SPEC_C / FEDERAL WAY · AADU · Verified 2026-01-15
Federal Way AADU Builders — Attached ADU Permits, Cost & Layout
Federal Way has aligned with HB 1337 and offers some of the most affordable land in south King County for an ADU project. 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. Federal Way permits attached adu (aadu) on most single-family lots under its current code. DADUs on flatter West Campus and Twin Lakes lots, garage conversions in older stock, and AADUs above existing garages.
SPEC_C.01 / ZONING — Federal Way, King County
Is a aadu allowed in Federal Way?
Federal Way 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 within ½-mile of major transit per HB 1337; otherwise city standard parking rules apply.
- Owner-occupancy
- Not required
- Height
- DADU heights per FWRC 19; commonly 24 ft.
- Setbacks
- 5 ft side/rear typical for DADUs.
- Lot coverage
- Coverage per FWRC zone.
- Minimum lot size
- No explicit ADU minimum lot size under current FWRC.
- Tree retention
- FWRC tree-retention rules trigger arborist review above thresholds; significant canopy is common.
- Critical areas
- Hylebos Creek watershed wetlands; ravine slopes.
- Alley access
- Limited alleys; most lots front-load.
Feasibility blockers to map early
- wetland buffers
- mature canopy
- slope along ravines (Hylebos area)
- Hylebos Creek watershed wetlands; ravine slopes.
- FWRC tree-retention rules trigger arborist review above thresholds; significant canopy is common.
SPEC_C.02 / PERMIT — Federal Way Community Development
Federal Way aadu permit timeline
Plan review (typical)
8–18 weeks
Federal Way Community Development standard ADU intake.
With one correction cycle
10–26 weeks
Includes one typical correction cycle.
Permit difficulty
4/10
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)
- 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
- 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 Federal Way reviews can slow
- wetland buffer reviews
- tree-retention plans
- ravine geotech
SPEC_C.03 / COST — Federal Way cost tier 3/5
Federal Way aadu cost range
$151k–$319k · typical $218k
Existing-space conversions sit at the low end; new additions at the high end.
What moves your number
- Demising-wall scope
- Existing structural capacity
- Egress / window scope
- Mechanical separation
Utility & site notes for this city
- Lakehaven Water & Sewer District coordination required; PSE for electric.
- Panel upgrades typically $4–7k.
- Lakehaven side-sewer review; stormwater per Ecology manual.
- Ravine-adjacent parcels add $10–30k in geotech and foundation work.
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 ~58 weeks
Federal Way aadu schedule, phase by phase
- 01FeasibilityParcel + zoning + utility verification.1–4 weeks
- 02DesignSchematic → permit-ready set.4–12 weeks
- 03EngineeringStructural runs in parallel with design.2–6 weeks
- 04Permit reviewFederal Way Community Development standard ADU intake.8–18 weeks
- 05Corrections & resubmittalCycle depends on correction depth.2–8 weeks
- 06ConstructionDetached scope; conversions sit at the lower end.18–38 weeks
- 07Final sign-offFinal inspections and certificate of occupancy.1–4 weeks
Federal Way-specific delay risks · wetland buffer reviews · tree-retention plans · ravine geotech
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 Federal Way
Why pick a aadu over the alternatives in Federal Way?
| 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 Federal Way lot?
Best-fit profile for a Federal Way 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 Federal Way ADU rules
- Federal Way Community DevelopmentPermit authority for Federal Way ADUs.
- Federal Way ADU infoCity landing page; ADU pages move — confirm current path on visit.
- Federal Way Revised CodeFWRC chapters covering residential zones and ADU standards.
- 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.
- WA State Energy Code (Residential)ADUs must meet WSEC; the residential provisions drive insulation, glazing, and HVAC requirements.
Federal Way AADU FAQ
Does Federal Way allow two ADUs per lot?
Yes — under HB 1337 alignment, one AADU and one DADU.
Go deeper: Read the DADU vs AADU in Puget Sound: which to build (2026) guide
Is owner-occupancy required in Federal Way?
No. The requirement was removed to align with state law.
Do I need to coordinate with Lakehaven for an ADU?
Yes — Lakehaven Water & Sewer District serves most of Federal Way and reviews side-sewer and water-meter changes.
How long does Federal Way ADU permit review take?
Standard ADU reviews run 8–18 weeks.
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 Federal Way
Same aadu in nearby cities
SPEC_C.08 / NEXT
Plan your Federal Way 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 Federal Way aadu planning
INTERNAL_LINKS / Deep Map