Olympia ADU rules
✓ Verified against code · August 19, 2026
Current rules under OMC 18.04.060 (Olympia Municipal Code), adopted by Ordinance 7444 (amended by Ord 7457), in effect since December 22, 2025. Olympia adopted its combined HB 1110/1337 package in November 2025 — about five months after the state deadline — making its rules the freshest of the region's major cities. Its ADU liberalization runs deep, though: zero parking has been the rule here since 2019, and the city's ADU history dates to 1995.
Common questions, answered from the code
Can I sell my Olympia ADU separately from my house?
Yes — via condominium or unit-lot subdivision, expressly allowed by the 2025 code package.
OMC 18.04.060(A)(8) · Permalink
Do I need parking for an Olympia ADU?
No — zero off-street parking is required citywide, with no transit-proximity condition. And this isn't new: Olympia dropped ADU parking in 2019, six years before the state's mandate.
OMC 18.38.100 · Permalink
Is there a tax break for building an ADU in Olympia?
Yes — a 3-year property-tax exemption for ADU construction up to 30% of the original structure's value. The catch: you must file with the Thurston County Assessor before construction begins.
RCW 84.36.400; city ADU page · Permalink
How many ADUs can I build, and how big?
Two per residential lot, 1,000 sq ft of gross floor area each — plus up to 120 sq ft of covered porch per unit, and an ADU can pair with an accessory structure up to 800 sq ft.
OMC 18.04.060(A) · Permalink
When did Olympia's current rules take effect?
The HB 1110/1337 package was adopted November 18, 2025 and took effect in late December 2025 — five months after the state deadline, which means guides published in mid-2025 describe transitional rules. A cleanup ordinance followed in 2026.
Ord 7444; Ord 7457 · Permalink
The rules at a glance
| ADUs allowed per lot | 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 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 | 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: 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 | 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 — 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 | 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 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 |
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.
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.
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:
Primary sources
Other Washington cities
Bellevue · Bothell · Burien · Edmonds · Everett · Issaquah · Kent · Kirkland · Lynnwood · Redmond · Renton · Sammamish · Seattle · 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.