How to Finance a DADU in King County: Loans, HELOCs, and More

Financing a $400K–$650K DADU Project

Building a DADU in King County is a significant investment — typically $400,000 to $650,000 all-in. Most homeowners don’t have that kind of cash on hand, which is why financing is a critical piece of every DADU project. The good news: several financing options work well for DADU construction, and King County’s strong real estate market makes lenders increasingly comfortable with ADU projects.

The right financing strategy depends on your equity position, income, timeline, and risk tolerance. Here’s how each option works.

Home Equity Line of Credit (HELOC)

A HELOC is the most common financing choice for King County DADU projects — and often the most cost-effective.

How It Works

• Borrow against the equity in your primary home
• Draw funds as needed during construction (you only pay interest on what you’ve drawn)
• Variable interest rate, typically 1–2% above prime rate
• Draw period usually 10 years, repayment period 20 years

Pros

• Lower interest rates than construction loans
• Flexible draw schedule matches construction milestones
• No need to refinance your existing mortgage
• Interest may be tax-deductible (consult your tax advisor)

Cons

• Requires significant existing equity (typically 80% combined loan-to-value)
• Variable rate means payments can increase
• Your primary home is collateral
• Some lenders cap HELOC amounts below what DADU projects need

Best For

Homeowners with substantial equity who want to minimize interest costs and maintain their existing mortgage rate.

Construction Loan

Construction loans are specifically designed for building projects and work differently from traditional mortgages.

How It Works

• Short-term loan (12–18 months) that covers construction costs
• Funds disbursed in stages as construction milestones are met
• Lender inspects progress before each draw
• Converts to a permanent mortgage or is paid off after construction

Pros

• Designed for construction — lenders understand the process
• Disbursement schedule protects your investment
• Can include soft costs (design, permits) in the loan amount
• Construction-to-permanent loans avoid double closing costs

Cons

• Higher interest rates than HELOCs (typically 1–3% above standard mortgage rates)
• More paperwork and approval requirements
• Lender oversight adds administrative steps during construction
• May require refinancing your existing mortgage

Best For

Homeowners who want structured disbursement controls or who don’t have enough equity for a HELOC.

Cash-Out Refinance

A cash-out refinance replaces your existing mortgage with a larger one, giving you the difference in cash for your DADU project.

How It Works

• Refinance your primary mortgage at current market rates
• Borrow more than you currently owe, up to 80% of your home’s value
• Receive the excess as cash to fund DADU construction

Pros

• One loan, one payment — simpler than layering a HELOC on top of a mortgage
• Fixed rate locks in your cost for the life of the loan
• May get a better rate than a HELOC if current rates are favorable

Cons

• Replaces your existing mortgage — if you have a low rate, you’ll lose it
• Closing costs on a full refinance are significant ($5,000–$15,000)
• Longer approval process than a HELOC
• You receive all funds at closing, not in stages — you’re responsible for managing disbursement to your builder

Best For

Homeowners whose current mortgage rate is close to or above market rates, making the refinance cost-neutral or beneficial.

ADU-Specific Loan Programs

As ADU construction has grown, several lenders have created products specifically for accessory dwelling units.

How They Work

• Specialized loan products that consider projected ADU rental income in underwriting
• Some programs allow the future rental income to help qualify for the loan
• May offer higher loan-to-value ratios than standard construction loans

Programs to Research

• Fannie Mae HomeStyle Renovation Loan: Allows financing for ADU construction as part of a renovation
• FHA 203(k) Loan: Government-backed renovation loan that can include ADU construction
• Local credit unions: Several Puget Sound credit unions have developed ADU-specific products
• ADU lending specialists: Companies like RenoFi and Point specialize in ADU financing

Best For

Homeowners who need the projected rental income counted in their qualification, or who want a product designed specifically for ADU construction.

Personal Savings and Family Loans

Some homeowners finance all or part of their DADU with personal savings, retirement funds, or family loans.

• Cash payment: Eliminates interest costs entirely and simplifies the process
• Partial cash: Combining savings with a smaller loan reduces interest expense
• Family loan: Informal or formal loans from family members — document these properly for tax purposes
• Retirement account: Some homeowners use 401(k) loans or IRA distributions — consult a financial advisor about tax implications

Financing Strategy: What to Consider

Choosing the right financing option comes down to several factors:

• Current mortgage rate: If your existing rate is low (under 4%), a HELOC preserves that rate while adding DADU financing on top.
• Available equity: King County home values have appreciated significantly. Check your current equity position — you may have more than you think.
• Income qualification: Construction loans and ADU-specific programs may count projected rental income, which can help with qualification.
• Risk tolerance: Fixed-rate options (cash-out refi, construction-to-permanent) provide payment certainty. Variable-rate options (HELOC) may cost less initially but carry rate risk.
• Timeline: HELOCs can close in 2–4 weeks. Construction loans and refinances take 4–8 weeks. Factor this into your project schedule.

The ROI Case for DADU Financing

DADU financing makes financial sense when the return exceeds the cost of borrowing:

• Rental income: $1,500–$3,000/month ($18,000–$36,000/year) from a well-located King County DADU
• Property appreciation: DADUs typically add 20–30% to property value
• Loan cost: At 7% interest on $500,000, annual interest is approximately $35,000 — roughly offset by rental income alone
• Net return: Property appreciation on top of rental income makes the investment positive from day one in most scenarios

See our rental income guide and property value guide for detailed projections.

Tips for DADU Financing Success

1. Talk to multiple lenders. Rates, terms, and ADU-friendliness vary significantly.
2. Get pre-approved early. Know your budget before you design.
3. Include all costs. Make sure your financing covers permits, design, construction, landscaping, and contingency.
4. Factor in carrying costs. You’ll pay interest during construction before rental income starts.
5. Keep a cash reserve. Don’t finance 100% — keep 10–15% in reserve for contingencies.

APEX DADU Helps You Plan Financially

We’ve helped dozens of King County homeowners structure their DADU financing. APEX DADU provides accurate cost estimates early in the process so you can work with lenders on the right financing strategy for your project. We also work with lender-required inspections and disbursement schedules to keep construction on track.

Contact APEX DADU to discuss your DADU financing plan