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 6 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 |
|---|---|---|---|---|---|---|
| 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 |
| 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 |
| 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 |
Summarized from each city's ordinance text, most recently read July 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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