SPEC_C / SAMMAMISH · DADU · Verified 2026-01-15
Sammamish DADU Builders — Detached ADU Cost, Permits & Timeline
Sammamish has aligned with HB 1337 in policy, but the city's strict tree-retention rules and the network of streams and slope edges across the plateau make ADU feasibility studies essential. A DADU is a standalone backyard cottage on the same lot as the main house — separate roof, separate entry, full kitchen and bath. Sammamish permits detached adu (dadu) on most single-family lots under its current code. DADUs on interior plateau lots with low canopy, basement conversions in newer plateau homes, and AADUs above existing garages.
SPEC_C.01 / ZONING — Sammamish, King County
Is a dadu allowed in Sammamish?
Sammamish allows detached ADU (DADU) 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 SMC 21A; commonly 24 ft.
- Setbacks
- 5 ft side/rear typical for DADUs.
- Lot coverage
- Coverage per SMC 21A zone.
- Minimum lot size
- No explicit ADU minimum lot size under current SMC 21A.
- Tree retention
- Sammamish tree-retention rules among the strictest in the region; significant trees on most lots.
- Critical areas
- Lake Sammamish shore, stream buffers (Ebright, Pine Lake), and plateau slope edges.
- Alley access
- Effectively no alleys.
Feasibility blockers to map early
- plateau steep edges
- stream and wetland buffers
- extensive tree canopy
- Lake Sammamish shore, stream buffers (Ebright, Pine Lake), and plateau slope edges.
- Sammamish tree-retention rules among the strictest in the region; significant trees on most lots.
SPEC_C.02 / PERMIT — Sammamish Community Development
Sammamish dadu permit timeline
Plan review (typical)
10–24 weeks
Sammamish Community Development standard ADU intake via MyBuildingPermit.
With one correction cycle
12–32 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)
- DADU site plan showing separation from primary structure and any easements
- Independent utility connections (sewer, water, electric) or shared-service 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
- Setback or coverage error on detached structure
- Stormwater BMP missing for added impervious
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 Sammamish reviews can slow
- tree-retention reviews
- stream/wetland buffers
- slope geotech on plateau edges
SPEC_C.03 / COST — Sammamish cost tier 5/5
Sammamish dadu cost range
$302k–$562k · typical $410k
Drivers: site access, foundation, utility upgrades, and finish level.
What moves your number
- Foundation type (slab vs stepped)
- Crane / site access
- Sewer routing length
- Finish level
Utility & site notes for this city
- Sammamish Plateau Water & Sewer District coordination; PSE for electric.
- Panel upgrades typically $5–9k.
- Sammamish Plateau side-sewer review; stormwater per Ecology manual.
- Tree-retention and buffer setbacks can shrink usable build envelope significantly.
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 ~62 weeks
Sammamish dadu 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 reviewSammamish Community Development standard ADU intake via MyBuildingPermit.10–24 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
Sammamish-specific delay risks · tree-retention reviews · stream/wetland buffers · slope geotech on plateau edges
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 — DADU vs the other ADU types in Sammamish
Why pick a dadu over the alternatives in Sammamish?
| vs. | Cost | Speed | Privacy | Resale | Best when |
|---|---|---|---|---|---|
| AADU | Higher — full new foundation + envelope. | Slower — ground-up build. | Better — no shared walls. | Strongest resale separation. | Backyard space + alley/side access available. |
| Garage Conversion | Higher — no reused shell. | Slower — full new structure. | Better — siting flexibility. | Higher — true second dwelling. | Lot can absorb a new footprint. |
| Basement ADU | Higher — no reused shell. | Slower — full new structure. | Much better — separate building. | Higher — visible second unit. | You want a true backyard cottage, not a downstairs unit. |
SPEC_C.06 / FIT — Is a dadu right for your Sammamish lot?
Best-fit profile for a Sammamish dadu
Pick a DADU when privacy, long-term rentability, and resale value matter more than speed or lowest cost. Avoid it on very tight lots, lots with deep critical-area buffers, or sites with no alley access and a long sewer run.
- Rental income
- Strongest of the four for long-term rental income — full privacy, full appliance set, dedicated yard space and parking when allowed.
- Aging parents / multigen
- Excellent for aging parents who want independence with proximity — single-level layouts with no shared walls or stairs.
- Tight lot
- Hardest of the four on a tight lot — DADU footprint plus required setbacks and tree retention can consume most rear-yard space.
- Alley access
- Ideal when alley access exists — separate guest entry, easier construction staging, and cleaner utility tie-ins.
- Lower budget
- Highest baseline budget of the four — foundation, framing, roof, siding, and full MEP are all new.
- Privacy
- Best privacy of all four — no shared walls, no shared entry, no shared mechanical chase.
- Speed to occupancy
- Slowest of the four — full ground-up build, full inspection sequence, full weather exposure during framing.
- Utilities already nearby
- Sewer routing length and side-sewer condition drive a meaningful share of the budget.
SPEC_C.07 / SOURCES — verified .gov / city / state references
Official sources for Sammamish ADU rules
- Sammamish Community DevelopmentPermit authority for Sammamish ADUs.
- Sammamish ADU infoCity landing page; ADU standards in SMC 21A.
- MyBuildingPermit.comPermit intake for Sammamish.
- Sammamish Critical AreasLakeshore, steep slope, and stream buffers drive ADU feasibility here.
- 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.
Sammamish DADU FAQ
Does Sammamish allow two ADUs per lot?
Yes — under HB 1337 alignment, one AADU and one DADU.
How big can my Sammamish DADU be?
Up to 1,000 sf of gross floor area.
Do I need a separate sewer line for a DADU?
Not always. Most Puget Sound cities allow a shared side-sewer with a cleanout and an as-built that proves capacity. A separate line is required only when the existing line is undersized, in failing condition, or the city explicitly demands one.
Can I rent the DADU short-term?
Short-term rental rules are city-specific and change. Seattle, Bellevue, Tacoma and most Tier-1 cities allow long-term rentals; short-term (STR) usually requires a separate license, an owner-occupancy attestation, or both. Confirm with the city before underwriting an STR pro forma.
How big can a DADU be in Washington?
HB 1337 sets a statewide floor of at least 1,000 sq ft of allowed gross floor area for an ADU. Individual cities may allow more (Seattle and Bellevue both go above 1,000 sf on many lots), but the city's specific max controls — always verify against the linked municipal code.
Go deeper: Read the DADU vs AADU in Puget Sound: which to build (2026) guide
What kind of foundation does a DADU need?
A stem-wall foundation on continuous footings is most common in the Puget Sound, with stepped foundations on sloped sites. Pin or pier foundations are sometimes used on tree-protection lots to limit critical-root-zone disturbance, but they require engineering.
Go deeper: Read the Foundation inspection checklist for Puget Sound homes guide
Other ADU types in Sammamish
Same dadu in nearby cities
SPEC_C.08 / NEXT
Plan your Sammamish dadu 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 Sammamish dadu planning
INTERNAL_LINKS / Deep Map