Seattle vs. Bellevue ADU rules
Seattle and Bellevue diverge sharply on how tall and how easy. Seattle allows detached ADUs up to 32 ft (plus 5 ft for a pitched roof) with zero off-street parking required anywhere in the city under SMC 23.42.022. Bellevue only legalized detached ADUs in July 2025 under Ordinance 6851 — before that they were banned outright — and caps them at 24 ft, with parking waived only for units under 1,000 sq ft or within a half mile of transit. Size caps are close (Seattle's 1,000–1,200 sq ft vs. Bellevue's flat 1,200), so height and parking, not square footage, are what actually separate these two builds.
✓ Verified against code · August 19, 2026
Seattle rules run 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.
✓ Verified against code · August 19, 2026
Bellevue rules run under LUC 20.20.120 (Land Use Code), adopted by Ordinance 6851, in effect since July 1, 2025.
Rule by rule
| Rule | Seattle | Bellevue |
|---|---|---|
| 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 | Two ADUs per lot in any district that allows a single-family dwelling — attached, detached, or a mix. No new ADU on a unit lot created by unit-lot subdivision.LUC 20.20.120.C.1, .C.7Previously: Detached ADUs were prohibited entirely; attached only — changed by Ord 6851, adopted June 24, 2025 |
| 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 | 1,200 sq ft maximum per ADU. Up to 300 sq ft of parking or unheated storage per ADU is excluded. The Director may approve more for a single-floor attached ADU, an addition to an existing detached accessory structure, or a conversion; guest-cottage conversions are exempt from the cap.LUC 20.20.120.C.3 |
| 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 | 24 ft to the highest point for a detached ADU; 28 ft when built as an addition over an existing accessory structure. (Principal units get 32–35 ft.)LUC 20.20.120.E.1; Table 20.20.538.C.1 fn 9 |
| 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 | Same dimensional standards as middle housing: side 5 ft; rear 15 ft (LL-1 through SR-2 districts) or 10 ft (SR-3 through MDR-2); front 10–25 ft by district. A DADU may be sited at a lot line that abuts an alley. Conversions of existing structures are allowed despite nonconforming setbacks.LUC 20.20.120.E.2, .C.4; Table 20.20.538.C.1 |
| 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 | None for ADUs under 1,000 sq ft; none within a half mile of a major transit stop; otherwise one space per ADU in addition to the primary structure's spaces.LUC 20.20.120.C.5, .B.1 |
| Owner-occupancy | None. Seattle removed owner-occupancy in 2019, six years before the state required it.Ord 125854 (2019) | None. Removed by Ordinance 6746 in July 2023 — owners are not required to live on site, and the separate ADU registration was abolished at the same time.Ord 6746 §1 |
| 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 | Lot coverage 40–45% by district (the middle-housing table applies to DADUs). FAR treatment is asymmetric: attached ADUs are fully exempt from FAR and unit counts, while detached ADUs count toward both — making attached units 'cheaper' in zoning currency.LUC 20.20.390.A–.B; Table 20.20.538.C.1 fns 5, 10 |
| 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 | Permitted use — no conditional use permit and no ADU registration. Built under a standard building permit. The city publishes no ADU-specific review timeline; secondary sources suggest roughly 4–6 months for custom single-family-track review (estimate only).LUC 20.20.120.C.2, .C.4 |
Seattle: 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.
- Ordinance 127376 also collapsed the old NR1/NR2/NR3 subzones into a single Neighborhood Residential (NR) zone effective January 21, 2026. King County assessor records still carry the superseded subzone, so a parcel listed as NR2 in county data is simply NR today.
Seattle: 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.
Bellevue: worth knowing
- Bellevue is the newest DADU market in the region — detached ADUs were illegal here until mid-2025, so almost no local DADU precedent or content exists.
- ADUs can be sold separately via unit-lot subdivision (LUC 20.45A.065 / 20.45B.057).
- An ADU cannot coexist with a Home Occupation Permit business on the same site (LUC 20.20.120.C.6).
- A pre-approved DADU plan program is in development (up to ~10 plans, launch anticipated 2026) but is NOT live yet — claims that Bellevue already has approved plan sets are wrong.
Bellevue: on our watchlist
Pre-approved DADU plan program launch anticipated in 2026, tied to the next state building code adoption. Watch for housekeeping Land Use Code amendments following Ord 6851.
Full detail: Seattle · Bellevue · back to the Washington hub. 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.