The Complete Guide to Building a DADU in King County, Washington

What Is a DADU?

A DADU — detached accessory dwelling unit — is a self-contained living space built as a separate structure on the same lot as your primary home. Think of it as a fully independent home in your backyard, complete with its own kitchen, bathroom, living area, and private entrance.

In King County, Washington, DADUs have become one of the most practical ways homeowners can add living space, generate rental income, or create multigenerational housing without buying a second property.

Why King County Homeowners Are Building DADUs

The Puget Sound housing market has made traditional homeownership increasingly difficult. DADUs offer a solution that benefits homeowners and the community alike:

• Rental income: A well-designed DADU in King County can generate $1,500–$3,000+ per month in rental income, depending on location and size.
• Property value: Adding a DADU typically increases property value by 20–30%, often exceeding the construction cost.
• Family flexibility: Keep aging parents close, provide space for adult children, or create a private home office.
• Housing impact: Washington State actively encourages ADU construction through legislation like HB 1337, making permits easier and faster to obtain.

King County DADU Regulations at a Glance

King County allows detached accessory dwelling units on single-family residential lots that meet minimum size requirements. Here are the key rules:

• Maximum size: 1,000 square feet of heated floor area, plus up to 1,000 square feet of unheated space. Basement floor area in a detached ADU does not count toward the maximum.
• Lot size: Urban lots must be at least 3,200 square feet. Rural lots must meet the minimum for their zone.
• Multiple ADUs: King County allows up to two ADUs per lot — one attached and one detached, two attached, or two detached.
• Setbacks: Minimum 5 feet from side and rear property lines in urban areas. Street-facing DADUs must maintain the same front yard setback as the primary home.
• Height: Must comply with base height limits for your zone.
• Owner occupancy: Under HB 1337, owner occupancy requirements have been relaxed across most jurisdictions.

For a deep dive into permit requirements, read our complete permitting guide.

How Much Does a DADU Cost in King County?

The total cost to build a DADU in King County ranges from $400,000–$650,000 for a typical 800 square foot unit, depending on location, site conditions, and finishes. Here’s a rough breakdown:

• Design and engineering: $14,000–$31,000
• Permits and fees: $43,000–$81,000
• Construction: $205,000–$322,000
• Project management and extras: $48,000–$87,000

Smaller units under 500 square feet have higher per-square-foot costs due to fixed expenses like kitchens, bathrooms, and permits. Larger units over 1,000 square feet trend lower per square foot.

Get the full cost picture in our DADU cost breakdown guide.

The DADU Building Process

Building a DADU in King County typically takes 10–18 months from initial design to move-in. Here’s what to expect:

1. Feasibility assessment: Evaluate your lot size, zoning, setbacks, utilities, and site conditions.
2. Design: Work with an architect or designer to create plans that meet code requirements and your needs.
3. Permit application: Submit plans through the MyBuildingPermit portal. Processing takes 2–6 months.
4. Construction: Foundation, framing, mechanical systems, interior finishes, and exterior work. Typically 4–8 months.
5. Inspections and occupancy: Final inspections and certificate of occupancy.

Follow along with our step-by-step building process guide.

Choosing the Right DADU Builder

Not every contractor has experience with DADUs, and the permitting complexity in King County makes experience essential. When evaluating builders, look for:

• Specific DADU and ADU project experience in King County
• Knowledge of local zoning and permit requirements
• A portfolio of completed projects you can visit
• Transparent pricing with detailed cost breakdowns
• Licensed, bonded, and insured in Washington State

APEX DADU specializes in detached dwelling units across King County, handling every step from initial feasibility through final move-in.

DADU Design Considerations

Your DADU should complement your primary home and work within your lot’s constraints. Key design decisions include:

• Layout: Studio, one-bedroom, or two-bedroom configurations each serve different purposes.
• Height: Single-story units minimize neighbor impact; two-story designs maximize square footage on small lots.
• Style: Pacific Northwest aesthetics — natural materials, large windows, and covered outdoor spaces — tend to blend well with existing neighborhoods.
• Accessibility: If you’re building for aging parents or future accessibility, plan single-floor living, wider doorways, and walk-in showers from the start.

Explore design inspiration in our floor plan guide.

Financing Your DADU

Most King County homeowners finance their DADU through one of these options:

• Home equity line of credit (HELOC): Leverage existing home equity at competitive rates.
• Construction loans: Short-term financing that converts to a mortgage after completion.
• Cash-out refinance: Refinance your primary mortgage and pull out equity for construction.
• ADU-specific loan programs: Some lenders offer products designed specifically for ADU construction.

Learn about all your options in our DADU financing guide.

Getting Started With Your DADU Project

The best first step is a feasibility assessment. A DADU expert can evaluate your property, review zoning requirements, and give you a realistic picture of what’s possible — and what it will cost.

APEX DADU helps King County homeowners through every phase of the DADU process — from that first feasibility conversation through design, permitting, construction, and final move-in. Whether you’re building for rental income, family, or future flexibility, we’ll help you get there.

Contact APEX DADU for a free consultation and find out what’s possible on your property.