Complete 2026 cost breakdown: construction, permits, financing, and expected rental ROI.
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 $0-$0, 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 |
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 taking approximately 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 property value appreciation, which typically adds 20-35% to home value.
1. Garage conversions save 40-60% vs new construction since the shell already exists. Average cost in Washington: $0-$0.
2. Prefab/modular ADUs offer predictable pricing and faster timelines. Average: $100,000-$150,000.
3. Pre-approved plans (Standard basement conversion and detached plans available) can save $3,000-$10,000 in design costs and 2-3 months in review time.
4. State grants — California's CalHFA program offers up to $40K. Check your state for available 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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