Everything homeowners ask about building an ADU in Seattle — answered with 2026 data.
Yes, ADUs are allowed in All residential zones (Neighborhood Residential and other residential zones) in Seattle, WA. The maximum size for a detached ADU is 1,000 sq ft of floor area above grade (size also constrained by FAR and lot coverage). 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 $2,000-$5,000+ in SDCI permit fees (varies with valuation; hourly review rates apply) to your budget. See our detailed Seattle ADU cost guide.
Detached ADUs in Seattle can be up to 1,000 sq ft of floor area above grade (size also constrained by FAR and lot coverage). Attached ADUs are limited to 1,000 sq ft. Junior ADUs (JADUs) have a cap of Seattle does not use a separate JADU category; second unit treated as an AADU/DADU. Maximum height is Up to 32 ft in Neighborhood Residential and most residential zones (raised by 2025 reform; height also governed by zone limits). You are allowed up to 2 ADUs per lot (up to ~3 units total on many lots under middle-housing rules) on a qualifying lot.
Rear setback: 5 feet. Side setback: 5 feet (reduced setbacks allowed along an alley/easement). Front setback: Per zone; DADU generally behind primary structure or 20 ft from front lot line. Note that garage conversions may be exempt from setback requirements if the existing structure is already within the setback area.
Not required (no parking minimum for ADUs). 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 Several weeks to a few months depending on review queue. Plan check: 1-3 weeks for pre-approved (ADUniverse) plans; longer for custom. Pre-approved plans availability: Yes — ADUniverse pre-approved DADU plan library.
Average ADU rental income in Seattle: 1-bedroom units rent for $1,600-$2,100/month, 2-bedroom units for $2,000-$2,800/month. Short-term rentals: Allowed with a Seattle STR operator license ($75/yr); a 'short-term rental' is under 30 consecutive nights, and an ADU/DADU can be a licensed STR subject to the operator/primary-residence rules. Expected ROI: 6-8% annual return.
The minimum lot size for an ADU in Seattle is No minimum lot size for an ADU (varies by zone). 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.