Complete 2026 cost breakdown: construction, permits, financing, and expected rental ROI.
Calculate your Seattle ADU cost →Building an ADU in Seattle costs between $160,000-$320,000 for new construction, with per-square-foot costs averaging $130-$270. These figures include design, permitting, site preparation, and construction. The most affordable option is a garage conversion at $75,000-$170,000, while prefab/modular ADUs fall in between at $110,000-$260,000.
| Cost Category | Estimated Range |
|---|---|
| Architectural Plans | $3,000 – $15,000 |
| Permits & Fees | $2,000-$5,000+ in SDCI permit fees (varies with valuation; hourly review rates apply) |
| Impact Fees | ADUs exempt from MHA affordability contributions and most impact fees |
| 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 Seattle — no obligation.
Get Seattle builder quotes →Permit fees in Seattle are $2,000-$5,000+ in SDCI permit fees (varies with valuation; hourly review rates apply). The review timeline is Several weeks to a few months depending on review queue, with plan check: 1-3 weeks for pre-approved (ADUniverse) plans; longer for custom. Impact fees are ADUs exempt from MHA affordability contributions and most impact fees. Pre-approved plans: Yes — ADUniverse pre-approved DADU plan library.
| 1-Bedroom Rent | $1,600-$2,100/month |
| 2-Bedroom Rent | $2,000-$2,800/month |
| Annual Rental Income (est.) | $22,200/year |
| Payback Period (est.) | 10.8 years |
| Short-Term Rental | Allowed with a Seattle STR operator license ($75/yr); a 'short-term rental' is under 30 consecutive nights, and an ADU/DADU can be a licensed STR subject to the operator/primary-residence rules |
| Overall ROI Estimate | 6-8% annual return |
A typical ADU in Seattle generates approximately $22,200 per year in rental income based on average 1-bedroom rents. At an average construction cost of $240,000, the estimated payback period is 10.8 years — before accounting for the resale value the ADU adds, which an appraiser determines from comparable sales in Seattle 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 Seattle: $75,000-$170,000.
2. Prefab/modular ADUs offer predictable pricing and faster timelines. Average: $110,000-$260,000.
3. Pre-approved plans — available in Seattle, 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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