SPEC_C / REDMOND · BASEMENT ADU · Verified 2026-01-15
Redmond Basement ADU — Egress, Ceiling Height, Cost & Permits
Redmond has aligned with HB 1337 and is one of the most permissive ADU markets in east King County. 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. Redmond permits basement adu on most single-family lots under its current code. DADUs on flatter Overlake-adjacent lots, AADUs above garages in established neighborhoods, and basement conversions in older Education Hill stock.
SPEC_C.01 / ZONING — Redmond, King County
Is a basement adu allowed in Redmond?
Redmond 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 RZC 21; commonly 24 ft.
- Setbacks
- 5 ft side/rear for DADUs typical; primary-structure setbacks for AADUs.
- Lot coverage
- Coverage varies by RZC zone; verify per parcel.
- Minimum lot size
- No explicit minimum lot size for ADU under current RZC.
- Tree retention
- RZC tree-retention rules trigger arborist review on lots above the tree-unit threshold.
- Critical areas
- Bear Creek, Evans Creek, and various wetlands drive critical-area review.
- Alley access
- Limited alley access in Redmond's modern subdivisions.
Feasibility blockers to map early
- critical-area streams (Bear, Evans creeks)
- Education Hill slope
- wetland buffers in Overlake
- Bear Creek, Evans Creek, and various wetlands drive critical-area review.
- RZC tree-retention rules trigger arborist review on lots above the tree-unit threshold.
SPEC_C.02 / PERMIT — Redmond Planning & Community Development
Redmond basement adu permit timeline
Plan review (typical)
8–20 weeks
Redmond Planning & Community Development standard ADU intake.
With one correction cycle
10–28 weeks
Includes one typical correction cycle.
Permit difficulty
5/10
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 Redmond reviews can slow
- critical-area / stream-buffer reviews
- geotech on Education Hill
- tree-retention plans
SPEC_C.03 / COST — Redmond cost tier 5/5
Redmond basement adu cost range
$156k–$354k · typical $239k
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
- PSE coordination standard; modern subdivisions usually have adequate panel capacity.
- Panel upgrades typically $4–8k when needed.
- Redmond stormwater per Ecology manual; critical-area parcels add water-quality treatment.
- Stream-buffer parcels can require enhanced foundations and limit ADU placement.
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
Redmond 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 reviewRedmond Planning & Community Development standard ADU intake.8–20 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
Redmond-specific delay risks · critical-area / stream-buffer reviews · geotech on Education Hill · tree-retention plans
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 Redmond
Why pick a basement adu over the alternatives in Redmond?
| 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 Redmond lot?
Best-fit profile for a Redmond 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 Redmond ADU rules
- Redmond Planning & Community DevelopmentPermit authority for Redmond ADUs.
- Redmond ADU permitsADU permit information page.
- MyBuildingPermit.comPermit intake for Redmond.
- Redmond Zoning Code (RZC)Zoning code with ADU dimensional 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.
Redmond Basement ADU FAQ
Does Redmond allow two ADUs per lot?
Yes — one AADU and one DADU under HB 1337 alignment.
Is owner-occupancy required in Redmond?
No. Redmond removed the requirement to align with state law.
How long does a Redmond ADU permit take?
Standard ADU reviews run 8–20 weeks.
Do critical-area streams affect my Redmond ADU?
Yes when the parcel is within mapped buffers — site layout adjustments and water-quality stormwater treatment are common.
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 Redmond
SPEC_C.08 / NEXT
Plan your Redmond 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).
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