DADU Regulations in Washington State: What Seattle Homeowners Need to Know (2026 Guide)

Detached Accessory Dwelling Units (DADUs) are becoming one of the most powerful ways for homeowners in Washington—especially in Seattle—to increase property value, generate rental income, and maximize land use.

If you’re considering building a DADU, understanding the regulations is critical. This guide breaks down everything you need to know, from statewide laws to Seattle-specific rules.

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What Is a DADU?

A Detached Accessory Dwelling Unit (DADU) is a secondary housing unit located on the same lot as a primary residence, but built as a separate structure.

A legal DADU must include:

• A kitchen
• A bathroom
• A sleeping area
• A separate entrance

DADUs are often referred to as:

• Backyard homes
• Granny flats
• Carriage houses

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Washington State DADU Laws (Overview)

Washington State has recently passed legislation to increase housing supply, making it easier for homeowners to build DADUs.

Key Statewide Rules:

• Cities must allow at least two accessory units per lot in many residential zones
• Minimum size allowances are expanding (typically up to 1,000 sq ft or more)
• Parking requirements are being reduced or eliminated
• Local governments cannot impose overly restrictive barriers

What this means:
Washington is actively encouraging DADU construction—especially in urban areas like Seattle.

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Seattle DADU Regulations (Most Important Section)

If you're building in Seattle, these are the rules that matter most.

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1. Lot Requirements

• Minimum lot size: typically 3,200 sq ft
• Property must already have a primary residence

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2. Size Limits

• Maximum DADU size: 1,000 sq ft (standard)
• Some flexibility depending on lot and design

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3. Height Limits

• Up to 16–18 ft traditionally
• In some updated zones, can go up to 30+ ft

This allows for two-story DADUs, which is a major opportunity.

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4. Setbacks (Distance from Property Lines)

• Typically, 5 feet from the rear and side property lines
• Larger structures may require more space

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5. Parking Requirements

• No off-street parking required in many Seattle zones
• Especially if:
  • Near transit
  • Located in urban neighborhoods

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6. Owner Occupancy

• No longer required

This means:

• You can rent out both the main house and the DADU
• You can use the property as a full investment

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7. Number of Units Allowed

• Up to 2 ADUs per lot
  • Example:
    • 1 Attached ADU (basement or internal)
    • 1 Detached ADU (backyard)

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8. Permits Required

To build a DADU, you must obtain:

• Construction permit
• Electrical permit
• Plumbing/sewer approvals

All permits are handled through Seattle’s permitting system.

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New Opportunity: DADU Subdivision

One of the biggest recent changes:

Some DADUs can now be legally separated and sold independently

This means:

• Build a DADU
• Divide the lot
• Sell the unit as its own property

This is a major shift and creates opportunities for:

• Developers
• Investors
• Homeowners looking to unlock equity

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Cost of Building a DADU in Seattle

Typical cost range:

• $350,000 – $650,000+

Cost depends on:

• Size
• Design complexity
• Site conditions
• Utilities and foundation work

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Why Homeowners Are Building DADUs

DADUs are gaining popularity for several reasons:

1. Rental Income

Generate $1,500–$3,000/month depending on location

2. Increased Property Value

Adds significant resale value to your property

3. Flexible Living

• Guest house
• Home office
• Family housing

4. Long-Term Investment

With new subdivision rules, DADUs can become sellable assets

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DADU Regulations Outside Seattle (Quick Note)

If you're in nearby areas like Bellevue, Kirkland, or Renton:

• Rules vary by city
• Some allow larger units (up to 1,200 sq ft)
• Some still require parking or owner occupancy

Always check local zoning before starting

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How to Get Started

Building a DADU involves several steps:

1. Site evaluation (lot size, zoning, access)
2. Design and planning
3. Permit approval
4. Construction
5. Final inspection and occupancy approval

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Final Thoughts

DADUs are one of the fastest-growing housing trends in Washington State—and Seattle is at the center of it.

With relaxed regulations, increased size allowances, and new opportunities like subdivision, homeowners now have more flexibility than ever to build and benefit from backyard homes.