Everything homeowners ask about building an ADU in Washington — answered with 2026 data.
Yes, ADUs are allowed in Most R (residential) zones; varies by specific district in Washington, DI. The maximum size for a detached ADU is Greater of 30% of required rear yard OR 450 sq ft. Attached ADUs can be up to 35% of gross floor area of primary dwelling; maximum 1,000 sq ft total. Washington has an ADU-friendliness score of 7/10. Check our full Washington ADU rules page for complete zoning details.
ADU construction in Washington typically costs $150,000-$250,000. The cost per square foot averages $300-$400. Garage conversions are more affordable at $0-$0, while prefab ADUs cost $100,000-$150,000. Permit fees add $1,000-$3,000 building permits; additional utility costs to your budget. See our detailed Washington ADU cost guide.
Detached ADUs in Washington can be up to Greater of 30% of required rear yard OR 450 sq ft. Attached ADUs are limited to 35% of gross floor area of primary dwelling; maximum 1,000 sq ft total. Junior ADUs (JADUs) have a cap of N/A (DC uses basement apartments/ADUs). Maximum height is 2 stories; 22 ft maximum (detached). You are allowed up to 1 accessory apartment per lot only on a qualifying lot.
Rear setback: Minimum 7.5 ft from alley centerline (if applicable). Side setback: Standard zone requirements. Front setback: Standard zone requirements. Note that garage conversions may be exempt from setback requirements if the existing structure is already within the setback area.
Not required for ADU (only primary dwelling). Many cities and states have been relaxing ADU parking requirements in recent years. In California, no additional parking is required for ADUs near transit.
Required - either main house or ADU must be owner-occupied. 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 Washington is typically 6-12 months total (planning through final inspection); 8-12 months typical. The plan check process takes approximately 4-6 weeks; longer for complex projects. Pre-approved plans availability: Standard basement conversion and detached plans available.
Average ADU rental income in Washington: 1-bedroom units rent for $1,400-$2,000/month, 2-bedroom units for $1,800-$2,800/month. Short-term rentals: Rental license required if renting; 30-day minimum suggested but not mandated. Expected ROI: 10-15 year payback; strong appreciation in DC market.
The minimum lot size for an ADU in Washington is 1,200 sq ft gross land area minimum for detached. 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 Washington, a garage conversion typically costs $0-$0. The key advantages are lower cost (no foundation work), potential setback exemptions, and faster permitting. Read our complete garage conversion guide.