts days in the city’s plan review, as Seattle records it. It is not the full time from application to permit, and it leaves out time the applicant spends on corrections. For a California comparison measured differently, see ADU permit times by city.
Pre-approved plans in Seattle
291 ADU permits from 2018 to mid-2025 fall in the city’s “Pre-Approved DADU Plans” housing category. That is a small share of all ADU permits. For how pre-approved plan programs work and which cities offer them, see our pre-approved ADU plans research.
Method
Anyone can repeat this count from the city’s public API.
- 1 Download
We queried data.seattle.gov dataset 76t5-zqzr on 2026-10-27 for every permit whose
dwellingunittypecontains “Accessory Dwelling”. That returned 6,432 rows and 6,432 unique permit numbers. - 2 Classify
Permits listing only “Accessory Dwelling Attached” are attached; only “Accessory Dwelling Detached” are detached; permits listing both are their own group. Other unit types on the same permit, such as a new house, do not change the class.
- 3 Count by issue year
Each permit is counted once, in the year of its issue date. Permits with no issue date are left out.
- 4 Review time
The median of the city’s
totaldaysplanreviewfield across ADU permits issued in the year, where recorded.
How to cite this page
HyreADU Research Desk, “Washington ADU permits data: what Seattle’s records show,” HyreADU, October 2026, https://hyreadu.com/research/washington-adu-permits/. Data: City of Seattle SDCI Building Permits (76t5-zqzr), retrieved 2026-10-27. Licensed CC BY 4.0.
Questions
How many ADU permits does Seattle issue a year?
Is there statewide ADU permit data for Washington?
Are most Seattle ADUs attached or detached?
How long does Seattle take to review an ADU permit?
Can I download the data?
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 City of Seattle SDCI Building Permits (data.seattle.gov, dataset 76t5-zqzr), downloaded 2026-10-27; dataset last updated 2026-10-25. The dwelling unit type field is filled through 2025-06-30.. 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.
The data behind this page
Every figure on this page is computed from one file, and that file is published here so the analysis can be checked, disagreed with, or reused.
Sources & retrieval dates
- City of Seattle, SDCI Building Permits (dataset 76t5-zqzr) , data.seattle.gov Retrieved 2026-10-27.
- RCW 36.70A.681, Accessory dwelling units, limitations on local regulation , Washington State Legislature Retrieved 2026-10-27.
Thinking about an ADU in Washington?
Tell us about your lot. Matching is not live yet, so this is not a quote. We will send ADU options and the questions to ask your city.
HyreADU is a publisher. It does not design, permit or build ADUs. Figures are a HyreADU calculation from City of Seattle open data and are not an official city statistic.