SPEC_C / FEDERAL WAY · BASEMENT ADU · Verified 2026-01-15
Federal Way Basement ADU — Egress, Ceiling Height, Cost & Permits
Federal Way has aligned with HB 1337 and offers some of the most affordable land in south King County for an ADU project. A basement ADU is a self-contained dwelling carved out of the main house's existing basement — independent entry, kitchen, bath, with code-required egress and fire separation. Federal Way permits basement adu 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 basement adu allowed in Federal Way?
Federal Way allows basement ADU 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 basement adu 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)
- Egress window detail meeting current IRC sizing
- Ceiling height verification across each room
- Fire-separation and dwelling-unit separation 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
- Egress window undersized
- Ceiling height below 7 ft in habitable area
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 basement adu cost range
$126k–$286k · typical $193k
Egress, ceiling height, and waterproofing dominate.
What moves your number
- Egress window cuts
- Ceiling height workability
- Waterproofing scope
- Plumbing & fire 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 ~54 weeks
Federal Way basement adu 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.12–32 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 — Basement ADU vs the other ADU types in Federal Way
Why pick a basement adu over the alternatives in Federal Way?
| vs. | Cost | Speed | Privacy | Resale | Best when |
|---|---|---|---|---|---|
| DADU | Lower — reused shell. | Faster — interior work. | Lower — same building. | Lower — invisible second unit. | Existing basement is dry and tall enough. |
| AADU | Similar — often lower if egress already exists. | Similar. | Comparable. | Comparable. | You want a unit below grade, not as a side addition. |
| Garage Conversion | Often higher — egress cuts + waterproofing. | Comparable. | Lower — attached to main house. | Lower — counts as basement, not separate structure. | No usable garage exists. |
SPEC_C.06 / FIT — Is a basement adu right for your Federal Way lot?
Best-fit profile for a Federal Way basement adu
Pick a basement ADU when ceiling height and egress already meet code, and the basement is dry. Avoid it when you'd have to underpin the foundation just to reach ceiling height — at that scope, an AADU addition or a DADU is usually a better answer.
- Rental income
- Good for long-term rental — typically a 1-bedroom configuration aimed at single tenants or couples.
- Aging parents / multigen
- Limited — stair access to the main house can be a hard blocker for mobility-restricted occupants.
- Tight lot
- Best of the four on a tight lot — zero new footprint.
- Alley access
- Less relevant — entry is usually street-side or side-yard.
- Lower budget
- Often the lowest of the four when egress and ceiling height already work.
- Privacy
- Moderate — separate entry helps, but tenant lives below the primary unit.
- Speed to occupancy
- Fast — interior work, minimal weather exposure, predictable inspection sequence.
- Utilities already nearby
- Strong — 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 Basement ADU 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.
What's the minimum ceiling height for a basement ADU?
The IRC minimum is 7 ft in habitable rooms, with allowable reductions under structural members. Some jurisdictions allow as low as 6 ft 8 in under beams or ductwork — but the room average must still meet code.
Do I have to add egress windows in every bedroom?
Yes. Every sleeping room below grade needs an egress window meeting current IRC sizing, with a window well sized and graded for emergency exit. This is one of the most common single-line items in a basement-ADU budget.
What if my basement has moisture issues?
Resolve them before drywall. Interior drainage, sump pump upgrades, exterior re-grading, and dehumidification are all viable strategies — but they need to be designed in, not bolted on after final inspection.
Can I rent the basement ADU without the upstairs being occupied?
Generally yes — HB 1337 removed owner-occupancy requirements in Tier-1 cities. Outside Tier-1, owner-occupancy may still apply; confirm against the city's adopted ADU code.
Other ADU types in Federal Way
Same basement adu in nearby cities
SPEC_C.08 / NEXT
Plan your Federal Way basement adu 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 basement adu planning
INTERNAL_LINKS / Deep Map