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-$250,000 for new construction, with per-square-foot costs averaging $300-$400. These figures include design, permitting, site preparation, and construction. The most affordable option is a garage conversion at Uncommon (few homes have garages), while prefab/modular ADUs fall in between at $100,000-$150,000.
| Cost Category | Estimated Range |
|---|---|
| Architectural Plans | $3,000 – $15,000 |
| Permits & Fees | $1,000-$3,000 building permits; additional utility costs |
| Impact Fees | Utility connection fees apply |
| 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. Get up to 3 free quotes from ADU builders serving Washington — no obligation.
Get Washington builder quotes →Permit fees in Washington are $1,000-$3,000 building permits; additional utility costs. The review timeline is 6-12 months total (planning through final inspection); 8-12 months typical, with plan check: 4-6 weeks; longer for complex projects. Impact fees are Utility connection fees apply. Pre-approved plans: Standard basement conversion and detached plans available.
| 1-Bedroom Rent | $1,400-$2,000/month |
| 2-Bedroom Rent | $1,800-$2,800/month |
| Annual Rental Income (est.) | $20,400/year |
| Payback Period (est.) | 9.8 years |
| Short-Term Rental | Rental license required if renting; 30-day minimum suggested but not mandated |
| Overall ROI Estimate | 10-15 year payback; strong appreciation in DC market |
A typical ADU in Washington generates approximately $20,400 per year in rental income based on average 1-bedroom rents. At an average construction cost of $200,000, the estimated payback period is 9.8 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: Uncommon (few homes have garages).
2. Prefab/modular ADUs offer predictable pricing and faster timelines. Average: $100,000-$150,000.
3. Pre-approved plans — available in Washington, which can save $3,000-$10,000 in design costs and 2-3 months in review time.
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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