Seattle ADU rules
✓ Verified against code · August 19, 2026
Current rules under SMC 23.42.022 and the new Chapter 23.44 (Neighborhood Residential), adopted by Ordinance 127376 (CB 120993), in effect since January 21, 2026. Seattle rewrote its residential zoning twice in eight months: the HB 1337 compliance package (Ord 127211, May 2025), then the permanent state-compliance code signed December 22, 2025 and effective January 21, 2026, which repealed the old Chapter 23.44 and amended SMC 23.42.022. Most guides written in 2025 describe interim rules that no longer exist.
Common questions, answered from the code
Does a backyard cottage count against Seattle's floor-area limits?
Yes — as of January 21, 2026, it does. The interim rule that exempted ADU floor area from FAR was struck by Ordinance 127376. If a guide tells you ADUs are FAR-exempt, it's describing the 2025 interim code, which no longer exists. Model your FAR before assuming you can build two 1,000 sq ft units.
SMC 23.42.022.J, as amended by Ord 127376 · Permalink
How many ADUs can I build on my Seattle lot?
Two, in addition to your house — attached, detached, or both, and two detached units may stack in a single structure. Seattle has allowed two ADUs since 2019.
SMC 23.42.022.C–.D · Permalink
Do I need to add parking for an ADU in Seattle?
No. Seattle requires no off-street parking for ADUs anywhere in the city, regardless of transit proximity.
SMC 23.42.022.I · Permalink
Do I have to live on the property?
No. Seattle repealed its owner-occupancy requirement in 2019. Both the main house and the ADUs can be rentals.
Ord 125854 · Permalink
Can I sell a Seattle ADU separately from the house?
Yes — the code expressly allows ADUs to be condominiumized and sold separately.
SMC 23.42.022.K · Permalink
What's the fastest way to permit a DADU in Seattle?
Use a pre-approved plan from the city's ADUniverse gallery — SDCI estimates roughly 2–6 weeks for pre-approved designs versus an unpublished (longer) timeline for custom designs.
SDCI / ADUniverse · Permalink
The rules at a glance
| ADUs allowed per lot | Two ADUs per lot, in addition to the principal unit(s) — attached, detached, or stacked, in any combination. Two detached ADUs may even stack in one structure.SMC 23.42.022.C–.D |
| Maximum size | 1,000 sq ft of gross floor area for an ADU with up to 2 bedrooms; 1,200 sq ft with 3+ bedrooms; up to 1,500 sq ft in Lowrise zones in frequent-transit areas (with an anti-speculation condition). Up to 250 sq ft of attached garage, underground stories, and up to 35 sq ft of bike parking don't count.SMC 23.42.022.G.1–.2 |
| Detached ADU height | Detached ADUs follow the same height standards as the main house: 32 ft in Neighborhood Residential zones, plus 5 ft for a pitched roof ridge (4:12 or steeper).SMC 23.42.022.E; 23.44.070 |
| Detached ADU setbacks | Side: 5 ft average, 3 ft minimum (a flat 3 ft on lots under 5,000 sq ft in frequent-transit areas). Rear: 5 ft, or zero where the rear lot line abuts an alley. Structures containing floor area need 5 ft between them.Table A for SMC 23.44.090 fn 3; 23.44.100.A |
| Off-street parking | None. No off-street parking is required for an ADU anywhere in Seattle — no transit-proximity condition.SMC 23.42.022.I |
| Owner-occupancy | None. Seattle removed owner-occupancy in 2019, six years before the state required it.Ord 125854 (2019) |
| Lot coverage / floor area | Lot coverage is capped at 50% in NR zones (60% for stacked-unit and certain courtyard developments). Important 2026 change: ADU floor area now counts toward the floor-area ratio (FAR), and ADUs count toward density — the interim ADU FAR exemption was struck effective January 21, 2026.SMC 23.44.080; 23.42.022.J; 23.44.050 Table APreviously: ADU gross floor area was exempt from FAR limits (interim 2025 rule) — changed by Ord 127376, effective Jan 21, 2026 |
| Permit path & timeline | Permitted outright — no land-use or design review, no MHA contribution. Standard construction permit through SDCI. Pre-approved DADU plans through the ADUniverse program permit in roughly 2–6 weeks; no published timeline for custom designs.SMC 23.42.022.A; SDCI |
Worth knowing
- ADUs can be condominiumized and sold separately from the main house (SMC 23.42.022.K).
- Accessory structures that existed before July 23, 2023 get Type I waivers for conversions and are exempt from coverage and setback limits when converted.
- The odd 1,500 sq ft allowance applies only if the lot hasn't been sold for more than $1,000 in the past 20 years — an anti-speculation clause.
- King County's sewer capacity charge applies on top of city fees.
On our watchlist
Later One Seattle Plan implementation phases continue through 2026 but do not change the ADU standards above. SDCI Tips and some code-hosting mirrors were still catching up to the January 2026 recodification in early 2026 — old deep links to SMC 23.44.041 are dead.
Check your parcel
The rules above are citywide — what you can build depends on your lot’s zone, size, and setbacks. Look yours up on the official viewers:
- King County Parcel Viewer
- King County Assessor Property Search (eRealProperty)
- Seattle zoning/property map (SDCI Property Information Map)
Primary sources
- SMC 23.42.022 (current)
- Ordinance 127376 / CB 120993 (signed text)
- SDCI — ADU permits
- ADUniverse (pre-approved DADU plans)
Other Washington cities
Bellevue · Bothell · Burien · Edmonds · Everett · Issaquah · Kent · Kirkland · Lynnwood · Olympia · Redmond · Renton · Sammamish · Shoreline · Tacoma
Not legal advice. Every fact above cites the code section it comes from — check the cite, or ask your city’s planning department to confirm your specific lot. Spotted an error or a new ordinance? See about to reach us; corrections are logged in the changelog.