An ADU builder in Sammamish is a licensed contractor who designs and constructs accessory dwelling units — detached backyard units, garage conversions, attached additions, and junior ADUs (JADUs). Because an ADU is a complete second dwelling, the builder coordinates foundation, framing, plumbing, electrical, and HVAC work and pulls it through Sammamish's permit process and utility connections. Not every general contractor has built one, so the right builder is one with ADU-specific experience in Washington.
What ADU Builders Cost in Sammamish [10]
Based on our Sammamish cost data, expect the following ranges from local ADU builders. Quotes for the same scope typically vary 20-40% between contractors, so getting at least three bids is the single best way to control cost.
| New Detached ADU (all-in) | $265,000-$480,000 |
| Cost per Sq Ft | $320-$530 |
| Garage Conversion | $130,000-$260,000 |
| Prefab / Modular ADU (installed) | $190,000-$365,000 |
| City Permit & Plan Review | 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 |
| Impact Fees | 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 |
For a full line-item breakdown, see our Sammamish ADU cost guide. To confirm what you're allowed to build before you hire, review the Sammamish ADU rules (max detached size: 1,000 sq ft. "The ADU must not exceed a floor area of 1,000 square feet" (Sammamish Municipal Code 21.05.010.C(5)(a)(iii)) — exactly the floor RCW 36.70A.681(1)(f) sets, since a city "may not establish a maximum gross floor area requirement for accessory dwelling units that is less than 1,000 square feet." Sammamish has matched the minimum rather than exceeded it. Two things stop that 1,000 sq ft from being eaten by other rules: ADUs are shown as "Exempt" from the maximum residential floor to area ratio in R-1 through R-8, where a single-family house is held to 50 percent, and SMC 21.04.030.C development condition (10) provides that "Accessory Dwelling Units are exempt from hardscape coverage and floor area ratio calculations when constructed on a lot with a single-family home" and built to SMC 21.05.010.C(5)(a). Hardscape coverage is capped at 30 percent in R-1 and 40 percent in R-4, so that exemption is worth a great deal on a large-lot plateau parcel. The separate 3,000 sq ft maximum building footprint still applies per building; owner-occupancy: Not required, and recently repealed. The current SMC 21.05.010.C(5)(a) contains no owner-occupancy condition, no recorded notice and no conversion-or-removal clause. The previous version of the code did: it required that "[t]he primary dwelling unit or the accessory dwelling unit shall be owner occupied," that "[t]he accessory dwelling unit shall be converted to another permitted use or shall be removed if one of the dwelling units ceases to be owner occupied," and that the applicant file a notice with the records and elections division running with the land before any permit would issue. All of it is gone. Sammamish could not reinstate it: "The city or county may not require the owner of a lot on which there is an accessory dwelling unit to reside in or occupy the accessory dwelling unit or another housing unit on the same lot" (RCW 36.70A.681(1)(b))).
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How to Vet an ADU Builder in Sammamish
- License: Confirm a current contractor's license in Washington and that it's in good standing. Ask for the license number and verify it with the state board.
- ADU-specific experience: Ask how many ADUs they've completed in or near Sammamish and request addresses or photos. ADU permitting and setbacks differ from a kitchen remodel.
- Portfolio & references: Get references from recent ADU clients and, if possible, visit a finished unit.
- Insurance: Verify general liability and workers' compensation coverage. Ask for a certificate of insurance.
- Written, detailed bid: Insist on an itemized, fixed-price or clearly-scoped bid — not a verbal estimate. It should state whether permits (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), 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), and impact fees (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) are included.
- Permit handling: Confirm they manage Sammamish's permit process ("A building permit, applied for online through MyBuildingPermit.com — Sammamish takes no paper applications and issues no permits over the counter. There is no conditional use permit, no hearing and, since the code update, no recorded notice running with the land. Start with Project Guidance, the city's formal request for zoning and development information about a parcel; on a plateau lot with any chance of a wetland, stream, steep slope or aquifer recharge designation this is the step that decides the project, because RCW 36.70A.681(4) removes every state protection from a lot designated with critical areas or their buffers and SMC 21.03.020 buffers then govern where anything can be built. State law bars the city from requiring public street improvements as a condition of permitting an ADU (RCW 36.70A.681(1)(l)). Permit Center: 425-295-0531, PermitTech@sammamish.us; walk-in hours Monday and Friday 9am-noon and Wednesday 1-4pm, virtual appointments available with building plans examiners, land use planners and development review engineers", typically 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) and who is responsible if revisions are required.
- Payment schedule: Avoid large upfront payments. A reasonable schedule ties payments to completed milestones.
Questions to Ask Before You Hire
- How many ADUs have you built in Sammamish in the last two years?
- Is your bid fixed-price, and what would trigger a change order?
- Are permits, plan check, and utility connection fees included?
- What's a realistic timeline, including permit review (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) and construction?
- Who is my point of contact and how are delays communicated?
- What warranty do you provide on workmanship?
- Can you share three recent Washington ADU client references?
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How much do ADU builders charge in Sammamish?
ADU builders in Sammamish typically quote $265,000-$480,000 for a new detached unit, or about $320-$530 per square foot, including design, permitting, and construction. A garage conversion runs $130,000-$260,000 and a prefab/modular ADU averages $190,000-$365,000. Add city permit and plan-review fees (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) plus impact fees of 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. Quotes for the same scope commonly vary 20-40% between builders, so collect at least three bids.
How do I find a good ADU builder in Sammamish?
Look for a licensed contractor with completed ADU projects in Sammamish specifically — ADUs involve local zoning, setback rules, and utility connections that general remodelers may not have handled. Verify the license, ask for an ADU-specific portfolio and references, confirm they carry liability and workers' comp insurance, and get a written fixed-price or detailed bid. Submit a request below and we'll forward it to local ADU builders serving Sammamish for free quotes.
Do ADU builders in Sammamish handle permits?
Most full-service ADU builders manage the permit process for you. In Sammamish the approval process is "A building permit, applied for online through MyBuildingPermit.com — Sammamish takes no paper applications and issues no permits over the counter. There is no conditional use permit, no hearing and, since the code update, no recorded notice running with the land. Start with Project Guidance, the city's formal request for zoning and development information about a parcel; on a plateau lot with any chance of a wetland, stream, steep slope or aquifer recharge designation this is the step that decides the project, because RCW 36.70A.681(4) removes every state protection from a lot designated with critical areas or their buffers and SMC 21.03.020 buffers then govern where anything can be built. State law bars the city from requiring public street improvements as a condition of permitting an ADU (RCW 36.70A.681(1)(l)). Permit Center: 425-295-0531, PermitTech@sammamish.us; walk-in hours Monday and Friday 9am-noon and Wednesday 1-4pm, virtual appointments available with building plans examiners, land use planners and development review engineers". On timing: 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. Confirm in writing whether permit fees, 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), and impact fees are included in the bid or billed separately.
How long does it take to build an ADU in Sammamish?
Plan for the permit review (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 in Sammamish) plus construction. Most detached ADUs take 4-8 months of construction after permits are issued; garage conversions and prefab units are often faster. Ask each builder for a realistic schedule and what could delay it.
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