An ADU builder in Bothell 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 Bothell'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 Bothell [9]
Based on our Bothell 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) | $235,000-$430,000 |
| Cost per Sq Ft | $290-$480 |
| Garage Conversion | $110,000-$235,000 |
| Prefab / Modular ADU (installed) | $170,000-$330,000 |
| City Permit & Plan Review | At submittal (2026 fees, from the city's ADU checklist): a plan check fee based on project valuation, a Single Family Planning Review fee of $231.00, and a 5 percent technology fee. At permit issuance: the permit fee, plumbing, mechanical and state fees; a Residential Site Review and Inspection fee of $834.00 where you are constructing an addition or a new building; a stormwater facility charge if you are adding square footage; and the 5 percent technology fee on everything except the state fee, impact fees and facility charges. For a new detached ADU the plumbing and mechanical permit fees are each 8 percent of the building permit fee, 16 percent in total; for a remodel or addition creating an ADU they are a base fee of $82.00 plus a per-fixture fee, most residential fixtures running $27.00 each |
| Impact Fees | Traffic, fire and park impact fees apply to a new dwelling unit, and Bothell has written the state cap into its own code: "The city will not assess impact fees on the construction of accessory dwelling units that are greater than 50 percent of the impact fees that would be imposed on the principal unit" (Bothell Municipal Code 12.14.135(G)), tracking RCW 36.70A.681(1)(a). Current dollar amounts are not published on the ADU checklist — ask Development Services at 425-806-6400 for the impact fee form, and hold them to the 50 percent cap |
For a full line-item breakdown, see our Bothell ADU cost guide. To confirm what you're allowed to build before you hire, review the Bothell ADU rules (max detached size: 1,200 sq ft. "An accessory dwelling unit may be developed up to 1,200 square feet" (Bothell Municipal Code 12.14.135(D)(1)) — 200 sq ft above the 1,000 sq ft floor state law guarantees (RCW 36.70A.681(1)(f)). Conversion of an existing space may exceed 1,200 sq ft as approved by the community development director, provided it does not exceed 40 percent of the primary dwelling unit's gross floor area. Critically, "Detached accessory dwelling units are exempt from the provisions of BMC 12.14.130 that limit the size of accessory buildings to five percent of the lot area" (BMC 12.14.135(D)(2)) — without that exemption a 1,200 sq ft detached unit would need a 24,000 sq ft lot; owner-occupancy: Not required. BMC 12.14.135 contains no owner-occupancy condition, no rental restriction and no recorded covenant. Bothell could not impose one in any event: "The city or county may not require the owner of a lot on which there is an accessory dwelling unit to reside in or occupy the accessory dwelling unit or another housing unit on the same lot" (RCW 36.70A.681(1)(b))).
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How to Vet an ADU Builder in Bothell
- 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 Bothell 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 (At submittal (2026 fees, from the city's ADU checklist): a plan check fee based on project valuation, a Single Family Planning Review fee of $231.00, and a 5 percent technology fee. At permit issuance: the permit fee, plumbing, mechanical and state fees; a Residential Site Review and Inspection fee of $834.00 where you are constructing an addition or a new building; a stormwater facility charge if you are adding square footage; and the 5 percent technology fee on everything except the state fee, impact fees and facility charges. For a new detached ADU the plumbing and mechanical permit fees are each 8 percent of the building permit fee, 16 percent in total; for a remodel or addition creating an ADU they are a base fee of $82.00 plus a per-fixture fee, most residential fixtures running $27.00 each), plan check (Plan check fee is based on project valuation. Plans must be no larger than 24 by 36 inches, drawn to scale, and the site plan must show hard surface coverage calculations under BMC 12.14.140(B), distances from existing and proposed structures to property lines, footing and roof drain location and discharge method, and any environmentally sensitive areas on the site, adjacent to it, or within 200 feet — wetlands, steep slopes, top and toe of slope, rivers and streams. That last item is not busywork: critical areas are what decide whether you get two ADUs or one under BMC 12.14.135(C). New detached ADUs need full Washington State Energy Code compliance worksheets; King County or Snohomish Health District approval is required where the property is on septic and you are adding square footage or bedrooms), and impact fees (Traffic, fire and park impact fees apply to a new dwelling unit, and Bothell has written the state cap into its own code: "The city will not assess impact fees on the construction of accessory dwelling units that are greater than 50 percent of the impact fees that would be imposed on the principal unit" (Bothell Municipal Code 12.14.135(G)), tracking RCW 36.70A.681(1)(a). Current dollar amounts are not published on the ADU checklist — ask Development Services at 425-806-6400 for the impact fee form, and hold them to the 50 percent cap) are included.
- Permit handling: Confirm they manage Bothell's permit process ("A building permit, applied for online through MyBuildingPermit.com: select Building, then Single Family Residential, then Activity Type, then Accessory Dwelling Unit (City of Bothell Development Services, Accessory Dwelling Unit submittal checklist, form 3f, updated December 2025). There is no conditional use permit, no hearing and no recorded covenant. An assigned address letter from the City must be obtained first (form 54), and a water and sewer availability certificate plus a fire flow availability letter are required where you are adding square footage — if Bothell is your water provider, apply for Water Sewer Availability (form 36) first and allow up to two weeks. Where an ADU already exists without an approved application, the owner must apply to the city for approval, and where it was built without a building permit the city will require a building inspection to confirm the structure is sound and meets BMC 12.14.135 and BMC Title 20; the ADU application fee covers that inspection (BMC 12.14.135(F)). State law bars the city from requiring public street improvements as a condition of permitting an ADU (RCW 36.70A.681(1)(l)). Bothell City Hall, 18415 101st Ave NE: 425-806-6400", typically Confirm with Bothell Development Services at 425-806-6400 — no ADU review timeline is published on the city's ADU submittal checklist. Budget separately for the water and sewer availability certificate, which the checklist puts at up to two weeks where the City of Bothell is the provider) 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 Bothell 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 (Confirm with Bothell Development Services at 425-806-6400 — no ADU review timeline is published on the city's ADU submittal checklist. Budget separately for the water and sewer availability certificate, which the checklist puts at up to two weeks where the City of Bothell is the provider) 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 Bothell?
ADU builders in Bothell typically quote $235,000-$430,000 for a new detached unit, or about $290-$480 per square foot, including design, permitting, and construction. A garage conversion runs $110,000-$235,000 and a prefab/modular ADU averages $170,000-$330,000. Add city permit and plan-review fees (At submittal (2026 fees, from the city's ADU checklist): a plan check fee based on project valuation, a Single Family Planning Review fee of $231.00, and a 5 percent technology fee. At permit issuance: the permit fee, plumbing, mechanical and state fees; a Residential Site Review and Inspection fee of $834.00 where you are constructing an addition or a new building; a stormwater facility charge if you are adding square footage; and the 5 percent technology fee on everything except the state fee, impact fees and facility charges. For a new detached ADU the plumbing and mechanical permit fees are each 8 percent of the building permit fee, 16 percent in total; for a remodel or addition creating an ADU they are a base fee of $82.00 plus a per-fixture fee, most residential fixtures running $27.00 each) plus impact fees of Traffic, fire and park impact fees apply to a new dwelling unit, and Bothell has written the state cap into its own code: "The city will not assess impact fees on the construction of accessory dwelling units that are greater than 50 percent of the impact fees that would be imposed on the principal unit" (Bothell Municipal Code 12.14.135(G)), tracking RCW 36.70A.681(1)(a). Current dollar amounts are not published on the ADU checklist — ask Development Services at 425-806-6400 for the impact fee form, and hold them to the 50 percent cap. 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 Bothell?
Look for a licensed contractor with completed ADU projects in Bothell 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 Bothell for free quotes.
Do ADU builders in Bothell handle permits?
Most full-service ADU builders manage the permit process for you. In Bothell the approval process is "A building permit, applied for online through MyBuildingPermit.com: select Building, then Single Family Residential, then Activity Type, then Accessory Dwelling Unit (City of Bothell Development Services, Accessory Dwelling Unit submittal checklist, form 3f, updated December 2025). There is no conditional use permit, no hearing and no recorded covenant. An assigned address letter from the City must be obtained first (form 54), and a water and sewer availability certificate plus a fire flow availability letter are required where you are adding square footage — if Bothell is your water provider, apply for Water Sewer Availability (form 36) first and allow up to two weeks. Where an ADU already exists without an approved application, the owner must apply to the city for approval, and where it was built without a building permit the city will require a building inspection to confirm the structure is sound and meets BMC 12.14.135 and BMC Title 20; the ADU application fee covers that inspection (BMC 12.14.135(F)). State law bars the city from requiring public street improvements as a condition of permitting an ADU (RCW 36.70A.681(1)(l)). Bothell City Hall, 18415 101st Ave NE: 425-806-6400". On timing: Confirm with Bothell Development Services at 425-806-6400 — no ADU review timeline is published on the city's ADU submittal checklist. Budget separately for the water and sewer availability certificate, which the checklist puts at up to two weeks where the City of Bothell is the provider. Confirm in writing whether permit fees, plan check (Plan check fee is based on project valuation. Plans must be no larger than 24 by 36 inches, drawn to scale, and the site plan must show hard surface coverage calculations under BMC 12.14.140(B), distances from existing and proposed structures to property lines, footing and roof drain location and discharge method, and any environmentally sensitive areas on the site, adjacent to it, or within 200 feet — wetlands, steep slopes, top and toe of slope, rivers and streams. That last item is not busywork: critical areas are what decide whether you get two ADUs or one under BMC 12.14.135(C). New detached ADUs need full Washington State Energy Code compliance worksheets; King County or Snohomish Health District approval is required where the property is on septic and you are adding square footage or bedrooms), and impact fees are included in the bid or billed separately.
How long does it take to build an ADU in Bothell?
Plan for the permit review (Confirm with Bothell Development Services at 425-806-6400 — no ADU review timeline is published on the city's ADU submittal checklist. Budget separately for the water and sewer availability certificate, which the checklist puts at up to two weeks where the City of Bothell is the provider in Bothell) 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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