Everything homeowners ask about building an ADU in Bellevue — answered with 2026 data.
Yes, ADUs are allowed in Any lot in a land use district that allows a single-family dwelling, i.e. every residential district LL-1 through MDR-2 (LUC 20.20.120.C.1 as rewritten by Ord. 6851, effective July 1, 2025). Detached ADUs were not allowed before that date; the 2023 ADU Reform (Ord. 6746) covered attached units only. State floor: RCW 36.70A.681(1)(c)-(d) in Bellevue, WA. The maximum size for a detached ADU is 1,200 sq ft of floor area (LUC 20.20.120; confirmed on the City's 2026 Pre-Approved Plans page). The Director may approve a larger unit when it is an addition to, or a conversion of, an existing detached accessory structure; a converted guest cottage is exempt from the cap if not enlarged. The council draft also excluded up to 250 sq ft of garage or unheated storage per ADU - confirm that clause with Land Use Review. Attached ADUs can be up to 1,200 sq ft of floor area. Bellevue has an ADU-friendliness score of 8/10. Check our full Bellevue ADU rules page for complete zoning details.
ADU construction in Bellevue 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 Fees are set by the 2026 fee ordinance (Ord. 6879): a plan-review deposit at submittal and an inspection deposit at issuance, charged at the stated hourly rate with monthly invoices for overage. Use the City's Permit Fee Estimator (permitfeeestimator.bellevuewa.gov) for a figure to your budget. See our detailed Bellevue ADU cost guide.
Detached ADUs in Bellevue can be up to 1,200 sq ft of floor area. Attached ADUs are limited to 1,200 sq ft of floor area. Junior ADUs (JADUs) have a cap of Washington has no junior ADU category. A small unit carved out of the house is simply an attached ADU and counts as one of the two ADUs allowed per lot (LUC 20.20.120). Maximum height is Detached ADU: 24 ft measured to the highest point of the structure, or 28 ft when built as an addition over an existing or proposed accessory structure such as a garage (LUC 20.20.120.D.1; City Pre-Approved Plans page, 2026). Attached ADUs use the main house's height limit. State floor: a city may not cap ADU roof height below 24 ft (RCW 36.70A.681(1)(g)). You are allowed up to 2 ADUs per lot in any combination - two attached, two detached on a qualifying lot.
Rear setback: Detached ADUs use the middle-housing setbacks in LUC 20.20.538 Table C.1, which the council draft set at 10 ft in SR-3 through MDR-2 and 15 ft in LL-1, LL-2, SR-1 and SR-2; a DADU may sit on a rear lot line that abuts a public alley (LUC 20.20.120.D.2; RCW 36.70A.681(1)(i)). Attached ADUs use the main house's rear setback. Confirm the adopted table with Land Use Review, 425-452-4188. Side setback: Detached ADUs: 5 ft in every residential district under the middle-housing table in LUC 20.20.538 Table C.1 (council draft; adopted table not re-read, code host blocked). Attached ADUs use the main house's side setback. By state law an ADU setback may not be more restrictive than the main house's (RCW 36.70A.681(1)(h)). Front setback: Detached ADUs: the middle-housing front setback in LUC 20.20.538 Table C.1 - 10 ft in SR-1 through MDR-2, 20 ft in LL-2 and 25 ft in LL-1 per the council draft; an existing garage may be converted to an ADU even if it violates today's setbacks or lot coverage (LUC 20.20.120.C.4; RCW 36.70A.681(1)(j)). Confirm the adopted figures with Land Use Review. Note that garage conversions may be exempt from setback requirements if the existing structure is already within the setback area.
None for an ADU under 1,000 sq ft (garage area excluded) or within 1/2 mile of a major transit stop; 1 space for 1,000-2,000 sq ft; over 2,000 sq ft, 1 space on lots under 6,000 sq ft and 2 on larger lots, in addition to the main house's spaces (LUC 20.20.120.C.5; City ADU Reform page; RCW 36.70A.681(2)). 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. Bellevue removed its owner-occupancy rule with Ord. 6746 on July 17, 2023 (City ADU Reform page), and RCW 36.70A.681(1)(b) now forbids any such requirement statewide. 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 Bellevue is typically The City publishes no target review time for a BS permit; plan review is billed hourly against a submittal deposit, so timing follows the review queue. Ask Permit Processing (425-452-6800) for the current first-review estimate. Plan check: Standard building plan review by Development Services under the BS permit; reviewers charge hours against the deposit, and the middle-housing amendments removed design-review and design-matching controls for ADUs (Ord. 6746; RCW 36.70A.681(1)(h)). A pre-application meeting is optional. Pre-approved plans availability: None available yet. The council made a DADU and cottage pre-approved plans program a 2024-2026 priority; the March 10, 2026 study session laid out a three-phase program ending with up to 10 city-pre-approved plans sold at a set price, timed to the next state building code adoption (City Pre-Approved Plans page).
Average ADU rental income in Bellevue: 1-bedroom units rent for $2,500-$3,200/month, 2-bedroom units for $2,900-$3,750/month. Short-term rentals: No citywide ban identified. Stays under 30 days are regulated as 'short term stay uses' under LUC 20.20.800, which Ord. Expected ROI: 5-8% gross rental yield on a $400,000-$550,000 detached build renting at $2,700-$3,500/mo, before vacancy, taxes and financing; conversions of existing space at $100,000-$225,000 yield roughly twice that.
The minimum lot size for an ADU in Bellevue is No ADU-specific minimum: any lot that meets the minimum lot size for its main house may add ADUs (RCW 36.70A.681(1)(e); LUC 20.20.120.C.1). On lots of 2,000 sq ft or less the city may still cap the number at two ADUs plus the main house (RCW 36.70A.681(3)). 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 Bellevue, 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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