SPEC_C / SEATTLE · DADU · Verified 2026-01-15
Seattle DADU Builders — Detached ADU Cost, Permits & Timeline
Seattle has the deepest ADU policy in Washington — two units per lot, no owner-occupancy, no parking minimums, and a pre-approved DADU plan library that compresses permit time. A DADU is a standalone backyard cottage on the same lot as the main house — separate roof, separate entry, full kitchen and bath. Seattle permits detached adu (dadu) on most single-family lots under its current code. Alley-loaded DADUs in Ballard, Beacon Hill, and Capitol Hill remain the strongest fit. AADUs above garages, basement conversions in Wallingford / Roosevelt, and pre-approved DADU plans on flatter lots compete well.
SPEC_C.01 / ZONING — Seattle, King County
Is a dadu allowed in Seattle?
Seattle allows detached ADU (DADU) on single-family lots (up to 1,000 sf), no owner-occupancy requirement, and no off-street parking required near transit.
- Max ADU size
- 1,000 sf (verify against city code)
- Parking
- No off-street parking required citywide for ADUs.
- Owner-occupancy
- Not required
- Height
- DADUs typically 18–24 ft depending on lot width and roof form; refer to SMC 23.44.041 for the current matrix.
- Setbacks
- 5 ft side/rear typical for DADUs; reduced when alley-loaded. AADUs follow primary-structure setbacks.
- Lot coverage
- Max lot coverage in single-family zones generally 35%, with the DADU footprint counted separately under city accounting.
- Minimum lot size
- No minimum lot size for an AADU; DADU eligibility commonly requires ≥3,200 sf parcel — verify per current code.
- Tree retention
- Tree Protection Ordinance (SMC 25.11) — exceptional trees and trees ≥ 24" DBH trigger arborist review.
- Critical areas
- ECA review for liquefaction-prone areas, steep slopes ≥ 40%, peat settlement zones, and riparian buffers.
- Alley access
- Alley-served lots get reduced setbacks and easier utility routing; common in Ballard, Beacon Hill, and Capitol Hill.
Feasibility blockers to map early
- liquefaction (Duwamish, Interbay)
- steep slope (West Seattle, Magnolia)
- peat (parts of Roosevelt, Northgate)
- ECA review for liquefaction-prone areas, steep slopes ≥ 40%, peat settlement zones, and riparian buffers.
- Tree Protection Ordinance (SMC 25.11) — exceptional trees and trees ≥ 24" DBH trigger arborist review.
SPEC_C.02 / PERMIT — Seattle Department of Construction & Inspections (SDCI)
Seattle dadu permit timeline
Plan review (typical)
10–26 weeks
Standard DADU/AADU review; pre-approved DADU plans review faster (≈ 6–10 weeks).
With one correction cycle
12–34 weeks
Includes one typical correction cycle.
Permit difficulty
7/10
Higher = more plan-check + design-review friction.
Documents you'll submit
- Site plan with parcel dimensions, structures, setbacks, and tree retention
- Architectural plans (floor plans, elevations, sections)
- Structural plans stamped by a WA-licensed engineer where required
- WSEC energy compliance worksheet
- Stormwater / drainage review per city threshold
- Utility plan (sewer / water / electric)
- Tree inventory & protection plan (SMC 25.11)
- Side-sewer card from SPU when sewer connection is altered
- DADU site plan showing separation from primary structure and any easements
- Independent utility connections (sewer, water, electric) or shared-service detail
Common plan-check corrections
- Setback or lot-coverage encroachment on site plan
- Energy-code U-value / glazing percentage mismatch
- Missing structural calcs for shear walls or beams
- Stormwater BMP not selected or sized for the impervious area
- Tree retention plan missing or critical-root-zone fencing not shown
- ECA boundary not shown when parcel is within steep-slope or liquefaction overlay
- Pre-approved DADU plan modifications exceed allowable deviation envelope
- Setback or coverage error on detached structure
- Stormwater BMP missing for added impervious
Inspection sequence
- 01Foundation / footing
- 02Underfloor framing & insulation
- 03Rough-in: framing, mechanical, plumbing, electrical
- 04Insulation & vapor barrier
- 05Drywall / wallboard
- 06Final building, plumbing, mechanical, electrical
- 07Certificate of occupancy / final approval
Why Seattle reviews can slow
- Resubmittals for energy-code revisions on glazing or envelope
- Side-sewer capacity letter delays from SPU
- Tree-protection plan revisions when an exceptional tree is identified late
- Coordinated review for ECA parcels adding 4–8 weeks
SPEC_C.03 / COST — Seattle cost tier 5/5
Seattle dadu cost range
$280k–$520k · typical $380k
Drivers: site access, foundation type, sewer routing, finish level, and ECA review.
What moves your number
- Foundation type (slab vs stepped)
- Crane / site access
- Sewer routing length
- Finish level
Utility & site notes for this city
- 200A panel upgrades common; SPU side-sewer reviews can require capacity upsizing on older lines.
- Panel upgrades to 200A typically $4–8k including SCL coordination; sub-panels for DADUs $2.5–5k.
- SPU stormwater triggers above 750 sf new impervious; flow-control BMPs may be required.
- Hillside / crane access / shoring can add $30–80k; ECA review adds soft costs and weeks of timeline.
Ranges reflect typical 2026 project budgets and exclude land. Your exact cost depends on site access, foundation, utility upgrades, finish level, and city review depth — confirm with a feasibility study before committing.
SPEC_C.04 / TIMELINE — typical end-to-end ~64 weeks
Seattle dadu schedule, phase by phase
- 01FeasibilityParcel + zoning + utility check.1–4 weeks
- 02DesignSchematic → permit set.4–12 weeks
- 03EngineeringStructural runs in parallel with design.2–6 weeks
- 04Permit reviewStandard SDCI ADU intake.10–26 weeks
- 05Corrections & resubmittalMultiple correction cycles possible on ECA parcels.2–8 weeks
- 06ConstructionDetached scope; conversions on the faster end.20–40 weeks
- 07Final sign-offFinal inspections and CofO.1–4 weeks
Seattle-specific delay risks · ECA / steep slope review · exceptional tree designation · SPU side-sewer coordination
Weather note · Excavation, foundations, and exterior weatherization sequence best Apr–Oct; framing and finishes continue year-round under temporary weather protection.
SPEC_C.05 / COMPARE — DADU vs the other ADU types in Seattle
Why pick a dadu over the alternatives in Seattle?
| vs. | Cost | Speed | Privacy | Resale | Best when |
|---|---|---|---|---|---|
| AADU | Higher — full new foundation + envelope. | Slower — ground-up build. | Better — no shared walls. | Strongest resale separation. | Backyard space + alley/side access available. |
| Garage Conversion | Higher — no reused shell. | Slower — full new structure. | Better — siting flexibility. | Higher — true second dwelling. | Lot can absorb a new footprint. |
| Basement ADU | Higher — no reused shell. | Slower — full new structure. | Much better — separate building. | Higher — visible second unit. | You want a true backyard cottage, not a downstairs unit. |
SPEC_C.06 / FIT — Is a dadu right for your Seattle lot?
Best-fit profile for a Seattle dadu
Pick a DADU when privacy, long-term rentability, and resale value matter more than speed or lowest cost. Avoid it on very tight lots, lots with deep critical-area buffers, or sites with no alley access and a long sewer run.
- Rental income
- Strongest of the four for long-term rental income — full privacy, full appliance set, dedicated yard space and parking when allowed.
- Aging parents / multigen
- Excellent for aging parents who want independence with proximity — single-level layouts with no shared walls or stairs.
- Tight lot
- Hardest of the four on a tight lot — DADU footprint plus required setbacks and tree retention can consume most rear-yard space.
- Alley access
- Ideal when alley access exists — separate guest entry, easier construction staging, and cleaner utility tie-ins.
- Lower budget
- Highest baseline budget of the four — foundation, framing, roof, siding, and full MEP are all new.
- Privacy
- Best privacy of all four — no shared walls, no shared entry, no shared mechanical chase.
- Speed to occupancy
- Slowest of the four — full ground-up build, full inspection sequence, full weather exposure during framing.
- Utilities already nearby
- Sewer routing length and side-sewer condition drive a meaningful share of the budget.
SPEC_C.07 / SOURCES — verified .gov / city / state references
Official sources for Seattle ADU rules
- Seattle SDCIPermit authority for all Seattle ADU work.
- Seattle ADU programCurrent ADU/DADU rules, application checklists, pre-approved DADU plans.
- Seattle Services PortalOnline intake for ADU permit applications and inspections.
- SMC 23.44.041 — Accessory dwelling unitsCode citation for accessory dwelling units in single-family zones.
- Seattle Environmentally Critical AreasLiquefaction, steep slope, peat — checked against parcels before feasibility.
- RCW 36.70A.681 — Accessory Dwelling UnitsState law setting the floor for ADU regulations cities must meet.
- HB 1337 (2023) — Bill textAuthoritative text of the 2023 ADU reform: two ADUs per lot, 1,000 sf min, no owner-occupancy, parking limits near transit.
- WA Department of Commerce — ADU resourcesState guidance and model code for cities implementing HB 1337.
Seattle DADU FAQ
Can I build two ADUs on a single Seattle lot?
Yes. Single-family lots can host one AADU and one DADU subject to lot size, FAR, and ECA review.
Go deeper: Read the DADU vs AADU in Puget Sound: which to build (2026) guide
What is the maximum DADU size in Seattle?
1,000 sf of gross floor area. Garages and storage do not count, but check current SMC for accounting changes.
How long does an SDCI ADU permit take?
Standard reviews are typically 10–26 weeks. Pre-approved DADU plans land in roughly 6–10 weeks when the lot is clear of ECA constraints.
Go deeper: Read the Seattle SDCI ADU permit process, step by step (2026) guide
Are pre-approved DADU plans cheaper?
Yes — pre-approved plans cut design and permit time substantially. Total project cost still depends on site work, finishes, and utility upgrades.
Go deeper: Read: Seattle's Pre-Approved DADU plans — when they make sense
Do I need a separate sewer line for a DADU?
Not always. Most Puget Sound cities allow a shared side-sewer with a cleanout and an as-built that proves capacity. A separate line is required only when the existing line is undersized, in failing condition, or the city explicitly demands one.
Can I rent the DADU short-term?
Short-term rental rules are city-specific and change. Seattle, Bellevue, Tacoma and most Tier-1 cities allow long-term rentals; short-term (STR) usually requires a separate license, an owner-occupancy attestation, or both. Confirm with the city before underwriting an STR pro forma.
How big can a DADU be in Washington?
HB 1337 sets a statewide floor of at least 1,000 sq ft of allowed gross floor area for an ADU. Individual cities may allow more (Seattle and Bellevue both go above 1,000 sf on many lots), but the city's specific max controls — always verify against the linked municipal code.
Go deeper: Read the DADU vs AADU in Puget Sound: which to build (2026) guide
What kind of foundation does a DADU need?
A stem-wall foundation on continuous footings is most common in the Puget Sound, with stepped foundations on sloped sites. Pin or pier foundations are sometimes used on tree-protection lots to limit critical-root-zone disturbance, but they require engineering.
Go deeper: Read the Foundation inspection checklist for Puget Sound homes guide
Other ADU types in Seattle
Same dadu in nearby cities
SPEC_C.08 / NEXT
Plan your Seattle dadu the right way
A 45-minute feasibility call confirms zoning, utility, and budget reality before you spend on design. Golden State ADU has been licensed in Washington since 2012 (LIC SEATTI*752N6).
More for Seattle dadu planning
INTERNAL_LINKS / Deep Map