SPEC_C / SEATTLE · AADU · Verified 2026-01-15
Seattle AADU Builders — Attached ADU Permits, Cost & Layout
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. An AADU is a self-contained dwelling carved into the existing house — its own entry, its own kitchen, its own bath, with a fire-rated demising wall between units. Seattle permits attached adu (aadu) 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 aadu allowed in Seattle?
Seattle allows attached ADU (AADU) 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 aadu 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
- Demising-wall and fire-separation detail between primary and accessory units
- Egress plan showing independent entry for the AADU
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
- Fire-separation detail incomplete between units
- Egress missing for the accessory unit
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 aadu cost range
$180k–$380k · typical $260k
Conversions of existing space are cheaper; AADUs adding habitable area trend higher.
What moves your number
- Demising-wall scope
- Existing structural capacity
- Egress / window scope
- Mechanical separation
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 aadu 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 — AADU vs the other ADU types in Seattle
Why pick a aadu over the alternatives in Seattle?
| vs. | Cost | Speed | Privacy | Resale | Best when |
|---|---|---|---|---|---|
| DADU | Lower — reused shell. | Faster — less new structure. | Lower — shared wall. | Slightly less separation; still a permitted second dwelling. | Lot is tight or the main house already has separate-entry potential. |
| Garage Conversion | Similar — depends on existing-conditions scope. | Similar — both reuse a shell. | Better — usually inside the main house, separate entry. | Higher — counts as living area in the main house. | Existing house has unused basement or side wing. |
| Basement ADU | Slightly higher — AADU often touches more of the main envelope. | Similar. | Comparable. | Comparable. | You want a side or upper-level unit, not just a basement. |
SPEC_C.06 / FIT — Is a aadu right for your Seattle lot?
Best-fit profile for a Seattle aadu
Pick an AADU when the existing house has the structural capacity, separate-entry potential, and ceiling height to absorb a second unit. Avoid it when reusing the existing envelope would compromise fire separation, egress, or sound transmission below code.
- Rental income
- Strong for long-term rental, especially when the primary unit will be owner-occupied — shared envelope keeps operating costs low.
- Aging parents / multigen
- Excellent — proximity inside the same building, no weather exposure between units, easier caregiving access.
- Tight lot
- Best of the four on a tight lot — no new footprint, no new setbacks to fight.
- Alley access
- Less relevant — AADU entries usually face the street or side yard, not the alley.
- Lower budget
- Lower baseline than DADU — reused envelope, reused roof, reused foundation.
- Privacy
- Lower than DADU — shared wall, sometimes shared mechanical chase or service entry.
- Speed to occupancy
- Faster than DADU — less weather exposure, less framing, no roofing scope.
- Utilities already nearby
- Strong fit — typically extends existing service rather than running new lines.
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 AADU FAQ
Do I need to live on the lot to build an ADU in Seattle?
No. Seattle removed the owner-occupancy requirement in 2019; one or both units can be rented.
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
Do I need a separate electrical meter for an AADU?
Not in most Puget Sound cities. A single meter is acceptable as long as the panel has capacity, the AADU has dedicated branch circuits, and any required sub-panel is sized correctly. Separate meters are usually only required if you plan to bill the AADU tenant for utilities directly.
How thick does the demising wall have to be?
Most plan reviewers require a 1-hour fire-rated separation between dwelling units, with STC 50 sound transmission. The exact assembly depends on the existing wall — UL-listed assemblies and city-approved alternatives are both accepted, but the listing must be on the drawings.
Go deeper: Read the Eastside deck cost & permits: cedar, IPE, composite (2026) guide
Can the AADU share an entry with the main house?
Most cities require a separate, independent entry for the AADU at grade. A shared vestibule with two interior doors is sometimes acceptable, but a shared interior hallway usually is not.
Go deeper: Read the DADU vs AADU in Puget Sound: which to build (2026) guide
Does an AADU need its own HVAC?
Each dwelling unit must have an independent temperature-control source. A dedicated mini-split or fan-coil for the AADU is the common solution — central forced-air shared across both units is generally not allowed under the current Washington Energy Code.
Other ADU types in Seattle
Same aadu in nearby cities
SPEC_C.08 / NEXT
Plan your Seattle aadu 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 aadu planning
INTERNAL_LINKS / Deep Map