King County DADU Permit Requirements: Everything You Need to Know

Do You Need a Permit for a DADU in King County?

Yes — every DADU in King County requires a building permit, regardless of size. There are no exceptions. Unpermitted structures create legal, insurance, and resale problems that far outweigh the cost and time of doing it right.

The good news: Washington State’s HB 1337 has streamlined the ADU permitting process, and King County has responded with clearer guidelines and faster processing times than in previous years.

What You Need for a DADU Permit Application

A complete DADU permit application in King County typically requires the following:

Site Plan

A scaled drawing of your lot showing:

• Property boundaries and dimensions
• Location of the primary home and proposed DADU
• Setback measurements from all property lines
• Driveways, walkways, and access paths
• Existing trees over 6 inches in diameter
• Utility locations (sewer, water, electrical)

Architectural Drawings

• Floor plan: Room layout, dimensions, doors, windows, and fixtures.
• Elevations: All four exterior views showing height, materials, and roof pitch.
• Sections: Cross-section views showing foundation, wall construction, and roof structure.
• Details: Specific construction details for connections, waterproofing, and structural elements.

Structural Calculations

A licensed structural engineer must provide calculations proving the DADU meets current building code requirements for:

• Foundation design
• Framing loads (gravity, wind, seismic)
• Connection details
• Snow load (relevant in some King County areas)

Energy Compliance

Your DADU must meet Washington State Energy Code requirements. This includes documentation showing:

• Insulation R-values for walls, roof, and foundation
• Window and door U-values
• HVAC system efficiency ratings
• Air sealing and ventilation plans

Additional Reports (When Required)

• Geotechnical report: Required on slopes, in areas with known soil issues, or when the building department requests one.
• Drainage plan: Shows how stormwater from the new structure will be managed.
• Tree protection plan: Required when significant trees are near the construction zone.
• Septic evaluation: Required for properties on septic systems.

The Permit Application Process

Here’s what the process looks like from submission to approval:

1. Pre-application research: Verify your lot’s zoning, size, and utility access. Contact King County Department of Local Services or use the online parcel viewer.
2. Submit application: Applications go through the MyBuildingPermit portal for unincorporated King County. Include all required documents and fees.
3. Intake review: Staff checks that your application is complete. Incomplete applications are returned, which delays the process by weeks.
4. Plan review: Building, fire, zoning, and health department reviewers examine your plans. This is where most of the waiting happens.
5. Corrections and resubmittal: Most applications receive at least one round of correction requests. Respond quickly and completely to keep things moving.
6. Permit issuance: Once all reviews pass, your permit is issued and construction can begin.

How Long Does the Permit Process Take?

Permit processing in King County typically takes 2–6 months, depending on:

• Application completeness: Complete, well-prepared applications move faster. Missing documents can add weeks.
• Project complexity: Simple, single-story DADUs on flat lots with sewer access process faster than complex projects.
• Review workload: Processing times fluctuate with the volume of applications the county is handling.
• Corrections: Each round of corrections adds 2–4 weeks. Minimizing corrections is the single biggest thing you can do to speed up the process.

For tips on keeping your timeline short, read our guide on DADU permit timelines.

Permit Fees

King County DADU permit fees vary by project scope but typically include:

• Building permit fee: Based on project valuation, typically $15,000–$35,000.
• Plan review fee: Often 65–75% of the building permit fee.
• Impact fees: School, transportation, and park district fees vary by location. Budget $15,000–$30,000.
• Utility connection fees: Sewer, water, and electrical connection fees, typically $8,000–$16,000.

Note: King County updated permit fees in 2025, with increases of 12–19% over previous rates. Get current fee schedules from the King County fee breakdown.

Common Permit Mistakes to Avoid

These are the errors we see most often — and each one can delay your project by weeks or months:

• Incomplete applications: Missing a single required document sends the entire package back to the end of the queue.
• Incorrect setback calculations: Measuring from the wrong reference point is surprisingly common. Always measure from the property line, not the fence.
• Ignoring easements: Utility or access easements can restrict where you can build. Check your title report before designing.
• Skipping the pre-application meeting: A 30-minute conversation with the planning department can prevent months of redesign.
• Undersized septic: If you’re on septic, not accounting for the additional capacity can stop your project cold.

Learn more in our permit mistakes guide.

Sewer vs. Septic: A Critical Permit Factor

Whether your property is on public sewer or septic significantly impacts your DADU permit process:

• Sewer: You’ll need confirmation of sewer availability and may need to pay sewer capacity charges. Generally straightforward.
• Septic: Requires evaluation of your existing system’s capacity. A 2025 King County Department of Public Health memo clarified that DADUs sized for one bedroom require 150 gallons per day capacity. System upgrades may be necessary, adding cost and time.

We cover this in detail in our sewer vs. septic guide.

Let APEX DADU Handle Your Permits

The permitting process is where most DADU projects lose time — and where experience makes the biggest difference. APEX DADU manages the entire permit process for King County homeowners, from initial application through final approval. We know what reviewers look for, how to prepare complete applications, and how to respond to corrections efficiently.

Contact APEX DADU to get your permit process started right.