Seattle ADU + DADU Cost Calculator

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 23.42.022 allows up to 2 ADUs per lot (typically 1 DADU + 1 AADU).

A 600–1,000 sf Seattle DADU prices at about $199,000–$301,000 in this calculator with standard finishes, a crawl-space foundation, standard sitework, a 40-ft utility run, design and permits. 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 23.42.022 (1,200 sf with three or more bedrooms) is the ceiling for a single DADU's floor area — most Seattle DADUs hit somewhere between 600 and 1,000 sf to maximize allowable area.

An ADU recovers about 56% of its construction cost in resale value in the Seattle / Pacific Northwest market, per Remodeling Magazine Cost vs. Value data — and unlike most remodels it can also produce ongoing rental income, since Washington HB 1337 bars cities from requiring owner-occupancy. The SDCI pre-approved DADU plan track (about $9,700 in savings in this calculator) improves the payback math further.

Yes. Seattle no longer requires owner-occupancy to permit an ADU — Washington HB 1337 bars that condition — so a homeowner can rent out both the main house and the ADU at the same time. A DADU up to the 1,000 sq ft cap of SMC 23.42.022 can operate as a legal long-term rental; short-term-rental (Airbnb / VRBO) use can still carry separate city conditions.

Per Tip #314, using one of the 12 SDCI pre-approved DADU plans skips the full custom-design + plan-review cycle. The calculator credits the $6,000 design fee and takes 40% off permit review fees — about $9,700 saved on a DADU, contingency included. Trade-off: you pick from a fixed catalog (Cast Architecture, Bohlin Cywinski Jackson, FiveDot, Shape Architecture, etc.) instead of fully custom.

DADUs need their own sewer + water + electrical trenched from the main house. At about $194 per linear foot for the combined trench + pull + connect (contingency included), an 80-ft run adds about $11,700 over a 20-ft run. This is the #1 invisible cost driver — easy to underbudget at planning time.

Seattle SMC 25.11 protects 'exception trees' (6" diameter at breast height +). Removing one for a DADU buildable footprint requires permit + arborist + report + disposal — about $1,744 per tree in this calculator, contingency included. A 3-tree clear adds about $5,233 on top of the build. Count trees in your planned footprint, not the whole lot.

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.