SPEC_C / BELLEVUE · BASEMENT ADU · Verified 2026-01-15
Bellevue Basement ADU — Egress, Ceiling Height, Cost & Permits
Bellevue has aligned with HB 1337 to allow two ADUs per lot, removed owner-occupancy, and dropped parking minimums for ADUs near transit. 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. Bellevue permits basement adu on most single-family lots under its current code. DADUs on flatter R-5 lots in Lake Hills and Bel-Red, AADUs above garages, and basement conversions in older Bridle Trails stock.
SPEC_C.01 / ZONING — Bellevue, King County
Is a basement adu allowed in Bellevue?
Bellevue 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
- DADUs commonly capped near 24 ft; refer to LUC 20.30J.
- Setbacks
- 5 ft side/rear typical for DADUs; primary-structure setbacks for AADUs.
- Lot coverage
- Lot coverage varies by zone (R-5 / R-4); confirm with parcel data.
- Minimum lot size
- No minimum lot size mandated post-HB 1337 alignment; functional minimums driven by setbacks and coverage.
- Tree retention
- Bellevue's tree-retention rules and significant-tree definitions add arborist review when lots have mature canopy.
- Critical areas
- Stream buffers (Kelsey, Coal Creek tributaries) and steep slopes drive feasibility.
- Alley access
- Alley lots are rare; most DADUs front-load with driveway easements.
Feasibility blockers to map early
- steep slope (Somerset, Cougar Mountain)
- critical-area streams
- tree-rich infill lots
- Stream buffers (Kelsey, Coal Creek tributaries) and steep slopes drive feasibility.
- Bellevue's tree-retention rules and significant-tree definitions add arborist review when lots have mature canopy.
SPEC_C.02 / PERMIT — Bellevue Development Services
Bellevue basement adu permit timeline
Plan review (typical)
8–22 weeks
Bellevue Development Services standard ADU intake via MyBuildingPermit.
With one correction cycle
10–30 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)
- 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 Bellevue reviews can slow
- geotech reports on Somerset / Cougar Mountain slopes
- tree-retention plan revisions
- puget sound energy / utility coordination
SPEC_C.03 / COST — Bellevue cost tier 5/5
Bellevue basement adu cost range
$162k–$367k · typical $248k
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
- 200A panel upgrades common in older Bel-Red and Crossroads stock; PSE coordination 2–4 weeks.
- Panel upgrades typically $5–9k including PSE meter swap; sub-panels for DADUs $3–5k.
- Bellevue Utilities side-sewer review; stormwater triggers at 2,000 sf new + replaced impervious in many cases — confirm per parcel.
- Slope, crane access, and rock excavation can add $25–70k on plateau and hillside lots.
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 ~56 weeks
Bellevue 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 reviewBellevue Development Services standard ADU intake via MyBuildingPermit.8–22 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
Bellevue-specific delay risks · geotech reports on Somerset / Cougar Mountain slopes · tree-retention plan revisions · puget sound energy / utility 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 — Basement ADU vs the other ADU types in Bellevue
Why pick a basement adu over the alternatives in Bellevue?
| 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 Bellevue lot?
Best-fit profile for a Bellevue 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 Bellevue ADU rules
- Bellevue Development ServicesPermit authority for Bellevue ADUs.
- Bellevue ADU informationADU rules, max size, parking, and application paths.
- MyBuildingPermit.comShared eastside permit portal Bellevue uses for ADU intake.
- Bellevue Land Use Code 20.30JLand Use Code chapter covering 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.
Bellevue Basement ADU FAQ
Do I need owner-occupancy to build an ADU in Bellevue?
No. Bellevue follows the HB 1337 alignment that removes the owner-occupancy requirement.
Can I rent both my house and ADU in Bellevue?
Yes. Both units can be rented; short-term rental rules differ from long-term and should be checked separately.
How big can a Bellevue DADU be?
Up to 1,000 sf of gross floor area under the current LUC 20.30J framework.
Do I need off-street parking for my Bellevue ADU?
Not when the lot is within ½-mile of major transit, per HB 1337 alignment. Confirm transit-stop classification for the parcel.
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 Bellevue
SPEC_C.08 / NEXT
Plan your Bellevue 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 Bellevue basement adu planning
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