How to Finance a DADU in Washington: Acquisition, Construction & Exit Strategy

Phase One: Acquisition and Buildability

A DADU deal starts with the lot, not the loan application.

Washington jurisdictions have different rules for setbacks, lot coverage, height, parking, utilities, critical areas, and unit size. A property that works in Tacoma may not work in Spokane, Bellevue, unincorporated King County, or a smaller jurisdiction.

Before you finance the acquisition, verify:

• Zoning and permitted residential uses
• Lot coverage and setback requirements
• Height limits
• Critical areas, wetlands, steep slopes, and tree restrictions
• Parking rules
• Water, sewer, and electrical capacity
• Side sewer or septic requirements
• Legal access for construction equipment
• Whether the existing home has unpermitted work
• Local rules for long-term or short-term rentals

King County, for example, notes that detached ADUs may be limited by minimum lot sizes and zoning conditions. Seattle requires a permitted unit to meet applicable building, land use, energy, mechanical, electrical, environmentally critical area, and shoreline requirements.

The takeaway is simple: not every Washington parcel can support a DADU. Get a zoning and feasibility review before you fall in love with the backyard.

How Much Does a Washington DADU Cost in 2026?

A realistic statewide planning range for a detached ADU is approximately $250,000 to $650,000+ all-in.

The lower end may fit a smaller, straightforward unit on a simple site in a lower-cost market. The upper end becomes more realistic for larger units, difficult access, extensive utility work, higher-end finishes, or projects in Seattle, King County, and the Eastside. Your budget should account for more than framing and finishes.

Commonly overlooked costs include:

• Architectural design and engineering
• Structural, civil, and energy consultants
• City or county permits and plan review
• Impact and utility fees
• Washington sales tax
• Side sewer installation or upgrades
• Water, electrical, and gas connections
• Surveying and geotechnical work
• Demolition, excavation, grading, and site preparation
• Foundation and drainage
• Temporary utilities and construction insurance
• Financing and permit-period carrying costs
• A contingency reserve of roughly 8–12%

A useful planning model is:

• Design and engineering: approximately 6–10%
• Permits and fees: approximately 3–8%
• Site work and utilities: approximately 15–25%
• Core construction and finishes: approximately 55–70%
• Sales tax and contingency: approximately 8–12%

These are planning ranges, not contractor bids. Get a detailed scope of work before you ask a lender to size the loan.

Washington DADU Financing Options Compared

Investors typically consider five financing paths.

Financing path	Speed	Appraisal requirements	Income verification	Rate structure	Best fit
HELOC or home-equity loan	Moderate; often weeks	Usually a drive-by or full appraisal	Standard income, credit, and debt-to-income review	Variable rate, usually tied to Prime	Owners with substantial equity and a strong existing first mortgage
Construction-to-permanent loan	Slower; often several weeks or longer	As-completed appraisal using plans and specifications	Full documentation typically required	Construction-phase rate followed by permanent market rate	Larger ground-up DADUs where future value supports the financing
ADU-specific program	Varies by lender and program	Usually appraisal-based	May require income, credit, occupancy, or income-limit documentation	Program-dependent; may be fixed or reduced for eligible borrowers	Owner-occupants or borrowers who meet a specific program’s rules
Cash-out refinance	Moderate to slow	Full appraisal of the existing property	Full mortgage underwriting	Fixed market mortgage rate	Borrowers with strong equity who are comfortable replacing their current mortgage
Hard money or rehab loan	Fast; potentially as little as 48 hours	Depends on lender; Intrust does not require an appraisal for its standard process	Flexible asset-focused underwriting; no income verification with Intrust	Intrust rehab loans use 12% annual interest	Investors acquiring and improving property with a clear refinance, rental, or sale exit

 

HELOCs and cash-out refinances can be attractive when you already own the property and have enough equity. Construction-to-permanent loans may work better for a ground-up build when you need the lender to consider the completed value. ADU-specific programs can be useful, but eligibility may be limited. Some are designed primarily for owner-occupants or income-qualified borrowers, not investment acquisitions.

Hard money costs more than traditional financing, but the value is speed and execution. If a slow approval causes you to lose the property, or carry it for months while waiting, the cheapest quoted rate may not be the cheapest financing decision. For more context on using a fast close strategically, read Speed Matters: Why a 48-Hour Close Is Your Best Weapon Against Seattle’s Rising Rates.

How Intrust Funding’s Rehab Loan Works for a DADU

Intrust Funding structures rehab loans around the full investment, not just the existing house.

For a qualifying Washington investment property, the loan can cover:

• Acquisition of the property
• Approved renovation or DADU construction costs
• First construction draws at closing
• Ongoing progress draws as the work advances

The standard structure includes:

• 15–20% down
• 12% annual interest
• Funding in as fast as 48 hours
• In-house underwriting
• No income verification
• No appraisal required
• No inspection required for draws
• First draws available at closing

For progress draws, investors provide approximately 10–15 progress photos showing completed work. That keeps the project moving without waiting for a third-party inspector to schedule a site visit. Intrust also offers complimentary deal analysis. That matters because a DADU project can look profitable until the side sewer, permit delay, or retaining wall joins the conversation.

Apply now and send the purchase price, proposed scope, estimated rent or resale value, contractor budget, and intended exit. We’ll review the numbers with you.

Phase Two: Permitting, Construction, and Carrying Costs

Permitting is where many DADU budgets quietly lose their optimism.

In Seattle, a straightforward pre-approved plan may move through the process in approximately 2–6 weeks. Custom or more complex projects can place the review window closer to 4–12 weeks, depending on completeness, corrections, site conditions, and review complexity. Seattle’s official ADU guidance also points investors toward pre-approved ADUniverse plans for a faster, more predictable process.

In unincorporated King County, a practical planning range is often 2–6 months for permit review, with longer timelines possible when correction cycles, critical areas, septic, drainage, or other site issues are involved. King County’s ADU guidance confirms that zoning, lot size, setbacks, and development conditions affect what can be built.

Your financing plan should account for:

• Interest during the permit window
• Property taxes and insurance
• Utilities and security
• Contractor deposits
• Material price changes
• Weather delays
• Permit corrections
• A contingency for unexpected site work

Do not assume that “construction time” begins the day you close. In many DADU deals, the clock starts with a permit application, and the lender’s interest clock may start at closing.

Phase Three: Exit Strategy

A strong DADU loan starts with the exit strategy.

1. Rent and hold, then refinance

Build the DADU, stabilize the property, lease the unit, and refinance into long-term rental financing.

Many programs may use approximately 75% of market rent for qualification, although the exact treatment varies by lender, documentation, lease status, and property type.

Plan for the refinance before construction begins. Confirm:

• Minimum seasoning requirements
• Required lease or rental history
• Final certificate of occupancy
• Appraisal treatment of the DADU
• Debt-to-income or cash-flow requirements
• Whether the completed unit must be separately metered

2. BRRRR-style cash-out

A DADU can create value through both added rental income and improved property utility. After completing and stabilizing the project, you may refinance based on the property’s updated value and recover part of your invested capital. The numbers must support the new loan payment, reserves, and operating expenses. Appreciation alone is not an exit strategy.

3. Sell the property as an income-producing asset

Some buyers will pay a premium for a legally permitted home with a separate rental unit. The marketing story becomes stronger when you can provide:

• Final permits
• Certificate of occupancy
• Construction documentation
• Current lease or rent analysis
• Utility information
• Operating expense history
• Clear proof that the DADU is legal and habitable

A permitted DADU is an investment feature. An unpermitted structure is usually a negotiation problem wearing a cute backyard-cottage costume.

The Bottom Line

Financing a Washington DADU requires a full money story:

1. Acquire the right property.
2. Verify zoning and buildability.
3. Budget the full project, including utilities, permits, taxes, and contingency.
4. Choose financing that matches your timeline and documentation profile.
5. Carry the project through permitting and construction.
6. Refinance, hold, or sell based on a defined exit.

Intrust Funding has helped Washington investors execute real estate deals since 2009. With local underwriting, fast funding, photo-based draws, and a complimentary deal analysis, we focus on helping investors move from viable plan to finished project.

Ready to move? Send us the deal.

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