Side by side · Washington
Compare ADU rules across Washington
Flexible on where you build? The same backyard cottage can be routine in one city and impossible next door. This table puts 17 Washington cities side by side so you can shortlist the friendly ones before reading a single ordinance, or narrow it to just the cities you are deciding between. Every value is summarized from the city's own code; open a city's page for the full notes, citations, and fine print.
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| City | Detached ADU | Attached / internal | Max size | Owner occupancy | Extra parking | Approval |
|---|---|---|---|---|---|---|
| Auburn | Allowed | Allowed | 1,000 sq ft maximum, 300 sq ft minimum, no more than 2 bedrooms (garage space excluded) | Not required | None within 1/2 mile walking distance of a major transit stop; otherwise 1 on-site space per ADU | Permitted use |
| Bellevue | Allowed | Allowed | 1,200 sq ft (attached and detached); Director may approve more for an ADU on a single floor of the primary structure or for additions to/conversions of existing detached accessory structures; guest cottage conversions exempt; up to 300 sq ft of parking or unheated storage per ADU excluded from the cap | Not required | None for ADUs under 1,000 sq ft of floor area (excluding garage) or within 1/2 mile of a major transit stop (as defined in RCW 36.70A.696); otherwise 1 off-street space per ADU, in addition to spaces required for the primary structure | Permitted use |
| Bellingham | Allowed | Allowed | 1,000 sq ft per ADU (attached or detached); up to 2 ADUs per lot | Not required | None | Permitted use |
| Everett | Allowed | Allowed | Under 1,000 sq ft of interior habitable area (basements and attics count; a garage or accessory structure does not). No percent-of-primary-dwelling cap. Height follows the zone's principal-building limit: 35 ft and 3 floors in NR and NR-C. A unit of 1,000 sq ft or more is not an 'ADU', but it is still allowed as an additional principal dwelling unit, because Everett repealed maximum residential density in 2025. | Not required | None in practice. EMC 19.34.050(C) exempts every dwelling unit under 1,200 sq ft of gross floor area from off-street parking, and an ADU is by definition under 1,000 sq ft of interior habitable area. The baseline is otherwise 1 space per dwelling unit, and NR and NR-C development within a half mile walking distance of a bus rapid transit or rail stop is exempt regardless of size. Bicycle parking is still required. | Permitted use |
| Federal Way | Allowed | Allowed | 1,000 sq ft per ADU (excluding garage, workshop and similar nonliving areas) and no more than two bedrooms; an attached ADU may exceed 1,000 sq ft up to the largest floor area of the primary dwelling if creating it requires no exterior alteration. Height limit 30 ft above average building elevation. | Not required | 1 off-street space per ADU on top of the primary dwelling's spaces; 0 spaces if the site is within a half mile of a major transit stop. | Permitted use |
| Kent | Allowed | Allowed | 1,000 sq ft gross floor area per ADU (not transferable between the two units); detached ADUs also capped at 24 ft in height | Not required | None within one-half mile walking distance of a major transit stop; otherwise 1 off-street space per ADU, waivable by the planning director | Permitted use |
| King County | Allowed | Allowed | 1,000 sq ft heated floor area plus 1,000 sq ft unheated; no limit if wholly within a basement or attic of the primary dwelling; detached-ADU basement floor area does not count; in the urban area, unused unheated allowance can convert 1:1 into heated area up to 1,500 sq ft heated | Not required | None; no additional off-street parking spaces are required for accessory dwelling units | Permitted use |
| Kirkland | Allowed | Allowed | 1,200 sq ft gross floor area (attached or detached); for an attached ADU entirely within existing floor area on a single floor, the Planning and Building Director may allow a larger unit | Not required | None within 1/2 mile of a major transit stop, and none for a single ADU elsewhere; lots more than 1/2 mile from a major transit stop with more than one ADU must provide one off-street space, waived if on-street parking exists within 600 ft or the property is within 1/2 mile of transit with 15-minute commute-hour headways | Permitted use |
| Olympia | Allowed | Allowed | 1,000 sq ft gross floor area per ADU, up to 2 ADUs per lot; 24 ft height for detached | Not required | None | Permitted use |
| Redmond | Allowed | Allowed | 1,000 sq ft (attached or detached); Administrator may allow more if the ADU occupies an entire single floor of the primary dwelling, or on sites over 10,000 sq ft where the ADU is affordable housing or a public benefit is provided; tiny homes capped at 400 sq ft | Not required | None: minimum required off-street parking for an accessory dwelling unit is 0 spaces in all three location categories (near frequent transit or TOD focus areas, centers, and all other locations) | Permitted use |
| Renton | Allowed | Allowed | 1,000 sq ft gross floor area and 1,000 sq ft building area per ADU; a conversion occupying an entire single floor of the primary dwelling may go larger, up to 1,250 sq ft (beyond that it is classified as an attached dwelling, not an ADU); PRADU base-plan ADUs are exempt from the building area cap | Not required | Minimum 1.0 off-street space per ADU; no off-street parking required for ADUs on lots within a one-half mile walking distance of a major transit stop (Area A) | Permitted use |
| Seattle | Allowed | Allowed | 1,000 sq ft gross floor area (ADU with up to two bedrooms); 1,200 sq ft (three or more bedrooms); up to 1,500 sq ft on qualifying LR-zone lots in a frequent transit service area. Excluded from the limit: up to 250 sq ft of attached garage, all underground stories or portions, and up to 35 sq ft of long-term bicycle parking. | Not required | None. 'No off-street motor vehicle parking is required for an accessory dwelling unit.' | Permitted use |
| Spokane | Allowed | Attached and internal ADUs are permitted in the same five residential zones with no hearing and no discretionary review, but Spokane gates them on the size of the existing house: the principal structure must have at least 1,000 sq ft of floor area, excluding an attached garage, before an attached ADU can be established. That threshold bites in Spokane's older housing stock because SMC 17A.020.060 defines floor area as the above-ground portion of a building and excludes any floor sitting four feet or more below the adjacent right-of-way, so a full basement generally does not count toward the 1,000 sq ft. Owners of small bungalows can therefore be shut out of a basement or attic conversion and pushed toward a detached unit instead. No such minimum applies to detached ADUs. SMC 17C.300.110(A)(1); SMC 17C.300.130(A)(1)(a), (b), (d); SMC 17A.020.060 (Floor Area)Full rules for Spokane Ordinance read August 2026 | 1,000 sq ft for an attached ADU and 1,000 sq ft of floor area for a detached ADU. Detached units are also capped at a 1,200 sq ft maximum building footprint and 25 ft in height, and count against the zone's total building coverage (50% RA, 65% R1, 80% R2, 100% RMF and RHD). The attached cap excludes any related garage area, and a conversion of existing interior basement or attic space may exceed 1,000 sq ft. | No owner occupancy is required for an ordinary ADU, whether the owner lives on site or not, and no deed restriction is recorded any more. The requirement returns only if the lot also holds a short-term rental permit under Chapter 17C.316: then one of the dwelling units on the lot must be occupied by an owner as their permanent and principal residence for more than six months of each calendar year, and that owner may not collect rent on it. On such a lot the ADU approval is not transferable except to a new owner who will occupy it, and the approval expires if the titleholder stops owning and residing in either unit. Owners can ask the director to waive the requirement for up to a one-year absence for good cause such as a job dislocation, sabbatical, education or illness. Ordinance read August 2026 | None. Spokane has no off-street parking minimum for any use citywide, so an ADU adds no required spaces. | Permitted use |
| Spokane Valley | Allowed | Allowed | 50% of the primary dwelling's habitable square footage, with a 300 sq ft minimum and a two-bedroom cap. The footprint may not exceed the greater of 10% of the lot area or 1,000 sq ft. Height follows the zone's dwelling limit, 35 ft in R-1 through R-4. | Required | 1 additional off-street space for the ADU, paved, on top of the 2 spaces required for the house, so 3 on a typical single-family lot. No transit-proximity exemption. | Permitted use |
| Tacoma | Allowed | Allowed | 1,000 sq ft of habitable area, excluding garage and other non-living areas. No percent-of-primary-dwelling or percent-of-lot cap. No ADU-specific height limit: the zone and housing type control (35 ft in the UR districts, but only 25 ft within the rear 25 feet of the lot in UR-1 and UR-2, where a detached ADU normally sits). | Not required | None. 'No off-street parking is required for ADUs', and existing off-street spaces may be removed to make room for one. | Permitted use |
| Vancouver | Allowed | Allowed | 1,000 sq ft gross floor area; height follows the district (3 floors and 45 ft in Low-Scale Neighborhood) | Not required | None | Permitted use |
| Yakima | Allowed | Allowed | 1,000 sq ft of floor area, attached or detached (no percent-of-house cap, no per-zone split) | Not required | 1 off-street space for the ADU, in addition to the 2 required for the house (3 on a typical lot) | Permitted use |
Summarized from each city's ordinance text, most recently read August 2026. Tap a value marked with an info icon to see exactly what it depends on, straight from the ordinance. Nearly every value has zone, lot-size, or process caveats beyond that; the per-city pages carry the full notes and code citations. Treat this as orientation, not a permit determination.
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Reading the table
- Detached ADU is the make-or-break column for backyard cottages: minimum lot sizes (often 8,000 to 14,500 sq ft) hide behind many of the "Allowed" values, so check your lot before celebrating.
- Attached / internal (basement apartments, additions) is legal almost everywhere and is usually the path of least resistance.
- Approval: "Permitted use" means staff-level sign-off; "Conditional use" means a public hearing, which adds time, cost, and uncertainty.
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