Tacoma vs. Olympia ADU rules
Height and homeowner perks separate these two more than size does — both cap ADUs around 1,000 sq ft. Olympia holds every detached ADU to a flat 24 ft regardless of zone (OMC 18.04.080 Table 4.05), while Tacoma follows Urban Residential zone heights: up to 35 ft, though only 25 ft in the rear 25 ft of UR-1 lots. Olympia also offers six pre-approved DADU stock plans and a 3-year property-tax exemption for ADU construction (RCW 84.36.400) that Tacoma's file doesn't mention. Both cities dropped ADU parking in 2019 and never required owner-occupancy, so financing and siting, not permission, are the questions to solve first.
✓ 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.
✓ Verified against code · August 19, 2026
Olympia rules run under OMC 18.04.060 (Olympia Municipal Code), adopted by Ordinance 7444 (amended by Ord 7457), in effect since December 22, 2025.
Rule by rule
| Rule | Tacoma | Olympia |
|---|---|---|
| ADUs allowed per lot | 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 | Two ADUs per residential lot in all residential districts — added to, created within, or detached from the principal unit; an ADU may even attach to a garage, shop, or another ADU.OMC 18.04.060(A)(1)–(4) |
| Maximum size | 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 | 1,000 sq ft of gross floor area each. Covered porches up to 120 sq ft per ADU don't count, and an ADU may combine with an accessory structure of up to 800 sq ft. The director may allow an entire single floor of an existing home.OMC 18.04.060(A)(3)–(7) |
| Detached ADU height | 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 The old 18-ft DADU cap was repealed.TMC 13.06.080.A.5.c; 13.06.020.F.1 | 24 ft for detached ADUs — an exception carved out of every district's height row, including districts that otherwise allow 35–60 ft.OMC 18.04.080 Table 4.05 |
| Detached ADU setbacks | 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 | Side: the zone's required side yard (typically 5 ft). Rear: 5 ft, reduced to zero where the lot abuts an alley. Any garage attached to the ADU must also conform.OMC 18.04.080(H)(5)(a)–(b) |
| Off-street parking | 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 | Zero. No off-street parking is required for ADUs anywhere in Olympia — and that has been true since 2019 (Ord 7187), not just since the 2025 package. Optional parking is allowed if the owner wants it.OMC 18.38.100 (most recently amended by Ord 7444) |
| Owner-occupancy | 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) | None — and ADUs may be sold separately from the main house via condominium or unit-lot subdivision.OMC 18.04.060(A)(8) |
| Lot coverage / floor area | 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 | ADUs meet the same lot-coverage and tree-density standards as a single-family residence in the zone. They're exempt from maximum density except in zones capped at 12 units/acre or less. Structures built before December 22, 2025 (or 10+ years old) may convert even if nonconforming.OMC 18.04.060(A)(5)–(6); 18.04.080(E)(3) |
| Permit path & timeline | 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 | Permitted outright in all residential districts — a building permit, with design review folded into it inside the Infill & Design Review District (the ADU must reflect the primary home's character; every unit needs a lit, 3-ft paved walkway). No published review timeline; the city urges early pre-application contact.OMC 18.04.060(A); Table 4.01; city ADU page |
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.
Olympia: worth knowing
- A 3-year property-tax exemption is available for ADU construction worth up to 30% of the original structure's value — but you must file with the Thurston County Assessor BEFORE building (RCW 84.36.400).
- Six pre-approved DADU stock plans, from a 480 sq ft studio to a 1,000 sq ft 3-bedroom, including a carriage-house-over-garage option.
- An existing home of 1,200 sq ft or less may be reclassified as the ADU when a new principal unit is built — useful for tear-down-averse lots.
- Fire sprinklers are required in the ADU if the main house has them.
- Park impact fees changed July 1, 2026 (Ord 7470, implementing SB 5258): now $3.92 per sq ft of habitable space (min $3,187.93), always capped at 50% of the principal unit's fee — and ADUs remain exempt from school impact fees.
- Ordinance 7471 (June 2026) added residential lot splitting under HB 1096 — a new path to put an ADU on its own sellable lot, alongside the condo and unit-lot-subdivision routes.
Olympia: on our watchlist
Home-based business code amendments were at Planning Commission public hearing August 17, 2026 — they will touch OMC 18.04.060's home-occupation provisions when they reach Council.
Ord 7472 (June 2026) shifted some design-review applicability language; we re-check the ADU design-criteria scope (OMC 18.175.080–.090) once the codifier publishes it.
Full detail: Tacoma · Olympia · 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.