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ADU Rules in Seattle, Washington

Complete guide to building an Accessory Dwelling Unit in Seattle — zoning, setbacks, permits, costs, and more.

Max Size: 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 Setbacks: 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) No Parking Required Verified: 2026-09-19

Researched by ADU Zoning Hub · verified 2026-09-19 against Seattle planning code and Washington ADU law · methodology · 25 sources cited ↓

ADU Score: 9/10
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Are ADUs allowed in Seattle, WA? [1][2][3][4][5]

Yes. Seattle allows ADUs on most residential lots. A detached unit can be up to 1,000 sq ft, set 5 ft back from the rear line. No extra parking is required. Owner-occupancy is not required, so the unit can be rented out. A building permit is required and applications are reviewed administratively, without a public hearing.

Maximum size1,000 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
SetbacksRear 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) · Side 5 ft average with a 3 ft minimum in Neighborhood Residential zones; a flat 3 ft on lots under 5,000 sq ft inside a frequent transit service area (Table A for 23.44.090). Eaves and gutters may project 2 ft if they stay 3 ft from the line. Lowrise zones use the principal-unit minimum setback with no averaging (23.42.022.E)
ParkingNone required for any ADU (SMC 23.42.022.I and Table B for 23.54.015, footnote 4). If you do build a space, it must be reached from an improved street, alley or easement, may not sit within 20 ft of the front lot line except on the driveway, and alley access is required where an improved alley exists (23.44.160). The state floor is the same: RCW 36.70A.681(2) bars any parking requirement for ADUs within a half mile of a major transit stop
Owner-occupancyNot required. SMC 23.42.022 contains no owner-occupancy rule and RCW 36.70A.681(1)(b) forbids one. Owners of ADUs permitted before 2019 may still have an owner-occupancy covenant recorded on title; SDCI publishes a 'Release of Owner Occupancy Covenant for ADUs' form to remove it

Do I need a permit to build an ADU in Seattle? [6][7][8][9][10][11][12][13][14]

Yes. Every ADU in Seattle needs a building permit. Applications are handled administratively — no public hearing.

Permit fees$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)
Typical timelineNo published SDCI target for ADU review. Pre-approved ADUniverse plans: 2-6 weeks to permit on most sites (SDCI and OPCD). Custom plans: SDCI's Tip 116B warns the intake appointment alone may be 2-4 months out, and 2026 industry guides report 4-8 weeks of review for a straightforward custom DADU and 12-18 months from design start to move-in overall (8-10 months with a pre-approved plan). SDCI's Construction Permit Timeline tool (April 2026) shows the actual review history of any permit since 2018
Official Seattle links:

Three things to know before you build in Seattle

  • Pick one of the 7 ADUniverse pre-approved designs if your lot is Neighborhood Residential, has no environmentally critical area and the cottage plus driveway disturbs under 750 sq ft: SDCI permits these in 2-6 weeks at a reduced plan-review fee, versus months for a custom set
  • Adding ADUs raises your allowed floor area, not just the unit count: on a 5,000 sq ft NR lot the FAR is 0.6 (3,000 sq ft) with one unit, 0.8 with two and 1.0 with three (Table A for 23.44.050), and lots under 5,000 sq ft always get at least 2,500 sq ft; run the FAR and 50 percent lot-coverage math before assuming two 1,000 sq ft cottages fit
  • The cottage may sit 5 ft from the rear line and on the lot line along an alley, but it has to stay out of the side and rear setbacks (structures inside a setback are capped at 12 ft) and 5 ft from the house; there is no longer a rule that it must sit behind the house

Last verified 2026-09-19 against Seattle planning code. How we verify

Yes: every Seattle lot that allows housing may add up to two accessory dwelling units (attached, detached or stacked) under Seattle Municipal Code 23.42.022, as rewritten by Ordinance 127211 (effective June 30, 2025) and Ordinance 127376 (signed Dec 22, 2025, in force since Jan 21, 2026). An ADU may be 1,000 sq ft of gross floor area with up to two bedrooms, or 1,200 sq ft with three or more, not counting up to 250 sq ft of attached garage, basement stories or 35 sq ft of bike storage. In Neighborhood Residential zones a backyard cottage follows the same 32-ft height limit as the house, sits 5 ft from the rear line (0 ft on an alley) and 3-5 ft from side lines, and needs no parking, no owner-occupancy and no street improvements. The permit is an ordinary SDCI construction permit with no public notice or appeal: SDCI's own 2026 fee example prices a 500 sq ft DADU at $3,453 in plan-review and permit fees, a pre-approved ADUniverse design can be permitted in 2-6 weeks, and King County adds a $46.01-per-month sewer capacity charge for 15 years.

In Seattle you will also hear an ADU called a DADU — short for detached accessory dwelling unit, the usual term across Washington. The rules on this page apply either way.

Outside city limits? If your property is in unincorporated King County (not within Seattle's boundaries), different rules apply — see our King County (unincorporated) ADU rules.

Zoning & Size Requirements [1][2][3][4][5]

Yes — Seattle allows ADUs. A detached ADU can typically be up to 1,000 sq ft under current Washington and Seattle zoning rules.

See what this means for your lot — ask Seattle ADU builders for a quote →

Allowed ZonesEvery zone where housing uses are allowed (SMC 23.42.022.A): Neighborhood Residential NR1-NR3, Lowrise LR1-LR3, Midrise, Highrise, Neighborhood Commercial, Seattle Mixed and downtown zones. In the Shoreline District an ADU must also meet Chapter 23.60A. ADUs may only be accessory to a housing use, and both units must be on the same lot (23.42.022.B and F)
Max Size (Detached)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
Max Size (Attached)Same 1,000 / 1,200 sq ft cap as a detached unit (23.42.022.G). Exception: an attached ADU may exceed 1,000 sq ft if the part of the house it occupies existed on July 23, 2023 (23.42.022.H.4), so a basement or upper-floor conversion of an older house is not size-capped
Max Size (JADU)Seattle has no junior ADU category. A second unit carved out of the house is simply a second attached ADU and counts toward the two-ADU limit
Max Height32 ft in Neighborhood Residential zones, the same limit as the main house (SMC 23.44.070.A.1); a pitched roof of 4:12 or steeper may rise 5 ft more, a shed or butterfly roof 3 ft. The limit is 42 ft on lots that keep a Tier 1 or Tier 2 tree or meet the tree-point score, or for developments with three or more principal units and a 20-ft front setback (23.44.070.A.2). In Lowrise and other zones the ADU takes the height allowed for a principal unit in that zone (23.42.022.E). An accessory structure placed inside a required setback is limited to 12 ft, which is why a DADU must stay out of the setbacks
Max UnitsTwo ADUs per lot in any mix: two attached, two detached (in one or two structures), or stacked (SMC 23.42.022.C-D). ADUs count toward density, which in Neighborhood Residential zones is 1 unit per 1,250 sq ft of lot, with a floor of 4 units on any lot under 5,000 sq ft and 6 units on lots under 7,500 sq ft within a quarter mile of a major transit stop (23.44.060), so a house plus two ADUs fits on every ordinary lot. A lot may instead be developed with 4-6 principal units (townhouses, cottages, stacked flats); only two of the units on any lot may be ADUs

Setback Requirements [1][2][3][4][5]

Seattle requires a rear setback of about 5 ft for a detached ADU, and no additional parking is required. Owner-occupancy is not required — you can rent it out without living on the property.

Rear Setback5 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)
Side Setback5 ft average with a 3 ft minimum in Neighborhood Residential zones; a flat 3 ft on lots under 5,000 sq ft inside a frequent transit service area (Table A for 23.44.090). Eaves and gutters may project 2 ft if they stay 3 ft from the line. Lowrise zones use the principal-unit minimum setback with no averaging (23.42.022.E)
Front Setback15 ft on a lot with one or two dwelling units, 10 ft on a lot with three or more (Table A for 23.44.090). The table counts dwelling units, which include ADUs, so a house plus two ADUs appears to qualify for the 10-ft figure; confirm with SDCI zoning review. Nothing in the code requires the cottage to sit behind the house any more; it simply has to meet the lot's setbacks
Parking RequiredNone required for any ADU (SMC 23.42.022.I and Table B for 23.54.015, footnote 4). If you do build a space, it must be reached from an improved street, alley or easement, may not sit within 20 ft of the front lot line except on the driveway, and alley access is required where an improved alley exists (23.44.160). The state floor is the same: RCW 36.70A.681(2) bars any parking requirement for ADUs within a half mile of a major transit stop
Owner OccupancyNot required. SMC 23.42.022 contains no owner-occupancy rule and RCW 36.70A.681(1)(b) forbids one. Owners of ADUs permitted before 2019 may still have an owner-occupancy covenant recorded on title; SDCI publishes a 'Release of Owner Occupancy Covenant for ADUs' form to remove it
Min Lot SizeNo minimum lot size for an ADU. Ordinance 127211 repealed former SMC 23.44.041, whose Table A required a 3,200 sq ft lot at least 25 ft wide and 70 ft deep for a backyard cottage; SDCI Tip 116B (Aug. 2023) still prints those numbers. Lots under 5,000 sq ft without wetlands, riparian corridors or steep-slope non-disturbance areas may hold up to four units (23.44.060.C.1); lots with those critical areas get a reduced unit count (23.44.060.C.5) and lose the state ADU guarantees (RCW 36.70A.681(4))

Permit Process & Fees [6][7][8][9][10][11][12][13][14]

An ADU permit in Seattle goes through a standard permit review. No published SDCI target for ADU review. Pre-approved ADUniverse plans: 2-6 weeks to permit on most sites (SDCI and OPCD). Custom plans: SDCI's Tip 116B warns the intake appointment alone may be 2-4 months out, and 2026 industry guides report 4-8 weeks of review for a straightforward custom DADU and 12-18 months from design start to move-in overall (8-10 months with a pre-approved plan). SDCI's Construction Permit Timeline tool (April 2026) shows the actual review history of any permit since 2018. Fees range from $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).

Approval ProcessAdministrative SDCI construction permit, no Master Use Permit, no public comment period and no appeal (SDCI Tip 116B). In order: 1) research the parcel on the Seattle Services Portal for zone, ECA and shoreline flags; 2) file a Building & Land Use Pre-Application with a site plan, which returns a record number and any pre-application site visit; 3) book an intake appointment and upload PDF plans (site plan, floor plans for house and ADU, elevations, structural, energy forms); 4) pay 75 percent of plan-review and permit fees at intake; 5) SDCI zoning, ordinance/structural and other reviews with correction cycles; 6) pay the balance, permit issued, inspections, final inspection before a tenant moves in. A new DADU or a unit inside the house is a Construction Addition/Alteration permit; legalizing an existing unit is a Construction Permit to Establish Use. A separate SDCI electrical permit, a King County Public Health plumbing permit and usually a side sewer permit are needed, and SDCI reports every ADU permit to King County for the sewer capacity charge
Review TimelineNo published SDCI target for ADU review. Pre-approved ADUniverse plans: 2-6 weeks to permit on most sites (SDCI and OPCD). Custom plans: SDCI's Tip 116B warns the intake appointment alone may be 2-4 months out, and 2026 industry guides report 4-8 weeks of review for a straightforward custom DADU and 12-18 months from design start to move-in overall (8-10 months with a pre-approved plan). SDCI's Construction Permit Timeline tool (April 2026) shows the actual review history of any permit since 2018
Permit Fees$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)
Impact FeesNo City of Seattle impact fee is charged on an ADU permit, ADU floor area is excluded from Mandatory Housing Affordability payments (SMC 23.58C.040) and ADUs are exempt from public street improvement requirements (SMC 23.53.003). Budget instead for the King County sewage treatment capacity charge: 0.59 residential customer equivalent for an ADU, or $46.01 per month at the 2026 rate of $77.99 per RCE, billed quarterly for 15 years (about $8,280 total, payable early at a discount). Since Jan. 1, 2026 Seattle Public Utilities also charges water, wastewater and drainage System Development Charges when a project needs a new or larger water service or adds hard surface; the amounts are on SPU's SDC page and were not retrievable for this update
Plan CheckElectronic plan review through the Seattle Services Portal. Plans must state the number of ADUs, whether the unit is in a new or existing structure, the ADU size, total lot coverage and any parking provided (Tip 116B). The ADU must meet the Seattle Residential, Building, Energy, Stormwater and side sewer codes; a converted structure must fully meet the Energy Code; sleeping rooms need egress windows (5.7 sq ft opening, sill no higher than 44 in.); the ADU needs its own heating controls plus smoke and CO alarms. Tips 116A and 116B were last updated Aug. 18, 2023 and still print the pre-2025 rules (3,200 sq ft lot, 40 percent rear-yard rule, 650 sq ft in Lowrise), which the 2025-26 ordinances superseded
Pre-Approved PlansYes. The City's ADUniverse gallery lists 7 pre-approved DADU designs (10 floor plans, 288 to 1,000 sq ft, studio to 3 bedrooms). Using one gets a permit in 2-6 weeks on most sites and a reduced plan-review fee, but only on a Neighborhood Residential lot with no environmentally critical area and under 750 sq ft of ground disturbance (SDCI help article, May 2026). ADUniverse 2.0 selected 15 new ADU and middle-housing designs from 110 submissions in 2026; the City expects them to be available from January 2027

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Construction Costs [15][16][17][18][19]

Building an ADU in Seattle typically costs $325,000-$550,000 ($400-$650 per sq ft); a garage conversion runs $100,000-$225,000.

See what this means for your lot — ask Seattle ADU builders for a quote →

Average Build Cost$325,000-$550,000
Cost per Sq Ft$400-$650
Garage Conversion$100,000-$225,000
Prefab ADU (Installed)$300,000-$450,000

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Rental Income Potential [20][21]

A 2-bedroom ADU in Seattle rents for around $2,300-$3,000/mo, an estimated 6-8% gross rental yield on a $325,000-$450,000 detached build renting at $2,000-$2,600/mo, before vacancy, taxes, the King County capacity charge and financing; conversions of existing space at $100,000-$225,000 yield ro.

Avg Rent (1 Bedroom)$1,900-$2,600/mo
Avg Rent (2 Bedroom)$2,300-$3,000/mo
Short-Term RentalYes, with limits.
Est. Annual ROI6-8% gross rental yield on a $325,000-$450,000 detached build renting at $2,000-$2,600/mo, before vacancy, taxes, the King County capacity charge and financing; conversions of existing space at $100,000-$225,000 yield ro.

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Recent Law Changes

What's Changed Recently

  • January 21, 2026: Ordinance 127376 (CB 120993, passed Dec. 16 and signed Dec. 22, 2025) took effect, replacing the interim rules with a new Chapter 23.44 and amending SMC 23.42.022: ADU size 1,000 sq ft (up to 2 bedrooms) or 1,200 sq ft (3+ bedrooms); ADUs now count toward density, FAR and lot coverage; NR height 32 ft (42 ft with tree retention or 3+ principal units); ADU rear setback 5 ft, 0 ft on an alley; up to 4 units per lot, 6 near major transit
  • January 1, 2026: SDCI construction-permit fees rose 18 percent and the base hourly rate to $292; SDCI's example 500 sq ft DADU now pays $3,453 in plan-review and permit fees. King County's sewer capacity charge rose to $77.99 per RCE ($46.01 per month for an ADU), and Seattle Public Utilities began charging water, wastewater and drainage System Development Charges on new or larger connections
  • 2026: ADUniverse 2.0 selected 15 new pre-approved ADU and middle-housing designs from 110 submissions; the City expects them to be available for permits from January 2027. The current gallery holds 7 pre-approved DADU designs
  • June 30, 2025: Ordinance 127211 (HB 1337 compliance, signed May 7, 2025) and Ordinance 127219 (HB 1110 interim rules, signed May 28, 2025) took effect, consolidating all ADU standards into SMC 23.42.022, allowing two ADUs on every lot including two detached cottages, raising the size limit to 1,000 sq ft in every zone, repealing the old DADU section 23.44.041 (3,200 sq ft minimum lot, 25 ft width, 70 ft depth), and adding SMC 23.53.003 exempting ADUs from public street improvements
  • July 1, 2025: RCW 36.70A.681 (HB 1337, 2023) fully in force statewide: no owner-occupancy requirement, no parking requirement near major transit, impact fees on an ADU capped at half of the principal unit's, conversions of existing garages allowed regardless of setbacks, and no ban on selling an ADU separately as a condominium (mirrored in SMC 23.42.022.K)

Tips for Building an ADU in Seattle

  • Pick one of the 7 ADUniverse pre-approved designs if your lot is Neighborhood Residential, has no environmentally critical area and the cottage plus driveway disturbs under 750 sq ft: SDCI permits these in 2-6 weeks at a reduced plan-review fee, versus months for a custom set
  • Adding ADUs raises your allowed floor area, not just the unit count: on a 5,000 sq ft NR lot the FAR is 0.6 (3,000 sq ft) with one unit, 0.8 with two and 1.0 with three (Table A for 23.44.050), and lots under 5,000 sq ft always get at least 2,500 sq ft; run the FAR and 50 percent lot-coverage math before assuming two 1,000 sq ft cottages fit
  • The cottage may sit 5 ft from the rear line and on the lot line along an alley, but it has to stay out of the side and rear setbacks (structures inside a setback are capped at 12 ft) and 5 ft from the house; there is no longer a rule that it must sit behind the house
  • A garage, shed or studio that existed before July 23, 2023 can become a DADU even if it violates today's setbacks or lot coverage (SMC 23.42.022.H), and an attached ADU inside a house that existed on that date is not held to the 1,000 sq ft cap
  • Map the trees before you design: under SMC 25.11 as updated by Ordinance 127300 (effective Jan. 21, 2026) a Tier 2 tree on a Neighborhood Residential lot may be removed only if the summed tree-protection areas leave too little buildable area to reach the allowed lot coverage, and keeping a Tier 1 or Tier 2 tree instead unlocks the 42-ft height limit
  • Budget beyond the SDCI fee: the King County capacity charge is $46.01 a month for 15 years on every new ADU sewer connection (about $8,280, or less if paid off early), and a new or larger water service triggers SPU System Development Charges from 2026
  • If you want short-term rental income, the City license allows an owner who lives on the property to list the ADU, but an owner living elsewhere may list only one unit anywhere in Seattle; a long-term rental is registered instead with SDCI's Rental Registration and Inspection program (RRIO)
  • Cost basis (2026-09-19 re-verification): avg build cost: $325,000-$550,000 all-in for a new detached DADU of typical size, per three Seattle industry guides published between November and May; low-end estimates start near $250,000 and design-build firms quote $400,000-$600,000 for a mid-size cottage | cost per sqft: $400-$650 per sq ft all-in for new detached construction (Seattle design-build guide, 2025-26), including design, permits, utilities and site work; site slope, seismic engineering and a new sewer or water service push a project toward the top of the range | garage conversion: $100,000-$225,000 for a garage, basement or other conversion inside an existing structure (2026 industry guides); an existing accessory structure built before July 23, 2023 may be converted even if it violates today's setbacks or lot coverage (SMC 23.42.022.H) | prefab adu: $300,000-$450,000 installed for a one-bedroom factory-built unit once foundation, crane, utilities and permits are added, on a factory price of roughly $150,000-$220,000 (national prefab guide, May 2026); Seattle-specific installed prices are not published, so get a site-specific quote. A prefab unit still needs the full SDCI permit and a permanent foundation | avg rent 1br: $1,900-$2,600/mo. HUD's fair market rents for the current fiscal year put Seattle ZIP codes between about $1,850 in south Seattle and $2,570 in the north-end neighborhoods, with a metro-wide FMR of $2,146; a January industry guide quotes $1,800-$2,500

ADU Builders in Seattle

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Sources & References

All data on this page is sourced from official government records and verified market data. View our full methodology.

  1. Ordinance 127376 (CB 120993), signed Dec. 22, 2025: SMC 23.42.022 Accessory dwelling units (Section 21) and new Chapter 23.44 (23.44.050 floor area, .060 density, .070 height, .080 lot coverage, .090 setbacks, .100 separations, .160 parking) | Seattle City Clerk / Legistar Municipal Code — Accessed 2026-09-19
  2. Seattle Municipal Code 23.42.022 Accessory dwelling units (current codification) | Municode Municipal Code — Accessed 2026-06-28
  3. New Middle Housing Codes to Boost Housing Supply (Ordinances 127375 and 127376), March 2, 2026 | SDCI Building Connections Planning Department — Accessed 2026-09-19
  4. ADUs and Middle Housing (Ordinances 127211 and 127219 effective June 30, 2025), July 7, 2025 | SDCI Building Connections Planning Department — Accessed 2026-09-19
  5. RCW 36.70A.681 Accessory dwelling units, limitations on local regulation (HB 1337, 2023 c 334 s 4) | Washington State Legislature State Law — Accessed 2026-09-19
  6. Accessory Dwelling Units, Common Projects (permit types, exemptions, King County capacity charge, ADUniverse) | Seattle SDCI Planning Department — Accessed 2026-09-19
  7. How To Apply for a Detached Accessory Dwelling Unit Using a Pre-Approved Standard Plan (updated May 14, 2026) | Seattle Services Portal Help Center Planning Department — Accessed 2026-09-19
  8. 2026 Fee Changes summary (18 percent construction-permit increase, $292 base hourly rate, 500 sq ft DADU example $3,453) | Seattle SDCI Government Data — Accessed 2026-09-19
  9. 2026 Fee Estimator (Fee Subtitle Table D-1 and Building Valuation Data) | Seattle SDCI Government Data — Accessed 2026-09-19
  10. ADUniverse pre-approved DADU gallery and ADUniverse 2.0 notice | Seattle OPCD Planning Department — Accessed 2026-09-19
  11. Tip 116B, Establishing a Detached Accessory Dwelling Unit (updated Aug. 18, 2023; process steps and code requirements) | Seattle SDCI Planning Department — Accessed 2026-09-19
  12. Tip 310, Standard Plans (pre-approved DADU eligibility) | Seattle SDCI Planning Department — Accessed 2026-09-19
  13. About the capacity charge: 2026 rate $77.99 per RCE, ADU 0.59 RCE | King County Wastewater Treatment Division Government Data — Accessed 2026-09-19
  14. System Development Charge and Participatory Latecomer Agreement Program (SPU charges from Jan. 1, 2026), Oct. 1, 2025 | SDCI Building Connections Government Data — Accessed 2026-09-19
  15. Cost to Build a DADU in Seattle (2025-2026 Update) | Ballard Backyard Cottages industry — Accessed 2026-09-19
  16. Seattle ADU Cost 2026 by Type and Size (updated May 12, 2026) | ADU Glossary industry — Accessed 2026-09-19
  17. How Much Does It Cost to Build an ADU in Seattle? 2026 Guide (Jan. 8, 2026) | Kolmo industry — Accessed 2026-09-19
  18. Seattle ADU Guide 2026: Costs, Permits, ROI (March 9, 2026) | Nine8 Redevelopment industry — Accessed 2026-09-19
  19. Prefab ADU 2026: Brands, True Costs (updated May 19, 2026) | OwnADU industry — Accessed 2026-09-19
  20. FY 2026 Fair Market Rents and Small Area FMRs, Seattle-Bellevue HUD Metro FMR Area | HUD User Government Data — Accessed 2026-09-19
  21. Ordinance 125490 (2017), SMC Chapter 6.600 Short-Term Rentals: 6.600.040.B unit limits, 6.600.090.B $75 operator license | Seattle City Clerk / Legistar Municipal Code — Accessed 2026-09-19
  22. Council Bill 120993 / Ordinance 127376 legislative record (passed Dec. 16, 2025; signed Dec. 22, 2025) | Seattle Legistar Government Data — Accessed 2026-09-19
  23. Encouraging Backyard Cottages (ADU program, ADUniverse, annual reports) | Seattle OPCD Planning Department — Accessed 2026-09-19
  24. 2026 Fee Changes (Jan. 5, 2026) | SDCI Building Connections Planning Department — Accessed 2026-09-19
  25. Washington HB 1337 (2023) bill summary | Washington State Legislature State Law — Accessed 2026-02-24

Frequently Asked Questions

Can I build an ADU in Seattle?

Yes. ADUs are allowed in Every zone where housing uses are allowed (SMC 23.42.022.A): Neighborhood Residential NR1-NR3, Lowrise LR1-LR3, Midrise, Highrise, Neighborhood Commercial, Seattle Mixed and downtown zones. In the Shoreline District an ADU must also meet Chapter 23.60A. ADUs may only be accessory to a housing use, and both units must be on the same lot (23.42.022.B and F) in Seattle. The maximum size for a detached ADU is 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. Owner-occupancy is not required, so you can rent out the ADU.

How much does an ADU cost in Seattle?

ADU construction in Seattle typically costs $325,000-$550,000. Garage conversions run $100,000-$225,000, and prefab ADUs cost $300,000-$450,000.

How much can I rent an ADU for in Seattle?

Average ADU rents in Seattle are $1,900-$2,600/mo for a 1-bedroom and $2,300-$3,000/mo for a 2-bedroom unit.

Do I need parking for an ADU in Seattle?

None required for any ADU (SMC 23.42.022.I and Table B for 23.54.015, footnote 4). If you do build a space, it must be reached from an improved street, alley or easement, may not sit within 20 ft of the front lot line except on the driveway, and alley access is required where an improved alley exists (23.44.160). The state floor is the same: RCW 36.70A.681(2) bars any parking requirement for ADUs within a half mile of a major transit stop. Check with Seattle planning department for the latest Washington ADU parking rules, as many states have relaxed requirements in recent years.

How long does it take to get an ADU permit in Seattle?

Seattle does not publish a standard ADU review timeline. No published SDCI target for ADU review. Pre-approved ADUniverse plans: 2-6 weeks to permit on most sites (SDCI and OPCD). Custom plans: SDCI's Tip 116B warns the intake appointment alone may be 2-4 months out, and 2026 industry guides report 4-8 weeks of review for a straightforward custom DADU and 12-18 months from design start to move-in overall (8-10 months with a pre-approved plan). SDCI's Construction Permit Timeline tool (April 2026) shows the actual review history of any permit since 2018. Fees range from $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).

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Seattle ADU FAQ

Answers to every common question about building an ADU in Seattle — costs, rules, permits, rental income.

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Seattle ADU Costs

Detailed cost breakdown: construction $325,000-$550,000, garage conversion $100,000-$225,000, plus ROI analysis.

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Garage Conversion ADU Guide

Complete guide to costs ($80K-$180K), impact fee savings, and permits for garage-to-ADU conversions.

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ADU vs Mother-in-Law Suite

Legal differences that affect what you can build, how you can use it, and rental rights.

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About this information: This page is an informational summary of publicly available regulations - it is not legal, financial, or professional advice, and it is not a substitute for confirming requirements with your local planning department. Rules change frequently; always verify current requirements before designing, budgeting, or building. Last verified: 2026-09-19. Contractors we introduce are independent businesses - verify license and insurance directly before hiring. Spotted an error? Tell us - we verify against primary sources and correct quickly.

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