An ADU builder in Woodinville 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 Woodinville'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 Woodinville [21][22][23][24]
Based on our Woodinville 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 | 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 |
| Impact Fees | School impact fee: ADUs approved under WMC 21.41.020 are fully exempt (WMC 3.38.040(3), Ord. 800, 2025); a new three-bedroom house pays $17,550 (WMC 3.38.060), so the exemption is real money. Park impact fee: an ADU pays 50% of the fee for the largest primary unit on the lot (WMC 3.36.080(2), Ord. 811, 4 Aug 2026); the schedule is $2,054 for a unit of 1,500 sq ft or less, $4,886 from over 1,500 to under 2,300 sq ft and $7,718 at 2,300 sq ft and over (WMC 3.36.060(2)), so an ADU behind a 2,000 sq ft house owes $2,443 and behind a 2,400 sq ft house $3,859. Transportation impact fee: $3,760 per p.m. peak-hour person trip (WMC 3.39.085, Ord. 772, 2024), trips predicted from the ITE Trip Generation manual (WMC 3.39.090(4)); Chapter 3.39 has no ADU exemption, but RCW 36.70A.681(1)(a) caps any ADU impact fee at 50% of the principal unit's, a cap RCW 36.70A.681(4) switches off on a critical-area lot, where only the city's own 3.36.080(2) and 3.38.040(3) apply. You may ask for a non-binding advance calculation (WMC 3.39.090(3)). Fees are fixed at building permit issuance; deferral to final inspection against a recorded lien is written for one-unit dwellings and single-family construction (WMC 3.36.060(3), 3.39.090(2)), so ask whether an ADU permit qualifies |
For a full line-item breakdown, see our Woodinville ADU cost guide. To confirm what you're allowed to build before you hire, review the Woodinville ADU rules (max detached size: 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%; owner-occupancy: Not required. WMC 21.41.020 has no owner-occupancy condition, and RCW 36.70A.681(1)(b) bars a city from requiring the owner to live in the ADU or the main house. On a critical-area lot the state bar switches off (RCW 36.70A.681(4)) but Woodinville's code still imposes nothing. RCW 36.70A.681(1)(k) requires the city to allow an ADU to be sold as a condominium unit (again, not on critical-area lots); WMC 21.41.020 is silent on it, so ask Development Services about the process).
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How to Vet an ADU Builder in Woodinville
- 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 Woodinville 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 (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), plan check (Standard building plan review under Chapter 21.62 WMC. Prefabricated units are allowed on a permanent foundation (WMC 21.41.020(4)(c)). Conversions of existing enclosed structures, detached garages included, are allowed even where the structure breaks current coverage or setback rules, provided it and the nonconformance were legally established (WMC 21.41.020(4)(a)). The Director may approve deviations from zoning standards for accessibility features (WMC 21.41.020(3)(e)). If the lot has a wetland, stream, steep slope, flood area or critical aquifer recharge area, expect a critical areas report (WMC 21.51.060), mitigation sequencing (WMC 21.51.050) and the 10-ft building setback from any riparian zone or wetland buffer (WMC 21.51.050(3)). Structural review sent to an outside consultant is billed at cost, with an advance deposit if it is expected to exceed $5,000 (2026 fee schedule, note 4)), and impact fees (School impact fee: ADUs approved under WMC 21.41.020 are fully exempt (WMC 3.38.040(3), Ord. 800, 2025); a new three-bedroom house pays $17,550 (WMC 3.38.060), so the exemption is real money. Park impact fee: an ADU pays 50% of the fee for the largest primary unit on the lot (WMC 3.36.080(2), Ord. 811, 4 Aug 2026); the schedule is $2,054 for a unit of 1,500 sq ft or less, $4,886 from over 1,500 to under 2,300 sq ft and $7,718 at 2,300 sq ft and over (WMC 3.36.060(2)), so an ADU behind a 2,000 sq ft house owes $2,443 and behind a 2,400 sq ft house $3,859. Transportation impact fee: $3,760 per p.m. peak-hour person trip (WMC 3.39.085, Ord. 772, 2024), trips predicted from the ITE Trip Generation manual (WMC 3.39.090(4)); Chapter 3.39 has no ADU exemption, but RCW 36.70A.681(1)(a) caps any ADU impact fee at 50% of the principal unit's, a cap RCW 36.70A.681(4) switches off on a critical-area lot, where only the city's own 3.36.080(2) and 3.38.040(3) apply. You may ask for a non-binding advance calculation (WMC 3.39.090(3)). Fees are fixed at building permit issuance; deferral to final inspection against a recorded lien is written for one-unit dwellings and single-family construction (WMC 3.36.060(3), 3.39.090(2)), so ask whether an ADU permit qualifies) are included.
- Permit handling: Confirm they manage Woodinville's permit process ("A building permit processed as a Type 1 administrative decision with no hearing, unless a SEPA threshold determination is needed, which makes it Type 2 (WMC 21.82.010(3)). Apply through the online Permit Portal or at the Development Services Permit Center, 17301 133rd Ave NE, 425-489-2754; the first pre-application meeting is free (2026 fee schedule). Two things add review: a critical area or buffer on the lot, which needs a critical areas report (WMC 21.51.060) and a critical area determination ($618) or alteration review ($2,805) in 2026; and, in R-1 to R-8, a third unit built onto the outside of a building, which triggers Chapter 21.38 design review inside the same Type 1 decision (WMC 21.38.010, 21.38.020). Street frontage improvements are not required for an ADU unless they would be required anyway (WMC 21.63.020(1)(b)). Electrical work is permitted separately; ask the Permit Center which agency issues it", typically WMC 21.80.210 (Ord. 782, 2025): the city must say whether the application is complete within 28 days and decide a construction permit within 120 days of the completeness date; the clock pauses while it waits for your corrections. Plan review covers two resubmittals; a critical areas report adds time) 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 Woodinville 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 (WMC 21.80.210 (Ord. 782, 2025): the city must say whether the application is complete within 28 days and decide a construction permit within 120 days of the completeness date; the clock pauses while it waits for your corrections. Plan review covers two resubmittals; a critical areas report adds time) 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 Woodinville?
ADU builders in Woodinville 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 (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) plus impact fees of School impact fee: ADUs approved under WMC 21.41.020 are fully exempt (WMC 3.38.040(3), Ord. 800, 2025); a new three-bedroom house pays $17,550 (WMC 3.38.060), so the exemption is real money. Park impact fee: an ADU pays 50% of the fee for the largest primary unit on the lot (WMC 3.36.080(2), Ord. 811, 4 Aug 2026); the schedule is $2,054 for a unit of 1,500 sq ft or less, $4,886 from over 1,500 to under 2,300 sq ft and $7,718 at 2,300 sq ft and over (WMC 3.36.060(2)), so an ADU behind a 2,000 sq ft house owes $2,443 and behind a 2,400 sq ft house $3,859. Transportation impact fee: $3,760 per p.m. peak-hour person trip (WMC 3.39.085, Ord. 772, 2024), trips predicted from the ITE Trip Generation manual (WMC 3.39.090(4)); Chapter 3.39 has no ADU exemption, but RCW 36.70A.681(1)(a) caps any ADU impact fee at 50% of the principal unit's, a cap RCW 36.70A.681(4) switches off on a critical-area lot, where only the city's own 3.36.080(2) and 3.38.040(3) apply. You may ask for a non-binding advance calculation (WMC 3.39.090(3)). Fees are fixed at building permit issuance; deferral to final inspection against a recorded lien is written for one-unit dwellings and single-family construction (WMC 3.36.060(3), 3.39.090(2)), so ask whether an ADU permit qualifies. 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 Woodinville?
Look for a licensed contractor with completed ADU projects in Woodinville 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 Woodinville for free quotes.
Do ADU builders in Woodinville handle permits?
Most full-service ADU builders manage the permit process for you. In Woodinville the approval process is "A building permit processed as a Type 1 administrative decision with no hearing, unless a SEPA threshold determination is needed, which makes it Type 2 (WMC 21.82.010(3)). Apply through the online Permit Portal or at the Development Services Permit Center, 17301 133rd Ave NE, 425-489-2754; the first pre-application meeting is free (2026 fee schedule). Two things add review: a critical area or buffer on the lot, which needs a critical areas report (WMC 21.51.060) and a critical area determination ($618) or alteration review ($2,805) in 2026; and, in R-1 to R-8, a third unit built onto the outside of a building, which triggers Chapter 21.38 design review inside the same Type 1 decision (WMC 21.38.010, 21.38.020). Street frontage improvements are not required for an ADU unless they would be required anyway (WMC 21.63.020(1)(b)). Electrical work is permitted separately; ask the Permit Center which agency issues it" with a typical review timeline of WMC 21.80.210 (Ord. 782, 2025): the city must say whether the application is complete within 28 days and decide a construction permit within 120 days of the completeness date; the clock pauses while it waits for your corrections. Plan review covers two resubmittals; a critical areas report adds time. Confirm in writing whether permit fees, plan check (Standard building plan review under Chapter 21.62 WMC. Prefabricated units are allowed on a permanent foundation (WMC 21.41.020(4)(c)). Conversions of existing enclosed structures, detached garages included, are allowed even where the structure breaks current coverage or setback rules, provided it and the nonconformance were legally established (WMC 21.41.020(4)(a)). The Director may approve deviations from zoning standards for accessibility features (WMC 21.41.020(3)(e)). If the lot has a wetland, stream, steep slope, flood area or critical aquifer recharge area, expect a critical areas report (WMC 21.51.060), mitigation sequencing (WMC 21.51.050) and the 10-ft building setback from any riparian zone or wetland buffer (WMC 21.51.050(3)). Structural review sent to an outside consultant is billed at cost, with an advance deposit if it is expected to exceed $5,000 (2026 fee schedule, note 4)), and impact fees are included in the bid or billed separately.
How long does it take to build an ADU in Woodinville?
Plan for the permit review (WMC 21.80.210 (Ord. 782, 2025): the city must say whether the application is complete within 28 days and decide a construction permit within 120 days of the completeness date; the clock pauses while it waits for your corrections. Plan review covers two resubmittals; a critical areas report adds time in Woodinville) 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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