A Seattle builder's October 2025 guide puts DADUs at about $350 to $500 per square foot, with garage conversions around $180,000 to $250,000, basement ADUs around $200,000 to $300,000 and detached cottages around $300,000 to $500,000 or more. We found no independent, dated survey for Seattle alone, as of November 2026.
Published ranges
| ADU type | Published range |
|---|---|
| Garage conversion | About $180,000 to $250,000 |
| Basement ADU | About $200,000 to $300,000 |
| Detached backyard cottage | About $300,000 to $500,000 or more |
| Per square foot, DADU | About $350 to $500 |
These are published ranges, not a quote for your lot.
What moves the number?
- Utility runs. Distance to the sewer and whether the electrical panel needs an upgrade.
- Foundation and soil. Slopes, expansive soil and tree roots.
- Size and finishes. Kitchens and bathrooms carry the most cost per square foot.
- Access. Narrow side yards slow every delivery.
- Fees. Plan check and permit fees, plus utility and drainage requirements.
- Slopes and drainage. Hillside lots need more foundation and stormwater work.
What lowers it?
- Using a pre-approved DADU plan where it fits the lot, to save design time.
- A simple roofline and a plan that fits the setbacks without custom engineering.
- Converting existing space instead of building new, when the structure is sound.
Sources, checked November 2026: New Image Construction Management, "Seattle DADU Cost: How Much Does It Cost to Build an ADU in 2025?" (October 2025). Published ranges, not a quote.
Updated November 2026