An ADU builder in Washington 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 Washington's permit process and utility connections. Not every general contractor has built one, so the right builder is one with ADU-specific experience in District of Columbia.
What ADU Builders Cost in Washington [10][11][12][13][14]
Based on our Washington 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) | $150,000-$550,000 |
| Cost per Sq Ft | $200-$500 |
| Garage Conversion | $110,000-$250,000 |
| Prefab / Modular ADU (installed) | $150,000-$300,000 |
| City Permit & Plan Review | New construction: $0.03 per cu ft of building plus a 10% enhanced fee and a $0.002/sq ft Green Building Fee. Alterations such as a basement conversion: $30 + 2% of construction value plus 10%, and a 0.13% green fee; a 50% filing deposit is paid up front (DOB Building Permit Fee Schedule). |
| Impact Fees | None (DC levies no development impact fee); only DOB's 10% enhanced fee and Green Building Fee apply, plus DC Water and DDOT charges if you add a service line or work in public space |
For a full line-item breakdown, see our Washington ADU cost guide. To confirm what you're allowed to build before you hire, review the Washington ADU rules (max detached size: Accessory-building footprint: the greater of 650 sq ft or 30% of the required rear yard in R-1/R-2, 550 sq ft in R-3 and the RF zones (D §5003, E §5003; raised from 450 sq ft on 7/10/2026). It counts toward the 40% lot occupancy (60% for an R-3 row house, D §210.1), may hold one dwelling unit and no roof deck (U §253.8(e)).; owner-occupancy: Required and non-waivable: the owner must live in either the house or the accessory apartment for as long as the apartment exists (U §253.5, §253.10(a)); the BZA may waive at most two other §253 conditions. Occupancy of the apartment is capped at 3 people (6 for house plus apartment in the GT zones) (U §253.6). RF-zone second principal units carry no owner-occupancy rule.).
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How to Vet an ADU Builder in Washington
- License: Confirm a current contractor's license in District of Columbia 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 Washington 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 (New construction: $0.03 per cu ft of building plus a 10% enhanced fee and a $0.002/sq ft Green Building Fee. Alterations such as a basement conversion: $30 + 2% of construction value plus 10%, and a 0.13% green fee; a 50% filing deposit is paid up front (DOB Building Permit Fee Schedule).), plan check (30 business days per DOB review cycle (DOB Service Level Agreements, Feb 2024); the next-day Homeowner's Center and Digital Walk-Thru tracks exclude new dwelling units), and impact fees (None (DC levies no development impact fee); only DOB's 10% enhanced fee and Green Building Fee apply, plus DC Water and DDOT charges if you add a service line or work in public space) are included.
- Permit handling: Confirm they manage Washington's permit process ("The Department of Buildings (DOB) issues the building permit through the Citizen Access Portal and ProjectDox; a new dwelling unit is a 'filed project' reviewed by zoning, structural, MEP, fire, green/energy and, in historic districts, OP Historic Preservation. Exterior work needs a DC Surveyor plat; the finished unit needs a Certificate of Occupancy; renting needs a DLCP Two-Family Rental Basic Business License.", typically DOB's published service level is 30 business days per review cycle for filed projects (new construction, additions, change of use); DOB publishes no end-to-end figure. DC builders quote 4-9 months for a basement unit and longer for a new carriage house.) 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 Washington 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 (DOB's published service level is 30 business days per review cycle for filed projects (new construction, additions, change of use); DOB publishes no end-to-end figure. DC builders quote 4-9 months for a basement unit and longer for a new carriage house.) 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 District of Columbia ADU client references?
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