Complete 2026 cost breakdown: construction, permits, financing, and expected rental ROI.
Calculate your Washington ADU cost →Building an ADU in Washington costs between $150,000-$550,000 for new construction, with per-square-foot costs averaging $200-$500. These figures include design, permitting, site preparation, and construction. The most affordable option is a garage conversion at $110,000-$250,000, while prefab/modular ADUs fall in between at $150,000-$300,000.
| Cost Category | Estimated Range |
|---|---|
| Architectural Plans | $3,000 – $15,000 |
| Permits & Fees | 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 |
| Site Prep & Foundation | $10,000 – $35,000 |
| Framing & Structure | $15,000 – $45,000 |
| Plumbing | $8,000 – $20,000 |
| Electrical | $5,000 – $15,000 |
| HVAC | $3,000 – $8,000 |
| Interior Finishes | $15,000 – $40,000 |
Quotes for the same ADU commonly vary 20–40% between builders. We pass your request to up to 5 local builders serving Washington, where we have them — free, no obligation.
Get Washington builder quotes →Permit fees in Washington are 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).. The review timeline is 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., with 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. Impact fees are 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. Pre-approved plans: None: DC has no pre-approved ADU plan program.
| 1-Bedroom Rent | $1,500-$3,300/month |
| 2-Bedroom Rent | $1,650-$3,650/month |
| Annual Rental Income (est.) | $28,800/year |
| Payback Period (est.) | 12.2 years |
| Short-Term Rental | Yes with limits: a Short-Term Rental endorsement on a Basic Business License, the property must be the host's primary residence, and an in-law suite qualifies (D.C. |
| Overall ROI Estimate | 6-17 year simple payback: a $150,000 basement unit at $2,200/mo (HUD FY2027 1BR FMR $2,204) recoups in about 6 years, a $500,000 carriage house at $2,500/mo in about 17 |
A typical ADU in Washington generates approximately $28,800 per year in rental income based on average 1-bedroom rents. At an average construction cost of $350,000, the estimated payback period is 12.2 years — before accounting for the resale value the ADU adds, which an appraiser determines from comparable sales in Washington and varies by market (often less than build cost).
1. Garage conversions save 40-60% vs new construction since the shell already exists. Average cost in Washington: $110,000-$250,000.
2. Prefab/modular ADUs offer predictable pricing and faster timelines. Average: $150,000-$300,000.
3. Pre-approved plans — available in Washington, which can save $3,000-$10,000 in design costs and 2-3 months in review time. See which cities offer pre-approved ADU plans.
4. State grants — California's CalHFA ADU grant (up to $40K) is currently paused; check calhfa.ca.gov/adu for status and your state for active programs.
5. Get multiple bids — ADU contractor pricing varies 20-40% for the same project. We recommend getting at least 3 quotes.
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