| ADU Type | Cost / sq ft | Typical Size | Typical Total |
|---|---|---|---|
| Garage conversion | $150–$300 | 400–600 sq ft | $80,000–$180,000 |
| Basement conversion | $150–$300 | 500–800 sq ft | $90,000–$220,000 |
| Attached addition | $250–$400 | 400–700 sq ft | $120,000–$260,000 |
| Detached cottage (DADU) | $300–$500 | 500–1,000 sq ft | $175,000–$450,000 |
| Phase | Share of Budget | What’s Included |
|---|---|---|
| Design & engineering | 8–12% | Plans, structural engineering, energy calcs |
| Permits & fees | 2–6% | Building permit, plan review, utility fees |
| Site & foundation | 15–20% | Excavation, utilities trenching, foundation |
| Structure & envelope | 25–30% | Framing, roofing, siding, windows |
| MEP systems | 15–18% | Electrical, plumbing, HVAC, water heater |
| Interior finishes | 20–25% | Insulation, drywall, kitchen, bath, flooring |
| City | Multiplier | Why |
|---|---|---|
| Everett | 1.00 | Baseline — strong contractor market, moderate fees |
| Seattle | 1.12 | Higher labor rates and permit fees |
| Lynnwood | 1.03 | Slightly above Everett baseline |
| Edmonds | 1.05 | Design review and site constraints |
| Mill Creek | 1.02 | HOA and design standards |
| Marysville | 0.97 | Lower fees and land prep costs |
Sources: published Puget Sound construction cost surveys and regional contractor pricing data, averaged for 2026. Last verified: January 2026. These are planning figures — not an ABM Construction quote. Run your own scenario: construction cost calculator →
What is the cheapest way to build an ADU?
Converting existing space — a garage or basement — at $150–$300/sq ft. The structure already exists, so your budget goes to finishes and systems instead of foundation and shell.
Does an ADU pay for itself?
Often, yes. A $250,000 DADU renting at $1,800/mo generates about $21,600/yr gross — frequently covering financing costs while building equity.
What extra costs should I expect beyond construction?
Budget for design/engineering (8–12% of construction), permits and fees ($5,000–$15,000 depending on city), utility connections, and landscaping.
Are these numbers an ABM quote?
No — this guide uses published 2026 regional averages for the Puget Sound. Your actual project depends on your lot, design and finishes; we provide exact pricing after a free lot check.