Complete 2026 cost breakdown: construction, permits, financing, and expected rental ROI.
Calculate your Sammamish ADU cost →Building an ADU in Sammamish costs between $325,000-$550,000 for new construction, with per-square-foot costs averaging $400-$650. These figures include design, permitting, site preparation, and construction. The most affordable option is a garage conversion at $100,000-$225,000, while prefab/modular ADUs fall in between at $300,000-$450,000.
| Cost Category | Estimated Range |
|---|---|
| Architectural Plans | $3,000 – $15,000 |
| Permits & Fees | Value-based (2026 Development Fee Schedule): $1,278.51 on the first $100,000 plus $8.65 per $1,000 above it, plus plan review at 65 percent, a $1,578 planning review, $544 plan-review support ($2,248 on a critical-area site) and a 15 percent technology fee — about $11,150 on a $400,000 ADU. |
| Impact Fees | Per square foot since July 1, 2026 (Ord. O2026-595, new SMC 21.06.045, adopted to meet SB 5258's amendment of RCW 82.02.060(1)): parks $3.13 and transportation $7.44 per sq ft, each with a 783 sq ft minimum ($2,450.79 and $5,825.06 — $8,275.85 together) and a 3,500 sq ft maximum ($10,955 and $26,040). A 1,000 sq ft ADU pays $10,570. School fees are per dwelling unit by district — Lake Washington SD $10,914.75, Issaquah SD $0.00, Snoqualmie Valley SD $19,690.27 at the single-family rate, plus a $266 administration fee; the schedule lists no ADU category, and RCW 36.70A.681(1)(a) caps any ADU impact fee at 50 percent of the principal unit's (at most $5,457.38 in Lake Washington SD), except on critical-area lots (subsection (4)). Fees are due at permit issuance but may be deferred to occupancy for up to 18 months against a recorded lien (SMC 21.06.045(I)); parks and transportation rates are indexed to the ENR Construction Cost Index each first quarter. |
| Site Prep & Foundation | $10,000 – $35,000 |
| Framing & Structure | $15,000 – $45,000 |
| Plumbing | $8,000 – $20,000 |
| Electrical | $5,000 – $15,000 |
| HVAC | $3,000 – $8,000 |
| Interior Finishes | $15,000 – $40,000 |
Quotes for the same ADU commonly vary 20–40% between builders. We pass your request to up to 5 local builders serving Sammamish, where we have them — free, no obligation.
Get Sammamish builder quotes →Permit fees in Sammamish are Value-based (2026 Development Fee Schedule): $1,278.51 on the first $100,000 plus $8.65 per $1,000 above it, plus plan review at 65 percent, a $1,578 planning review, $544 plan-review support ($2,248 on a critical-area site) and a 15 percent technology fee — about $11,150 on a $400,000 ADU.. The review timeline is Not published — the city sets no ADU-specific review clock, and in September 2026 warns of reduced service levels during a staff transition. Project Guidance is answered within one week; ask the Permit Center (425-295-0531) for the current building-permit review target before you submit., with plan check: Standard building plan review through MyBuildingPermit.com, plus critical-areas review on many Sammamish sites: the Building Plan Review Support fee is $2,248 for a "Single family house/ADU - Critical Area" against $544 otherwise, a third-party critical area report review takes a $5,000 deposit, and wetland, stream, landslide, erosion, habitat and aquifer-recharge worksheets are on the city's forms page. A minor clearing-and-grading permit (up to 100 cubic yards) is billed at $267 planning plus $543 public-works review. Tree removal is separately permitted ($139), and unlawful removal is charged at $1,500 per inch of diameter at breast height. Concurrency testing for a "Single family house/ADU" is $280 plus $144 processing and a $187 administrative fee.. Impact fees are Per square foot since July 1, 2026 (Ord. O2026-595, new SMC 21.06.045, adopted to meet SB 5258's amendment of RCW 82.02.060(1)): parks $3.13 and transportation $7.44 per sq ft, each with a 783 sq ft minimum ($2,450.79 and $5,825.06 — $8,275.85 together) and a 3,500 sq ft maximum ($10,955 and $26,040). A 1,000 sq ft ADU pays $10,570. School fees are per dwelling unit by district — Lake Washington SD $10,914.75, Issaquah SD $0.00, Snoqualmie Valley SD $19,690.27 at the single-family rate, plus a $266 administration fee; the schedule lists no ADU category, and RCW 36.70A.681(1)(a) caps any ADU impact fee at 50 percent of the principal unit's (at most $5,457.38 in Lake Washington SD), except on critical-area lots (subsection (4)). Fees are due at permit issuance but may be deferred to occupancy for up to 18 months against a recorded lien (SMC 21.06.045(I)); parks and transportation rates are indexed to the ENR Construction Cost Index each first quarter.. Pre-approved plans: None. As of September 20, 2026 the city's ADU page and its Applications and Forms index carry no pre-approved ADU plan set — only the new single-family residence submittal checklist (updated July 1, 2026) and the residential plan-set coversheet — and no King County or state pre-approved plan program was found that applies to permits inside the city. Budget for a full custom design set and confirm with the Permit Center at 425-295-0531..
| 1-Bedroom Rent | $2,900-$3,300/month |
| 2-Bedroom Rent | $3,400-$3,850/month |
| Annual Rental Income (est.) | $37,200/year |
| Payback Period (est.) | 11.8 years |
| Short-Term Rental | Confirm with the city (425-295-0531) before you list. No short-term rental chapter was found on the city's site, and the codified SMC is behind a bot wall, so a rule elsewhere cannot be ruled out. The ADU code sets none. |
| Overall ROI Estimate | 5-8% annual return |
A typical ADU in Sammamish generates approximately $37,200 per year in rental income based on average 1-bedroom rents. At an average construction cost of $437,500, the estimated payback period is 11.8 years — before accounting for the resale value the ADU adds, which an appraiser determines from comparable sales in Sammamish and varies by market (often less than build cost).
1. Garage conversions save 40-60% vs new construction since the shell already exists. Average cost in Sammamish: $100,000-$225,000.
2. Prefab/modular ADUs offer predictable pricing and faster timelines. Average: $300,000-$450,000.
3. Pre-approved plans — available in Sammamish, which can save $3,000-$10,000 in design costs and 2-3 months in review time. See which cities offer pre-approved ADU plans.
4. State grants — California's CalHFA ADU grant (up to $40K) is currently paused; check calhfa.ca.gov/adu for status and your state for active programs.
5. Get multiple bids — ADU contractor pricing varies 20-40% for the same project. We recommend getting at least 3 quotes.
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