Seattle ADU Requirements: A Complete Guide

An Accessory Dwelling Unit (ADU) is a secondary housing unit on a residential property in Seattle. Understanding the current requirements is essential before starting construction.

Basic ADU Types in Seattle:

• Attached ADU (AADU): Built within or attached to the main house
• Detached ADU (DADU): A separate structure in the backyard. Source: https://www.seattle.gov/sdci/permits/common-projects/accessory-dwelling-units

Key Seattle ADU Requirements:

Number of ADUs Allowed Seattle allows up to two ADUs per lot. You can have either:

• Two ADUs
• Two DADUs
• One ADU and one DADU Source: https://www.seattle.gov/documents/Departments/SDCI/Codes/ADUFactSheet.pdf

Size Requirements

• Maximum size: 1,000 square feet (excluding garage and storage areas)
• Minimum size: 220 square feet
• For DADUs, the footprint cannot exceed 60% of the main house’s footprint. Source: Seattle Municipal Code 23.44.041

Height Limits

• DADUs: Maximum 18 feet generally, up to 23 feet with green building features
• AADUs: Must comply with single-family home height limits. Source: https://www.seattle.gov/sdci/codes/common-code-questions/accessory-dwelling-units

Parking

• No off-street parking required for ADUs in Seattle
• Existing required parking can be removed to build an ADU. Source: https://www.seattle.gov/documents/Departments/SDCI/Codes/ADUFactSheet.pdf

Owner Occupancy

• As of 2019, owner occupancy is no longer required in Seattle
• Property owners can rent both the main house and ADU(s). Source: Seattle Ordinance 125854

Building Requirements

• Must meet current Seattle Residential Code standards
• Requires separate entrance from main house
• Must include bathroom and kitchen facilities
• Fire safety requirements include smoke alarms and carbon monoxide detectors. Source: Seattle Municipal Code 23.44.041

Permitting Process

1. Pre-application research
2. Site plan review
3. Building permit application
4. Construction inspections. Source: https://www.seattle.gov/sdci/permits/common-projects/accessory-dwelling-units

Setback Requirements

• Side yard: 5 feet minimum
• Rear yard: 5 feet minimum
• Front yard: Must meet underlying zone requirements. Source: Seattle Municipal Code 23.44.041

Rental Rules

• Both short-term and long-term rentals are allowed
• No minimum rental period requirements
• Must comply with Seattle rental registration requirements. Source: https://www.seattle.gov/rental-registration

Important Considerations:

• ADUs must maintain architectural compatibility with main house
• Solar panels are not specifically required for ADUs
• Utility connections may require separate meters
• HOA restrictions may apply (check your covenant). Source: https://www.seattle.gov/sdci/permits/common-projects/accessory-dwelling-units

Cost Considerations

• Permit fees vary based on project scope
• Impact fees may apply
• Utility connection fees may be required [Source: https://www.seattle.gov/sdci/codes/codes-we-enforce-(a-z)/fees]

Getting Started

1. Review Seattle’s ADU requirements thoroughly
2. Consult with us as your primary contractor to start the estimation process
3. We will check property zoning and restrictions
4. Apply for necessary permits. Source: https://www.seattle.gov/sdci/permits

This guide reflects Seattle’s current ADU regulations as of 2023. Requirements may change, so always verify current rules with the Seattle Department of Construction and Inspections (SDCI) before starting your project.