Seattle ADU + DADU Cost Calculator
Estimate your 2026 Seattle ADU project — detached DADU, attached AADU, or basement-conversion. The calculator handles the levers Seattle ADU budgets actually break on: foundation type, sitework scope on sloped lots, utility-run distance from the main house, exception-tree removal, and the SDCI pre-approved plan track that can save $20,000+ on a DADU.
ADU Type & Size
What you're building and how big it is
Seattle SMC 23.44.041 allows up to 2 ADUs per lot (typically 1 DADU + 1 AADU) on single-family-zoned lots.
Typical DADU: 400–1,000 sf (1,000 sf cap per SMC 23.44.041). Typical AADU: 300–800 sf.
Foundation & Sitework
DADU-specific — these are the biggest cost surprises on detached builds
Steep lots (>15% grade) and tight side-yard access push almost every Seattle DADU into "heavy."
Linear feet of trench from main house to DADU for combined sewer + water + electrical. ~$180/ft. Typical Seattle yards: 20–80 ft.
Permits & Site Conditions
The accuracy levers — be honest here.
Seattle SMC 25.11 protects "exception trees" (6"+ DBH). Removal needs a permit + arborist + $2,000/tree avg. Count trees in the buildable footprint.
-40% on permit fees + skip architect ($12k savings). See Tip #314 standard plans.
Your Estimate
Configure your project and click Calculate
Enter your ADU details to get started
What this calculator captures
Three Seattle ADU bands priced against current labor rates, supplier pricing, and our bid history. All ranges include foundation, permits, and standard sitework. Pre-approved plan track can knock $20k+ off the DADU bands.
- Basement-conversion ADU
- $95,000-$180,000
- AADU (attached) 600 sf
- $180,000-$280,000
- DADU (detached) 800 sf
- $280,000-$450,000+
Convert existing basement into legal independent unit. Egress, kitchenette, full bath, sub-panel. Lowest cost path — assumes existing shell.
Attached addition or in-law suite sharing main structure + utilities. Mid-cost path — common for adding a 1-bed unit to existing home.
Separate small home in your yard. Highest cost — full new structure, separate utility runs, foundation, sitework. Pre-approved plan saves $20k+.
Using this calculator
What's the difference between an ADU, DADU, and AADU?
ADU is the umbrella term — Accessory Dwelling Unit. A DADU is detached (separate building). An AADU is attached (shares walls with the main home, like an addition or in-law suite). A basement-conversion ADU is a third path — converting existing basement space into a legal independent unit. Seattle SMC 23.44.041 allows up to 2 ADUs per single-family lot (typically 1 DADU + 1 AADU).
How much does a DADU cost in Seattle?
A typical 700–1,000 sf Seattle DADU runs $280,000–$450,000 all-in. Below that on flat lots with the SDCI pre-approved plan track; above that on steep lots, premium finishes, or long utility runs. The 1,000 sf cap per SMC 23.44.041 is the ceiling for single-DADU footprint — most Seattle DADUs hit somewhere between 600 and 1,000 sf to maximize allowable area.
What does the SDCI pre-approved DADU plan track save?
Per Tip #314, using one of the 12 SDCI pre-approved DADU plans skips the full custom-design + plan-review cycle. Typical savings: $12,000 in architect fees + ~40% off permit review fees. Total ~$18k–$25k saved on a DADU. Trade-off: you pick from a fixed catalog (Cast Architecture, Bohlin Cywinski Jackson, FiveDot, Shape Architecture, etc.) instead of fully custom.
Why are utility runs priced separately?
DADUs need their own sewer + water + electrical trenched from the main house. At ~$180/linear foot for the combined trench + pull + connect, an 80-ft run adds $14,400 over a 20-ft run. This is the #1 invisible cost driver — easy to underbudget at planning time.
What about exception trees on my lot?
Seattle SMC 25.11 protects 'exception trees' (6" diameter at breast height +). Removing one for a DADU buildable footprint requires permit + arborist + report + disposal — roughly $2,000 per tree. A 3-tree clear adds $6,000 on top of the build. Count trees in your planned footprint, not the whole lot.
Can I model a basement-conversion ADU here?
Yes — choose basement-conversion as the type. But for that specific scope the basement finishing calculator at /calculator/basement gives a more accurate number — it handles egress windows (IRC R310), ceiling-height code, and moisture remediation. We recommend using both: this calculator for the rough envelope, basement for the detailed buildout.
Keep planning
Continue the planning once you have a number.