Almost everything a homeowner believes about ADUs is two or three years out of date
Washington rewrote the accessory dwelling unit law in 2023, every city rewrote its code on its own schedule, and Seattle changed the whole envelope in January 2026. Here is what actually changed, city by city, with the ordinance number next to each one.
| Where | What changed |
|---|---|
| Statewide HB 1337 · RCW 36.70A.681 | Two ADUs on any residential lot in a city over 25,000. No cap below 1,000 sq ft. No owner-occupancy requirement anywhere in Washington. No extra parking within half a mile of a major transit stop. Large cities were to comply by June 2025, smaller ones by December 2025. |
| Seattle One Seattle Plan · 21 January 2026 | The big one. Height went from 14–18 ft to 32 ft. Lot coverage to 50 percent. Two DADUs allowed in a single structure. Every plan in the current pre-approved gallery was drawn for the old envelope. |
| Bellevue Ordinance 6851 · effective 1 July 2025 | Detached ADUs became legal in Bellevue for the first time. Up to two per lot, 1,200 sq ft, 24 ft — 28 over an existing accessory structure — and an ADU may be sold to a separate owner. Most guides online still say detached units are banned here. |
| Kirkland KZC 115.07 · through Ord. 4936, May 2026 | Two ADUs, 1,200 sq ft detached, separate ownership allowed, impact fees capped at half the principal unit's. Median permit approval 8–16 weeks from a clean submittal. |
| Redmond RZC 21.04.1110 · Ord. 3261, June 2026 | Two ADUs or tiny homes per lot, 1,000 sq ft, and zero required parking. The old section 21.08.220 — one unit, owner occupancy, no separate sale — is superseded but still online, and most 2026 articles are still quoting it. |
| Sammamish DC 21.05.010 · through Ord. O2026-597, August 2026 | Two units, 1,000 sq ft, 24 ft, 5 ft setbacks all round. The city's own Handout 310 is dated 2023 and still says one unit at 18 ft with two parking spaces. |
| Issaquah LUC 18.500.090 · through Ord. 3144, June 2026 | Two units in any combination, 1,000 sq ft each, no parking minimum — and accessory dwelling units are exempt from impact fees entirely. |
| Kenmore KMC Ch. 18.73 · Ord. 25-0630, June 2025 | The most generous city on our list: up to 17 percent of the lot to a maximum of 1,500 sq ft, 35 ft tall, no parking, explicit permission to sell separately — and the building permit fee for a new ADU has been waived since July 2023 and still is. |
| Renton RMC 4-2-110C · amended June 2025, Ord. 6168 | Two per lot, 1,000 sq ft, 24 ft, and no public street improvements or sidewalks required. Renton's PRADU programme gives you eight permit-ready plans free with expedited review. |
| Newcastle Ordinance 2025-671 | Adopted the two-unit rule in its 2025 compliance ordinance. 1,000 sq ft, 24 ft detached, sale as a condominium or fee simple within a unit lot subdivision. |
| Mercer Island Interim Ordinance 25C-02 · March 2025, in force June 2025 | Two units, 1,000 sq ft, at least 24 ft — but these are interim regulations. Permanent ones are still to be adopted. Check anything you design against the code in force on the day you submit. |
| Lake Forest Park LFPMC 18.50.050 · Ord. 1310, 2025 | Two units, 1,000 sq ft, 25 ft, 15 ft rear setback or zero at an alley. The only city on this list that publishes a firm number: eight weeks for any building permit. |
| Woodinville WMC 21.41.020 · through Ord. 792, 2025 | Two per parent lot, 300–1,500 sq ft habitable. The code still lists one parking stall per dwelling with no transit exemption, which conflicts with state law — worth confirming at the counter. |
| Bothell BMC 12.14.135 · last amended Ord. 2277, 2019 | The outlier. Bothell has not touched its ADU section since 2019. It still says one unit, 800 sq ft, one mandatory parking stall and six months a year of owner occupancy — all preempted by state law. State law wins; the counter is reading the old text. |
Checked against each city's own municipal code on 4 September 2026, plus RCW 36.70A.681. Where a city's code is silent we say so rather than carrying a number over from the city next door. Codes in this region are being rewritten faster than any page keeps up with — we verify your parcel against the code in force on the day we submit.
What is coming, and what is worth waiting for
November 2026
Seattle ADUniverse 2.0. Submissions closed 15 July 2026. The refreshed catalogue is drawn to the January 2026 envelope and runs from single units up to fourplexes. If you are not in a hurry, waiting is worth real money.
Announced, no date
Bellevue pre-approved plans. A pilot of up to ten plans, detached to 1,200 sq ft. Tied to the state code update.
Ongoing
Mercer Island permanent regulations. The interim rules stay in force until the city adopts permanent ones after more public engagement.
See any of it drawn to scale
The Explorer redraws a 60 × 110 ft lot for whichever city you pick — the biggest unit that city allows, at its real setback, next to an elevation at its real height limit.