An ADU builder in Bellevue 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 Bellevue'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 Bellevue [16][17][18][19]
Based on our Bellevue 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 | 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 |
| Impact Fees | Transportation impact fee: single-family rate $8,185 per dwelling from Jan. 1, 2026 ($8,430 from Jan. 1, 2027, Ord. 6888); state law caps an ADU at 50% of the principal unit's fee (RCW 36.70A.681(1)(a)), so expect about $4,092 at most - the 2026 schedule has no ADU line, confirm the category with Transportation Development Review (425-452-4236). School impact fee: only in the Issaquah ($4,728 per single-family unit) and Renton ($1,003) school district parts of the city, and an ADU pays 50% (BCC 22.18.100, Ord. 6852); no school impact fee in the Bellevue School District. ADUs are exempt from street frontage improvements (BCC 14.60.110.G). Water and sewer connection charges apply separately - see Utilities Permit Fees |
For a full line-item breakdown, see our Bellevue ADU cost guide. To confirm what you're allowed to build before you hire, review the Bellevue ADU rules (max detached size: 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; owner-occupancy: Not required. Bellevue removed its owner-occupancy rule with Ord. 6746 on July 17, 2023 (City ADU Reform page), and RCW 36.70A.681(1)(b) now forbids any such requirement statewide).
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How to Vet an ADU Builder in Bellevue
- 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 Bellevue 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 (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), plan check (Standard building plan review by Development Services under the BS permit; reviewers charge hours against the deposit, and the middle-housing amendments removed design-review and design-matching controls for ADUs (Ord. 6746; RCW 36.70A.681(1)(h)). A pre-application meeting is optional), and impact fees (Transportation impact fee: single-family rate $8,185 per dwelling from Jan. 1, 2026 ($8,430 from Jan. 1, 2027, Ord. 6888); state law caps an ADU at 50% of the principal unit's fee (RCW 36.70A.681(1)(a)), so expect about $4,092 at most - the 2026 schedule has no ADU line, confirm the category with Transportation Development Review (425-452-4236). School impact fee: only in the Issaquah ($4,728 per single-family unit) and Renton ($1,003) school district parts of the city, and an ADU pays 50% (BCC 22.18.100, Ord. 6852); no school impact fee in the Bellevue School District. ADUs are exempt from street frontage improvements (BCC 14.60.110.G). Water and sewer connection charges apply separately - see Utilities Permit Fees) are included.
- Permit handling: Confirm they manage Bellevue's permit process ("By-right building permit: a one- and two-family residence (BS) permit for each building, filed online at MyBuildingPermit.com with PDF plans (City 'Middle Housing Building' permit guide). A separate Clearing and Grading (GD) site permit is required when the project has more than one building with dwelling units. No land-use review, ADU registration or neighbour notice applies (removed by Ord. 6746, 2023). Structures with 3-6 units use the Middle Housing (MH) permit instead", typically The City publishes no target review time for a BS permit; plan review is billed hourly against a submittal deposit, so timing follows the review queue. Ask Permit Processing (425-452-6800) for the current first-review estimate) 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 Bellevue 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 (The City publishes no target review time for a BS permit; plan review is billed hourly against a submittal deposit, so timing follows the review queue. Ask Permit Processing (425-452-6800) for the current first-review estimate) 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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