Everything homeowners ask about building an ADU in Seattle — answered with 2026 data.
Yes, ADUs are allowed in All residential zones in Seattle, WA. The maximum size for a detached ADU is 1,200 sq ft. Attached ADUs can be up to 1,000 sq ft. Seattle has an ADU-friendliness score of 9/10. Check our full Seattle ADU rules page for complete zoning details.
ADU construction in Seattle typically costs $160,000-$320,000. The cost per square foot averages $130-$270. Garage conversions are more affordable at $75,000-$170,000, while prefab ADUs cost $110,000-$260,000. Permit fees add $1,800-$2,800 to your budget. See our detailed Seattle ADU cost guide.
Detached ADUs in Seattle can be up to 1,200 sq ft. Attached ADUs are limited to 1,000 sq ft. Junior ADUs (JADUs) have a cap of 800 sq ft. Maximum height is 40 feet. You are allowed up to 3 units per lot possible (duplex + ADU) on a qualifying lot.
Rear setback: 5 feet. Side setback: 0 feet (easement). Front setback: 0 feet (alley lot). Note that garage conversions may be exempt from setback requirements if the existing structure is already within the setback area.
Optional. 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. 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 Seattle is typically 30-45 days. The plan check process takes approximately 1-2 weeks. Pre-approved plans availability: Extensive library available.
Average ADU rental income in Seattle: 1-bedroom units rent for $1,600-$2,100/mo/month, 2-bedroom units for $2,000-$2,800/mo/month. Short-term rentals: Restricted (30 days minimum). Expected ROI: 6-8% annual return.
The minimum lot size for an ADU in Seattle is 3,000 sq ft. 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 Seattle, a garage conversion typically costs $75,000-$170,000. The key advantages are lower cost (no foundation work), potential setback exemptions, and faster permitting. Read our complete garage conversion guide.