How Much Does an ADU Cost in Sammamish, WA?

Construction Cost Breakdown

Building an ADU in Sammamish costs between $265,000-$480,000 for new construction, with per-square-foot costs averaging $320-$530. These figures include design, permitting, site preparation, and construction. The most affordable option is a garage conversion at $130,000-$260,000, while prefab/modular ADUs fall in between at $190,000-$365,000.

Permit Costs & Timeline

Permit fees in Sammamish are Building permit fees run on a project-value sliding scale (2026 Development Fee Schedule). For a project valued between $100,001 and $500,000 the fee is $1,278.51 on the first $100,000 plus $8.65 for each additional $1,000 — so a $300,000 ADU is about $3,008 in permit fee. Plan review is charged at 65 percent of the permit fee on top, roughly $1,955 on the same project, and a 15 percent technology fee is applied to each item except impact fees, stormwater rates, surface water development charges and code penalties. Add the Planning Division's Building Application Plan Review for New Construction at $1,578; an Energy Code fee of $136 for a new single-family residence or $91 for a remodel/addition; flat mechanical and plumbing fixture fees of $314 each for a new residence; concurrency testing at $280 plus $144; and Building Plan Review Support, which is $544 for new construction generally but $2,248 where the site is a critical area. A surface water development charge of $3,798 applies to a new residential dwelling unit with up to 3,500 sq ft of impervious coverage, and the residential stormwater rate is $504.68. The review timeline is Confirm with the Sammamish Permit Center at 425-295-0531 — the city publishes no ADU-specific review clock, and Washington's permit review timelines under the 2023 permitting reform legislation vary by project type. Two items add real calendar time and are worth starting early: concurrency testing, which the fee schedule bills for a "Single family house/ADU" at $280 for the test and $144 for processing and informational analysis, and critical-areas review where any of the site is designated, with plan check: Standard building plan review, plus critical-areas review on most Sammamish sites. Budget for the Building Plan Review Support fee, which quadruples from $544 to $2,248 for a "Single family house/ADU - Critical Area," and for a clear and grade permit, which starts at $543 for up to 100 cubic yards of earth. If your existing house or an existing detached ADU already sits inside a wetland, stream, habitat or landslide hazard buffer, SMC 21.03.020 allows a one-time footprint increase of up to 1,000 sq ft within that buffer or setback, subject to a critical areas study and, for landslide hazard areas, a demonstration of no increased risk to life or property — but no further expansion is allowed where the structure sits over or within the wetland, stream or landslide hazard area itself. Tree removal is separately regulated, and unlawful removal is charged at $1,500 per inch of diameter at breast height. Impact fees are This changed on July 1, 2026, and the change matters. Ordinance O2026-595, adopted May 19, 2026, created a new SMC 21.06.045 replacing the city's flat per-unit parks and transportation impact fees with per-square-foot rates: Parks Impact Fee $3.13 per square foot, minimum $2,450.79 (783 sq ft) and maximum $10,955.00 (3,500 sq ft); Transportation Impact Fee $7.44 per square foot, minimum $5,825.06 (783 sq ft) and maximum $26,043.91 (3,500 sq ft). Scaling by size helps an ADU, but the 783 sq ft floor means even a 500 sq ft unit pays at least $8,275.85 across the two. School impact fees remain per dwelling unit and depend entirely on which district you are in: Lake Washington School District $10,914.75, Issaquah School District $0.00, Snoqualmie Valley School District $19,690.27, plus a $266 administration fee. Over all of it sits RCW 36.70A.681(1)(a): a city "may not assess impact fees on the construction of accessory dwelling units that are greater than 50 percent of the impact fees that would be imposed on the principal unit." Hold the city to it — and be aware that on a lot designated with critical areas or their buffers, RCW 36.70A.681(4) removes that cap along with everything else in the section. Pre-approved plans: None found in a City of Sammamish source as of September 1, 2026 — the city publishes handouts and a Master Builders Association member directory but no pre-approved ADU plan gallery of the kind Kirkland runs. Confirm with the Permit Center at 425-295-0531 before commissioning a full custom set.

Rental Income & ROI

A typical ADU in Sammamish generates approximately $29,100 per year in rental income based on average 1-bedroom rents. At an average construction cost of $372,500, the estimated payback period is 12.8 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).

Ways to Reduce ADU Costs in Sammamish