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 (LUC 20.20.120) in Bellevue, WA. The maximum size for a detached ADU is 1,200 sq ft (Director may approve more for single-floor units in the primary structure or conversions/additions to existing accessory structures). Attached ADUs can be up to 1,200 sq ft, or 40% of the structure, whichever is greater. 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 $180,000-$380,000. The cost per square foot averages $200-$350. Garage conversions are more affordable at $90,000-$190,000, while prefab ADUs cost $120,000-$280,000. Permit fees add Permit and plan review fees based on project valuation — Confirm with Bellevue Development Services to your budget. See our detailed Bellevue ADU cost guide.
Detached ADUs in Bellevue can be up to 1,200 sq ft (Director may approve more for single-floor units in the primary structure or conversions/additions to existing accessory structures). Attached ADUs are limited to 1,200 sq ft, or 40% of the structure, whichever is greater. Junior ADUs (JADUs) have a cap of Not a category in Washington (no JADU designation). Maximum height is Detached ADUs: 24 ft to the highest point (28 ft when built as an addition over an existing accessory structure). You are allowed up to 2 ADUs per lot (any combination of attached/detached); ADUs may be condominiumized and sold separately on a qualifying lot.
Rear setback: Subject to middle-housing dimensional standards (LUC 20.20.538) — Confirm with Bellevue Development Services. Side setback: Subject to middle-housing dimensional standards (LUC 20.20.538) — Confirm with Bellevue Development Services. Front setback: Per underlying district / middle-housing standards (LUC 20.20.538) — Confirm with Bellevue Development Services. Note that garage conversions may be exempt from setback requirements if the existing structure is already within the setback area.
None under 1,000 sq ft; 1 space for 1,000-2,000 sq ft; none required within a half mile of frequent transit (HB 1337 also caps ADU parking mandates). 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 (removed by Ordinance 6746 in 2023; prohibited by HB 1337). 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 Several weeks to a few months depending on review queue — Confirm with Bellevue Development Services. Plan check: Standard plan review; timing varies with queue — Confirm with Bellevue Development Services. Pre-approved plans availability: No city pre-approved plan library — Confirm with Bellevue Development Services.
Average ADU rental income in Bellevue: 1-bedroom units rent for $2,500-$2,900/month, 2-bedroom units for $3,100-$3,900/month. Short-term rentals: No citywide STR ban identified — Confirm current short-term rental licensing with the City of Bellevue. Expected ROI: 6-8% annual return.
The minimum lot size for an ADU in Bellevue is No ADU-specific minimum — allowed wherever a single-family dwelling is permitted. 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 $90,000-$190,000. The key advantages are lower cost (no foundation work), potential setback exemptions, and faster permitting. Read our complete garage conversion guide.