Seattle vs. Tacoma ADU rules
Seattle and Tacoma both scrapped owner-occupancy and parking requirements years apart from each other, but the details differ. Tacoma requires owner-occupancy only if you run an ADU or the main house as a short-term rental (TMC 13.06.080.A.4.g); Seattle has no such exception anywhere. Seattle now counts ADU floor area toward its floor-area ratio as of January 21, 2026 (Ordinance 127376), narrowing what two units can add up to; Tacoma has no comparable FAR limit for ADUs, only its zone-based ratios. Permitting is also more concrete in Seattle, where pre-approved DADU plans through ADUniverse take roughly 2–6 weeks; Tacoma publishes only a general 8-week goal for residential permits.
✓ 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
Tacoma rules run under TMC 13.06.080.A, under the Home in Tacoma Phase 2 zoning (UR-1/UR-2/UR-3), adopted by Home in Tacoma Phase 2 (adopted Nov 19, 2024), in effect since February 1, 2025.
Rule by rule
| Rule | Seattle | Tacoma |
|---|---|---|
| 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 residential lot, on any legally established lot regardless of size or width, in any zone permitting residential use — attached, detached, or a mix. All dwellings must be in single ownership.TMC 13.06.080.A.4.a, A.5.a |
| 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 | 1,000 sq ft of habitable living area, excluding garage and other non-living areas. An ADU occupying a single floor of an existing building may exceed this to use the whole floor efficiently.TMC 13.06.080.A.5.b |
| 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 | No ADU-specific cap — zone and housing-type limits apply: 35 ft in UR zones, but 25 ft within the rear 25 ft of UR-1 lots (where most DADUs sit). The old 18-ft DADU cap was repealed.TMC 13.06.080.A.5.c; 13.06.020.F.1 |
| 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 | Side: 5 ft (8 ft where a main entrance faces the side lot line). Rear: 15 ft in UR-1, 10 ft in UR-2/UR-3 — and none from alleys (rear may be measured from the alley centerline). Attached ADUs use the primary structure's setbacks.TMC 13.06.020.F.1; 13.06.100.F |
| 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. No off-street parking is required for ADUs anywhere in Tacoma — a local rule since 2019, not a transit-conditioned waiver. You may not remove the primary dwelling's required parking unless it's replaced.TMC 13.06.080.A.4.dPreviously: Off-street parking was required for ADUs — changed by Ord 28576 (2019), reaffirmed by Home in Tacoma Phase 2 |
| Owner-occupancy | None. Seattle removed owner-occupancy in 2019, six years before the state required it.Ord 125854 (2019) | None — removed by Tacoma's 2019 ADU ordinance. One exception: using an ADU (or the main house) as a short-term rental requires the owner to occupy one of the dwellings.TMC 13.06.080.A.4.g; Ord 28576 (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 | UR zones use floor-area ratio rather than lot coverage: UR-1 max 0.6 (1–2 units) / 0.8 (3+); UR-2 0.8/1.0; UR-3 1.0/1.2, higher with affordability bonuses. At least 10% of the lot must remain usable yard space.TMC 13.06.020.F.1, F.7 |
| 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 outright — building permit only, no discretionary land-use review. The city's stated issuance goal for residential new-building permits is 8 weeks.TMC 13.06.080.A.3.a; tacomapermits.org |
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.
Tacoma: worth knowing
- Tacoma liberalized ADUs citywide in 2019 — DADUs everywhere, no owner-occupancy, no parking — so HB 1337 changed almost nothing here. Guides crediting the state law for Tacoma's rules are flattening six years of local history.
- DADUs are now a 'Backyard Building' housing type that can stack behind houseplexes and rowhouses, not just single houses.
- A walkway to the street (minimum 3 ft, distinct materials) and a separate visible address are required.
- Legal lots of record are guaranteed a minimum of four dwellings under Home in Tacoma.
Tacoma: on our watchlist
The 2025 Annual Amendment package (minor Home in Tacoma refinements) was in public review in early 2025; the current tip sheet already reflects post-Phase-2 rules.
Full detail: Seattle · Tacoma · 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.