Everything homeowners ask about building an ADU in Bothell — answered with 2026 data.
Yes, ADUs are allowed in Permitted (P) in every residential zone — R-C, R-L1, R-L2, R-M1, R-M2, R-M3, R-M4, R-MHP and R-AC — and not permitted in the OP, NB, CB, GC and LI zones (Bothell Municipal Code 12.06.140(A), Residential Uses table). Development condition 4 states that "Accessory dwelling units are permitted when added to, created within, or detached from a primary dwelling unit(s) subject to the restrictions in BMC 12.14.135" (BMC 12.06.140(B)(4)). Bothell renamed its residential zones as part of its 2024-2025 code updates; older documents still use the R 40,000 / R 9,600 / R 8,400 / R 7,200 / R 5,400 / R 4,000 / R 2,800 labels. RCW 36.70A.681(1)(c) independently requires two ADUs in all zoning districts inside an urban growth area that allow single-family homes. Confirm your parcel's zone with Bothell Community Development at 425-806-6400 in Bothell, WA. The maximum size for a detached ADU is 1,200 sq ft. "An accessory dwelling unit may be developed up to 1,200 square feet" (Bothell Municipal Code 12.14.135(D)(1)) — 200 sq ft above the 1,000 sq ft floor state law guarantees (RCW 36.70A.681(1)(f)). Conversion of an existing space may exceed 1,200 sq ft as approved by the community development director, provided it does not exceed 40 percent of the primary dwelling unit's gross floor area. Critically, "Detached accessory dwelling units are exempt from the provisions of BMC 12.14.130 that limit the size of accessory buildings to five percent of the lot area" (BMC 12.14.135(D)(2)) — without that exemption a 1,200 sq ft detached unit would need a 24,000 sq ft lot. Attached ADUs can be up to Same 1,200 sq ft cap, with the same conversion allowance: an ADU created by converting existing space may exceed 1,200 sq ft as approved by the community development director, so long as it stays within 40 percent of the primary dwelling unit's gross floor area. Bothell has an ADU-friendliness score of 9/10. Check our full Bothell ADU rules page for complete zoning details.
ADU construction in Bothell typically costs $235,000-$430,000. The cost per square foot averages $290-$480. Garage conversions are more affordable at $110,000-$235,000, while prefab ADUs cost $170,000-$330,000. Permit fees add 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 to your budget. See our detailed Bothell ADU cost guide.
Detached ADUs in Bothell can be up to 1,200 sq ft. "An accessory dwelling unit may be developed up to 1,200 square feet". Attached ADUs are limited to Same 1,200 sq ft cap, with the same conversion allowance: an ADU created by converting existing space may exceed 1,200 sq ft as approved by the community development director, so long as it stays within 40 percent of the primary dwelling unit's gross floor area. Junior ADUs (JADUs) have a cap of Not a category in Washington — Bothell's code has no junior ADU designation. The nearest equivalent is an ADU created within the walls of the existing house, which is treated as an ordinary attached ADU and benefits from the conversion allowance at BMC 12.14.135(D)(1). Maximum height is 30 feet for a detached ADU, or 33 feet for a unit built over an existing accessory structure (Bothell Municipal Code 12.14.135(E)). That sits below the 35-foot principal-building height allowed in the R-C, R-L1, R-L2, R-M1 and R-M2 zones (45 feet in R-M3, 55 feet in R-M4 — BMC 12.14.030(A)), but it is comfortably above the 24-foot floor that RCW 36.70A.681(1)(g) protects, so it stands. An attached ADU is part of the principal building and takes the zone's height limit. You are allowed up to 2 ADUs per lot — "Two accessory dwelling units are allowed per lot" on a qualifying lot.
Rear setback: The same rear yard setback as the house: "Within all residential zone classifications, rear yard setbacks for accessory buildings and structures, other than detached accessory dwelling units, shall be five feet. Detached accessory dwelling units shall maintain the same rear yard setback as primary structures, except for those accessory dwelling units located on an alley, in which case the setbacks for structures adjoining alleys in BMC 12.14.060 shall apply" (Bothell Municipal Code 12.14.070(B)). The zone minimums where there is no alley are 35 feet in R-C, 15 feet in R-L1 and R-L2, and 10 feet in R-M1 through R-M4; where the lot adjoins an alley the rear setback drops to 0 feet, or 3 feet for garage doors (BMC 12.14.030(A)). Note the odd result: an ordinary garden shed gets a 5-foot rear setback while a detached ADU in R-C must sit 35 feet back. State law bars setbacks for ADUs more restrictive than those for principal units (RCW 36.70A.681(1)(h)), and Bothell's rule matches the principal unit exactly. Side setback: 5 feet in every residential zone — R-C, R-L1, R-L2 and R-M1 through R-M4 all carry a 5-foot minimum side yard setback for the principal building (Bothell Municipal Code 12.14.030(A)), and a detached ADU takes the principal-structure standards. Setbacks are measured under BMC 12.14.050 through 12.14.100; projections allowed into a setback are listed at BMC 12.14.090. Front setback: 15 feet in the R-C, R-L1 and R-L2 zones and 10 feet in R-M1 through R-M4, except that any garage door must be set back 20 feet (Bothell Municipal Code 12.14.030(A)). BMC 12.14.030(B)(4)(b) adds that garages, carports and other structures intended to house vehicles must be set back at least 20 feet from streets and access tracts. The area within the front yard setback may not contain hard surface except driveways, walkways and permitted projections. Note that garage conversions may be exempt from setback requirements if the existing structure is already within the setback area.
One off-street space per ADU as last verifiable, waived on transit or trail proximity — and possibly now zero. BMC 12.16.030(A) lists "Dwelling units, accessory — 1 per unit, unless waived per BMC 12.16.110(F)", and BMC 12.16.110(F) states the ADU parking requirement does not apply where the ADU is within one-half mile walking distance of a major or frequent transit stop, or within one-half mile walking distance of pedestrian access to a regional trail that provides a continuous two-way paved connection to employment centers or similar activity centers and amenities (Ord. 2415, 2024) — the trail waiver is Bothell's own addition and reaches much of the Sammamish River and Burke-Gilman corridor. Since then, on July 8, 2025, the City Council "approved the elimination of required parking minimums for new developments," citing SB 5184. Whether that repeal reached the single-family ADU row could not be verified for this entry because Bothell's code host blocks automated access — confirm with Bothell Community Development at 425-806-6400 before you give up a parking space. Either way, RCW 36.70A.681(2)(a) caps what Bothell may require at zero within half a mile walking distance of a major transit stop, one space per unit on lots under 6,000 sq ft, and two per unit on larger lots. Many cities and states have been relaxing ADU parking requirements in recent years. In California, no additional parking is required for ADUs near transit.
Not required. BMC 12.14.135 contains no owner-occupancy condition, no rental restriction and no recorded covenant. Bothell could not impose one in any event: "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)). Note that owner-occupancy rules vary by state law — California and several other states have eliminated this requirement for standard ADUs.
The ADU permit review timeline in Bothell is typically 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. 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. Pre-approved plans availability: 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.
Average ADU rental income in Bothell: 1-bedroom units rent for $1,800-$2,350/month, 2-bedroom units for $2,250-$2,950/month. Short-term rentals: Confirm with the City of Bothell at 425-806-6400 before you list. No short-term rental chapter of the Bothell Municipal Code could be verified for this entry — the code host blocks automated access — and BMC 12.14.135 itself imposes no minimum tenancy. Bothell's zoning does separately regulate "Rooms for roomers or boarders" as a permitted residential use in every residential zone (BMC 12.06.140(A)). Long-term letting of an ADU is unrestricted: there is no owner-occupancy rule and no cap on renting the unit. Expected ROI: 5-8% annual return.
The minimum lot size for an ADU in Bothell is No ADU-specific minimum, and state law requires a city to allow an ADU on any lot that meets the minimum lot size for the principal unit (RCW 36.70A.681(1)(e)). The zone minimums for the lot itself are 40,000 sq ft in R-C, 6,000 sq ft in R-L1 and 3,600 sq ft in R-L2, with no minimum in R-M1 through R-M4 or R-AC (Bothell Municipal Code 12.14.030(A)). Lot size averaging applies in R-L1 and R-L2 land divisions (BMC 12.14.030(B)(2)(b)). The practical size constraint on a detached ADU is hard surface coverage — 35 percent of the lot in R-C, 55 percent in R-L1, 60 percent in R-L2, 70 percent in R-M1 — not lot area, because BMC 12.14.135(D)(2) exempts detached ADUs from the five-percent accessory building cap. This applies to detached ADUs — garage conversions and JADUs may have different or no lot size requirements.
Garage conversions are one of the most popular and affordable ADU options. In Bothell, a garage conversion typically costs $110,000-$235,000. The key advantages are lower cost (no foundation work), potential setback exemptions, and faster permitting. Read our complete garage conversion guide.
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