An ADU builder in Seattle is a licensed contractor who designs and constructs accessory dwelling units — detached backyard units, garage conversions, attached additions, and junior ADUs (JADUs). Because an ADU is a complete second dwelling, the builder coordinates foundation, framing, plumbing, electrical, and HVAC work and pulls it through Seattle's permit process and utility connections. Not every general contractor has built one, so the right builder is one with ADU-specific experience in Washington.
What ADU Builders Cost in Seattle [15][16][17][18][19]
Based on our Seattle cost data, expect the following ranges from local ADU builders. Quotes for the same scope typically vary 20-40% between contractors, so getting at least three bids is the single best way to control cost.
| New Detached ADU (all-in) | $325,000-$550,000 |
| Cost per Sq Ft | $400-$650 |
| Garage Conversion | $100,000-$225,000 |
| Prefab / Modular ADU (installed) | $300,000-$450,000 |
| City Permit & Plan Review | $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 Fees | No 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 |
For a full line-item breakdown, see our Seattle ADU cost guide. To confirm what you're allowed to build before you hire, review the Seattle ADU rules (max detached 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; owner-occupancy: Not 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).
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How to Vet an ADU Builder in Seattle
- License: Confirm a current contractor's license in Washington and that it's in good standing. Ask for the license number and verify it with the state board.
- ADU-specific experience: Ask how many ADUs they've completed in or near Seattle and request addresses or photos. ADU permitting and setbacks differ from a kitchen remodel.
- Portfolio & references: Get references from recent ADU clients and, if possible, visit a finished unit.
- Insurance: Verify general liability and workers' compensation coverage. Ask for a certificate of insurance.
- Written, detailed bid: Insist on an itemized, fixed-price or clearly-scoped bid — not a verbal estimate. It should state whether permits ($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)), plan check (Electronic 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), and impact fees (No 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) are included.
- Permit handling: Confirm they manage Seattle's permit process ("Administrative 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", typically 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) and who is responsible if revisions are required.
- Payment schedule: Avoid large upfront payments. A reasonable schedule ties payments to completed milestones.
Questions to Ask Before You Hire
- How many ADUs have you built in Seattle in the last two years?
- Is your bid fixed-price, and what would trigger a change order?
- Are permits, plan check, and utility connection fees included?
- What's a realistic timeline, including 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) and construction?
- Who is my point of contact and how are delays communicated?
- What warranty do you provide on workmanship?
- Can you share three recent Washington ADU client references?
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How much do ADU builders charge in Seattle?
ADU builders in Seattle typically quote $325,000-$550,000 for a new detached unit, or about $400-$650 per square foot, including design, permitting, and construction. A garage conversion runs $100,000-$225,000 and a prefab/modular ADU averages $300,000-$450,000. Add city permit and plan-review 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)) plus impact fees of No 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. Quotes for the same scope commonly vary 20-40% between builders, so collect at least three bids.
How do I find a good ADU builder in Seattle?
Look for a licensed contractor with completed ADU projects in Seattle specifically — ADUs involve local zoning, setback rules, and utility connections that general remodelers may not have handled. Verify the license, ask for an ADU-specific portfolio and references, confirm they carry liability and workers' comp insurance, and get a written fixed-price or detailed bid. Submit a request below and we'll forward it to local ADU builders serving Seattle for free quotes.
Do ADU builders in Seattle handle permits?
Most full-service ADU builders manage the permit process for you. In Seattle the approval process is "Administrative 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". 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. Confirm in writing whether permit fees, plan check (Electronic 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), and impact fees are included in the bid or billed separately.
How long does it take to build an ADU in Seattle?
Plan for the 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 in Seattle) plus construction. Most detached ADUs take 4-8 months of construction after permits are issued; garage conversions and prefab units are often faster. Ask each builder for a realistic schedule and what could delay it.
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