Understanding ADUs in Seattle: A Complete Guide

What is an ADU?

An Accessory Dwelling Unit (ADU) is a secondary living space on a residential property. These can be either attached to your main house (AADU) or detached as a separate structure (DADU). ADUs are sometimes called backyard cottages, in-law apartments, or granny flats. Source: https://www.seattle.gov/sdci/permits/common-projects/accessory-dwelling-units

Seattle ADU Rules (Updated 2023)

In 2019, Seattle made major changes to its ADU rules (read full requirements here), making it easier for homeowners to build these units. Here are the key rules:

• You can build up to two ADUs on your property
• The maximum size is 1,000 square feet (not 1,200 as previously stated)
• No minimum lot size requirement
• No off-street parking requirement
• Owner-occupancy is not required Source: https://www.seattle.gov/documents/Departments/SDCI/Codes/ADUFactSheet.pdf

Size and Requirements

Every ADU must include:

• A kitchen
• A bathroom
• A sleeping area
• Its own entrance
• Minimum ceiling height of 7 feet. Source: https://www.seattle.gov/sdci/codes/common-code-questions/accessory-dwelling-units

Types of Seattle ADUs

1. Attached ADUs (AADUs):

• Built within or attached to your main house
• Common examples: basement conversions, above-garage apartments
• Must share a wall with the main house

2. Detached ADUs (DADUs):

• Separate structure from main house
• Also called backyard cottages
• Can be up to 1,000 square feet. Source: https://www.seattle.gov/sdci/permits/common-projects/accessory-dwelling-units

Permits and Process

All ADUs in Seattle require:

• Building permit
• Site plan review
• Zoning review
• Construction inspections The permit process typically takes 2-4 months for review and approval. Source: https://www.seattle.gov/sdci/permits/permits-we-issue-(a-z)/accessory-dwelling-unit

Benefits of Building an ADU

1. Additional Income: You can rent out your ADU for extra monthly income
2. Family Housing: Provide separate living space for family members
3. Property Value: ADUs typically increase property values
4. Housing Solutions: Help address Seattle’s housing shortage. Source: https://www.seattle.gov/housing/homeowners/accessory-dwelling-units

Cost Considerations

As of 2023, building an ADU in Seattle typically costs:

• Attached ADU: $100,000 – $300,000
• Detached ADU: $150,000 – $500,000 These costs include permits, design, and construction. Source: https://www.seattle.gov/documents/Departments/SDCI/Codes/ADUCostEstimate.pdf

Important Considerations

Before building an ADU, consider:

• Zoning requirements
• Building codes
• Utility connections
• Construction timeline
• Budget and financing options
• Impact on property taxes

Getting Started

To begin your ADU project:

1. Review Seattle’s ADU regulations
2. Consult with a licensed architect or contractor
3. Get a site evaluation
4. Apply for necessary permits
5. Begin construction with approved plans

For more information, visit the Seattle Department of Construction and Inspections website and contact us, your primary licensed local contractor specializing in ADU construction.