How Much Does an ADU Cost in Bothell, WA?

Construction Cost Breakdown

Building an ADU in Bothell costs between $235,000-$430,000 for new construction, with per-square-foot costs averaging $290-$480. These figures include design, permitting, site preparation, and construction. The most affordable option is a garage conversion at $110,000-$235,000, while prefab/modular ADUs fall in between at $170,000-$330,000.

Permit Costs & Timeline

Permit fees in Bothell are At submittal (2026 fees, from the city's ADU checklist): a plan check fee based on project valuation, a Single Family Planning Review fee of $231.00, and a 5 percent technology fee. At permit issuance: the permit fee, plumbing, mechanical and state fees; a Residential Site Review and Inspection fee of $834.00 where you are constructing an addition or a new building; a stormwater facility charge if you are adding square footage; and the 5 percent technology fee on everything except the state fee, impact fees and facility charges. For a new detached ADU the plumbing and mechanical permit fees are each 8 percent of the building permit fee, 16 percent in total; for a remodel or addition creating an ADU they are a base fee of $82.00 plus a per-fixture fee, most residential fixtures running $27.00 each. The review timeline is Confirm with Bothell Development Services at 425-806-6400 — no ADU review timeline is published on the city's ADU submittal checklist. Budget separately for the water and sewer availability certificate, which the checklist puts at up to two weeks where the City of Bothell is the provider, with plan check: Plan check fee is based on project valuation. Plans must be no larger than 24 by 36 inches, drawn to scale, and the site plan must show hard surface coverage calculations under BMC 12.14.140(B), distances from existing and proposed structures to property lines, footing and roof drain location and discharge method, and any environmentally sensitive areas on the site, adjacent to it, or within 200 feet — wetlands, steep slopes, top and toe of slope, rivers and streams. That last item is not busywork: critical areas are what decide whether you get two ADUs or one under BMC 12.14.135(C). New detached ADUs need full Washington State Energy Code compliance worksheets; King County or Snohomish Health District approval is required where the property is on septic and you are adding square footage or bedrooms. Impact fees are Traffic, fire and park impact fees apply to a new dwelling unit, and Bothell has written the state cap into its own code: "The city will 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" (Bothell Municipal Code 12.14.135(G)), tracking RCW 36.70A.681(1)(a). Current dollar amounts are not published on the ADU checklist — ask Development Services at 425-806-6400 for the impact fee form, and hold them to the 50 percent cap. Pre-approved plans: None found in a City of Bothell source as of September 1, 2026 — the city's ADU submittal checklist assumes a custom set of construction plans and lists no pre-approved plan gallery. Confirm with Development Services at 425-806-6400.

Rental Income & ROI

A typical ADU in Bothell generates approximately $24,900 per year in rental income based on average 1-bedroom rents. At an average construction cost of $332,500, the estimated payback period is 13.4 years — before accounting for the resale value the ADU adds, which an appraiser determines from comparable sales in Bothell and varies by market (often less than build cost).

Ways to Reduce ADU Costs in Bothell