Compare ADU zoning rules across 10 Washington cities. Average ADU score: 8.4/10.
Gathered from the 10 cities below, with the bill or ordinance each one names. Statewide measures are listed once with the cities they reached, rather than repeated for each.
Statewide law sets a floor; the numbers that decide your project are local. Here is the spread across the 10 Washington cities covered here, taken from each city's own ordinance rather than a state summary.
| Question | Across Washington |
|---|---|
| Are ADUs permitted? | 10 of 10 permit them. |
| Detached size cap | Median 1,000 sq ft, from 200 to 1,000 sq ft across the 10 cities publishing a numeric cap. |
| Owner-occupancy | Required in none of them. |
| Build cost | $130,000 to $550,000 depending on city and unit type. |
| Easiest and hardest here | Bothell scores 9/10; Woodinville scores 8/10. |
Recomputed whenever a city page changes, so this and the city pages below cannot disagree.
| City | Max Size | Setbacks | Permit Fees | Build Cost | 1BR Rent | Score |
|---|---|---|---|---|---|---|
| Bellevue | 1,200 sq ft of floor area (LUC 20.20.120; confirmed on the City's 2026 Pre-Approved Plans page). The Director may approve a larger unit when it is an addition to, or a conversion of, an existing detached accessory structure; a converted guest cottage is exempt from the cap if not enlarged. The council draft also excluded up to 250 sq ft of garage or unheated storage per ADU - confirm that clause with Land Use Review | Detached ADUs use the middle-housing setbacks in LUC 20.20.538 Table C.1, which the council draft set at 10 ft in SR-3 through MDR-2 and 15 ft in LL-1, LL-2, SR-1 and SR-2; a DADU may sit on a rear lot line that abuts a public alley (LUC 20.20.120.D.2; RCW 36.70A.681(1)(i)). Attached ADUs use the main house's rear setback. Confirm the adopted table with Land Use Review, 425-452-4188 | Fees are set by the 2026 fee ordinance (Ord. 6879): a plan-review deposit at submittal and an inspection deposit at issuance, charged at the stated hourly rate with monthly invoices for overage. Use the City's Permit Fee Estimator (permitfeeestimator.bellevuewa.gov) for a figure | $325,000-$550,000 | $2,500-$3,200/mo | 8/10 |
| Bothell | 1,200 sq ft: "An accessory dwelling unit may be developed up to 1,200 square feet" (BMC 12.14.135(D)(1)), 200 sq ft above the state floor (RCW 36.70A.681(1)(f)). A conversion of existing space may exceed 1,200 sq ft with the community development director's approval, up to 40 percent of the primary unit's gross floor area. Detached ADUs are exempt from the BMC 12.14.130 five-percent-of-lot cap on accessory buildings (BMC 12.14.135(D)(2)); hard surface coverage is the real limit | Same as the house. "Detached accessory dwelling units shall maintain the same rear yard setback as primary structures, except for those accessory dwelling units located on an alley, in which case the setbacks for structures adjoining alleys in BMC 12.14.060 shall apply" (BMC 12.14.070(B), quoted on the city's ADU checklist). Zone minimums: 35 ft in R-C, 15 ft in R-L1 and R-L2, 10 ft in R-M1 through R-M4; on an alley 0 ft, 3 ft for a garage door (BMC 12.14.030(A)) | 2026 fees: a valuation-based permit fee (Table 20.02.155-A: $1,514 plus $9 per $1,000 over $100,000), plan check at 65 percent, planning review $231, site review $834, energy review $230, plumbing and mechanical 16 percent, 5 percent tech fee. A $300,000 detached ADU: about $7,700 plus impact fees | $325,000-$550,000 | $2,300-$2,700/mo | 9/10 |
| Kirkland | 1,200 sq ft of gross floor area (KZC 115.07(4)), above the 1,000 sq ft floor in RCW 36.70A.681(1)(f). Not counted: area under 5 ft of ceiling, uncovered rooftop decks or at-grade patios, and up to 200 sq ft of other exterior elements. Ordinance 4905 took ADUs out of the KZC 115.08 accessory-structure cap. Floor area ratio still bites: only the first 500 sq ft (lot under 8,500 sq ft) or 800 sq ft (8,500 sq ft and up) of a detached ADU behind the house is exempt from F.A.R. (KZC 115.42(1)(c)-(d)). | 10 ft, the house standard (KZC 15.30.060), relaxed by KZC 115.115(3)(o): a detached ADU without alley access may sit 5 ft from the rear line if the part inside the required rear yard is no taller than 15 ft above ABE and the rear yard does not abut an access easement treated as a rear line; a detached ADU may extend to a rear line abutting an alley if its eaves stay on the lot and sight distance (KZC 115.135) is met (RCW 36.70A.681(1)(i)). A converted garage keeps its setbacks (KZC 115.07(6)). | 2026 building permit on valuation (KMC Table 3: $1,620 plus $8.22 per $1,000 over $100,000), plan review at 65% of it, a $101 + $0.01/sq ft energy-code fee and a 3.5% technology fee: about $5,700 on a $300,000 detached ADU before plumbing, fire and utility charges. Plan licence up to $1,000. | $325,000-$550,000 | $2,450-$2,850/mo | 9/10 |
| Redmond | 1,000 sq ft: "The total square footage of an ADU shall not exceed 1,000 square feet" (RZC 21.04.1110.C.3.a) — exactly the floor RCW 36.70A.681(1)(f) sets. On a site over 10,000 sq ft the Administrator may allow more if the ADU is an affordable unit or a public benefit is provided (C.3.c). Watch the accessory-structure rule: accessory footprints may not exceed 60 percent of the house's ground-floor living area, excluding garage (RZC 21.04.1120.C.2.a) | 5 ft in the Neighborhood Residential zone, the same as the house (Table 21.08.300.A); accessory structures in NR and NMU "shall comply with the underlying setback regulations of the zone" (RZC 21.04.1120.C.5). The alley setback is 2 ft, and RZC 21.04.1110.C.2.b goes further: detached ADUs may be sited immediately abutting a lot line that abuts a public alley or right-of-way, overriding other setbacks — broader than RCW 36.70A.681(1)(i), which covers alleys only | Valuation-based (Residential Building Permit Fees, effective Feb. 1, 2025): a $300,000 detached ADU is about $4,069 permit + $2,645 plan review (65%) + $795 planning + $767 fire + 4% technology surcharge, roughly $8,600 before impact and utility fees | $325,000-$550,000 | $2,650-$2,850/mo | 9/10 |
| Sammamish | 1,000 sq ft of floor area: "The ADU must not exceed a floor area of 1,000 square feet" (SMC 21.05.010.C(5)(a)(iii)) — the floor RCW 36.70A.681(1)(f) sets, not a foot more. ADUs are listed as "Exempt" from the 50 percent residential floor area ratio in R-1 through R-8, and SMC 21.04.030.C condition (10) exempts them from hardscape coverage (30 percent in R-1, 40 percent in R-4) and FAR calculations on a single-family lot. The 3,000 sq ft maximum building footprint per building still applies. | 5 ft for a stand-alone detached ADU in all six zones (SMC 21.04.030.C, Detached Accessory Dwelling Setbacks, conditions (4) and (7)). The house itself keeps 30 ft in R-1, 20 ft in R-8, R-12 and R-18, and a 12 to 20 ft minimum in R-4 and R-6 by building size. Condition (7): the 5-ft standard "only applies to stand-alone detached accessory dwelling units" — a DADU combined with a garage or covered patio takes the accessory-structure setback: 10 ft in R-1 through R-8, 5 ft in R-12 and R-18. | Value-based (2026 Development Fee Schedule): $1,278.51 on the first $100,000 plus $8.65 per $1,000 above it, plus plan review at 65 percent, a $1,578 planning review, $544 plan-review support ($2,248 on a critical-area site) and a 15 percent technology fee — about $11,150 on a $400,000 ADU. | $325,000-$550,000 | $2,900-$3,300/mo | 8/10 |
| Seattle | 1,000 sq ft of gross floor area for an ADU with up to two bedrooms; 1,200 sq ft with three or more bedrooms (SMC 23.42.022.G.1, as amended by Ord. 127376). Not counted: up to 250 sq ft of attached garage, any story that is underground, and up to 35 sq ft of long-term bike parking. A 1,500 sq ft ADU is allowed only on a Lowrise lot inside a frequent transit service area that has not been sold for more than $1,000 in 20 years (community land trust case). ADU floor area does count toward the lot's FAR, lot coverage and density | 5 ft for an ADU in Neighborhood Residential zones; 0 ft where the rear lot line abuts an alley (Table A for 23.44.090, footnote 3). The house itself needs 15 ft (10 ft with three or more principal units), so the cottage can sit closer to the back line than the house can. Structures containing floor area must be 5 ft apart, eave to eave (23.44.100.A) | $3,453 in plan-review and building-permit fees for a 500 sq ft DADU valued at $93,145, per SDCI's own 2026 fee example (up 18.7 percent from $2,908 in 2025). Fees are valuation-based (Fee Subtitle Table D-1); at SDCI's 2026 rate of $186.29 per sq ft for wood-frame R-3 construction, the 2026 Fee Estimator gives roughly $4,990 for a 1,000 sq ft DADU, 75 percent due at intake. Add a 5 percent technology fee, the $6.50 state surcharge, hourly reviews at the 2026 base rate of $292 per hour where triggered, plus separate electrical, plumbing and side sewer permits. A subject-to-field-inspection permit pays 40 percent of the plan-review fee, and a pre-approved standard-plan submittal gets a reduced plan-review fee (Tip 310) | $325,000-$550,000 | $1,900-$2,600/mo | 9/10 |
| Spokane | 1,000 sq ft of floor area (SMC 17C.300.110(A)(2)). Building footprint up to 1,200 sq ft in every residential zone (Table 17C.111.205-2), counted within the lot's total building coverage cap of 50% in RA, 65% in R1, 80% in R2 and 100% in RMF/RHD (Table 17C.111.205-1, note 5). | 0 ft where the rear lot line abuts an alley; 5 ft where there is no alley (Table 17C.111.205-2). An existing garage or shed already inside a rear or side setback may be converted in place (SMC 17C.300.130(B)(4), 17C.111.240), but any added floor area puts the whole structure under current setbacks. | About $2,000-$3,200 in city fees for a $150,000-$300,000 ADU (2025 New SFR/ADU fee table: $65 processing, plan review at 50% of the permit fee, $100 stormwater review, 2.5% technology fee, $6.50 state fee). A CPI-adjusted 2026 schedule now applies; confirm with the DSC. | $130,000-$260,000 | $1,050-$1,250/mo | 8/10 |
| Tacoma | 1,000 sq ft of habitable area, excluding garage and other non-living areas (TMC 13.06.080.A.5.b). A Backyard Building is also limited to 40 ft wide (75 ft in UR-3) and 40 ft deep, and the lot's total floor area must stay within the UR FAR cap (UR-1: 0.6 for 1-2 units, 0.8 for 3+; UR-2: 0.8/1.0; UR-3: 1.0/1.2; TMC 13.06.020.F.1). | 15 ft in UR-1/UR-2, 10 ft in UR-3; 0 ft from an alley, and the rear setback may be measured from the alley centerline (TMC 13.06.020.F.1; 13.06.100.F.4.b). Retaining a 12-in DBH tree cuts it to 10 ft (UR-1/2) or 7.5 ft (UR-3). Buildings on the same lot must be 6 ft apart, 10 ft if either is over 25 ft tall. | PDS Fee Estimator (2026): building and site permit plus plan review about $2,000 for a 300 sq ft studio, $3,500 for 520 sq ft and $6,600 for 1,000 sq ft (valuation $170.80/sq ft x 1.31%, plus 45% plan review, site development and 17% surcharges). Electrical, water and sewer fees are separate. | $150,000-$350,000 | $1,450-$2,050/mo | 8/10 |
| Vancouver | 1,000 sq ft or less to count as an ADU (VMC 20.150 definition; City ADU page). A larger detached unit is still allowed, but it is treated as a regular middle-housing unit and pays full impact fees. The only floor is the Washington State Building Code minimum for a dwelling unit. | 5 ft minimum from the rear lot line; 0 ft from an alley (VMC 20.420.100.D, Flex-Plex building type). A detached ADU must also sit 10 ft from every other structure on the lot - house, garage, sheds, decks and stairs - or meet extra fire-code separation such as sprinklers or 1-hour rated construction (City ADU page). | About $3,000-$4,600 in building and plan-review fees for a 500-1,000 sq ft ADU (2026 fee schedule: valuation-based Table I permit fee x 160% single permit, plus 50% plan review, $32 issuance, $6.50 state fee and $66.25 planning review), before impact fees and any water/sewer SDCs. | $150,000-$350,000 | $1,500-$1,800/mo | 8/10 |
| Woodinville | 300 to 1,500 sq ft of habitable floor area (WMC 21.41.020(3)(b)), 500 sq ft above the 1,000 sq ft floor in RCW 36.70A.681(1)(f). Habitable floor area is gross floor area minus vehicle parking, covered and open-air floors and space under 5-ft ceilings, so a garage bay or low attic does not count. Building coverage (Table 21.32.050(1)) is the practical cap: R-1 steps from 40% on lots under 5,000 sq ft to 15% at 43,560 sq ft and over; R-4 35%; R-6 50%; R-8 55%; R-12 and R-18 60-65% | No separate rear setback: Table 21.32.030(2) sets one "minimum setback from interior property lines", 10 ft in R-1 (5 ft on R-1 lots under 15,000 sq ft, note 2) and 5 ft in R-4, R-6, R-8, R-12 and R-18. A detached ADU "may have the zoning setback reduced to zero from any property line abutting a dedicated public alley" (WMC 21.41.020(3)(c)). Add a 10-ft building setback from the edge of any wetland or riparian buffer (WMC 21.51.050(3)) | 2026 schedule (Resolution 668): permit fee on valuation ($148.28/sq ft for wood-frame R-3, WMC 21.62.060), plan review 65% of it, fire plan review 13.2% of that, $6.50 state surcharge. A 1,000 sq ft detached ADU comes to about $3,480, plus $119 plus $25 per fixture for mechanical and plumbing | $325,000-$550,000 | $2,400-$2,700/mo | 8/10 |
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