Everything homeowners ask about building an ADU in Bothell — answered with 2026 data.
Yes, ADUs are allowed in All residential zones: R-C, R-L1, R-L2 and R-M1 through R-M4. The Residential Development Standards table carries a "Maximum ADUs per lot" row of 2 and BMC 12.14.135 permits ADUs with no zone limit (BMC 12.14.030(A), 12.14.135). The use table at BMC 12.06.140 listed ADUs as permitted in every residential zone when last read Sept 1, 2026. RCW 36.70A.681(1)(c) requires two ADUs wherever single-family homes are allowed inside the urban growth area. Confirm your zone: 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" (BMC 12.14.135(D)(1)), 200 sq ft above the state floor (RCW 36.70A.681(1)(f)). A conversion of existing space may exceed 1,200 sq ft with the community development director's approval, up to 40 percent of the primary unit's gross floor area. Detached ADUs are exempt from the BMC 12.14.130 five-percent-of-lot cap on accessory buildings (BMC 12.14.135(D)(2)); hard surface coverage is the real limit. Attached ADUs can be up to Same 1,200 sq ft cap. 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 $325,000-$550,000. The cost per square foot averages $400-$650. Garage conversions are more affordable at $100,000-$225,000, while prefab ADUs cost $300,000-$450,000. Permit fees add 2026 fees: a valuation-based permit fee (Table 20.02.155-A: $1,514 plus $9 per $1,000 over $100,000), plan check at 65 percent, planning review $231, site review $834, energy review $230, plumbing and mechanical 16 percent, 5 percent tech fee. A $300,000 detached ADU: about $7,700 plus impact fees 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. Junior ADUs (JADUs) have a cap of Washington has no junior ADU category and Bothell's code has none. An ADU built inside the existing house is an ordinary attached ADU under BMC 12.14.135 and may use the conversion allowance in 12.14.135(D)(1), so it can exceed 1,200 sq ft within 40 percent of the house's gross floor area. Maximum height is 30 ft for a detached ADU, or 33 ft for a unit built over an existing accessory structure (BMC 12.14.135(E)). The principal-building limit is 35 ft in R-C, R-L1, R-L2, R-M1 and R-M2, 45 ft in R-M3 and 55 ft in R-M4 (BMC 12.14.030(A)); an attached ADU is part of the house and takes that limit. Both sit above the 24-ft floor in RCW 36.70A.681(1)(g). You are allowed up to Two per lot: "Two accessory dwelling units are allowed per lot" on a qualifying lot.
Rear setback: Same as the house. "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" (BMC 12.14.070(B), quoted on the city's ADU checklist). Zone minimums: 35 ft in R-C, 15 ft in R-L1 and R-L2, 10 ft in R-M1 through R-M4; on an alley 0 ft, 3 ft for a garage door (BMC 12.14.030(A)). Side setback: 5 ft in every residential zone (BMC 12.14.030(A), Residential Development Standards table as amended by Ord. 2461, July 2025); a detached ADU takes the principal-structure standard. Measurement rules are at BMC 12.14.050 through 12.14.100 and permitted projections into a setback at BMC 12.14.090. State law bars ADU setbacks stricter than the house's (RCW 36.70A.681(1)(h)). Front setback: 15 ft in R-C, R-L1 and R-L2 and 10 ft in R-M1 through R-M4; any garage door must be 20 ft back (BMC 12.14.030(A)). The front setback may hold no hard surface other than a driveway no wider than three cars, a walkway up to 5 ft wide and projections allowed by BMC 12.14.090 (BMC 12.14.030(B)(6)). Note that garage conversions may be exempt from setback requirements if the existing structure is already within the setback area.
None. Ordinance 2461 (adopted July 8, 2025) struck the ADU parking subsection from BMC 12.14.135 and deleted the minimum-parking table in BMC 12.16.030, including "Dwelling units, accessory: 1 per unit"; the section now reads "Except as provided herein, there is no required minimum number of off-street parking spaces." The old transit and trail waivers in 12.16.110(F) went with it. You may still build a space; the site plan must show any you keep or add. State floor: RCW 36.70A.681(2)(a). 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, as codified through Ord. 2464 (Sept 16, 2025), has no owner-occupancy condition, rental restriction or covenant, and RCW 36.70A.681(1)(b) forbids one. State law also bars the city from blocking the separate sale of an ADU that has been made a condominium unit (RCW 36.70A.681(1)(k)). 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 Not published: the city's ADU checklist (form 3f, Dec 2025) gives no review time; ask Development Services at 425-806-6400. Add up to two weeks for the water and sewer availability certificate where the City is the provider, and get the address assignment first. Plan check: Plan check fee is 65 percent of the building permit fee and is due at submittal. Plans are 24 by 36 inches maximum, construction drawings at 1/8 inch to the foot or larger. The site plan must show a hard surface coverage table per BMC 12.14.140(B), distances from every structure to the property lines, footing and roof drain routing and discharge, existing and proposed parking, 2-ft contours (5-ft where slope is 15 percent or more), easements, and any environmentally sensitive area on site, adjacent, or within 200 ft (wetlands, steep slopes, streams). That last item decides whether you get two ADUs or one under BMC 12.14.135(C). A new detached ADU needs the full Washington State Energy Code worksheets (prescriptive compliance, heating system size, glazing schedule). Pre-approved plans availability: None. The city has no pre-approved ADU plan program: the ADU checklist assumes a custom plan set, and the draft Housing Action Plan presented to Council on June 9, 2026 (adoption by resolution expected December 2026) only lists "pre-designed layout plans for ADUs" as an idea to explore. Confirm with Development Services at 425-806-6400.
Average ADU rental income in Bothell: 1-bedroom units rent for $2,300-$2,700/month, 2-bedroom units for $2,700-$3,150/month. Short-term rentals: Not regulated by an ADU-specific rule. Expected ROI: 5-8% annual return.
The minimum lot size for an ADU in Bothell is No ADU-specific minimum; state law requires an ADU on any lot that meets the principal unit's minimum (RCW 36.70A.681(1)(e)). Lot minimums are 40,000 sq ft in R-C, 6,000 in R-L1, 3,600 in R-L2 and none in R-M1 through R-M4 (BMC 12.14.030(A)). The binding limit for a detached ADU is hard surface coverage: 35 percent of the lot in R-C, 55 in R-L1, 60 in R-L2, 70 in R-M1, 75 in R-M2, since Ord. 2461 dropped the building-coverage row and 12.14.135(D)(2) waives the 5 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 $100,000-$225,000. 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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