Seattle ADU Financing: HELOC, Construction Loans & Rates (2026)

For most Seattle homeowners, a HELOC or cash-out refinance against existing home equity is the cheapest way to finance an ADU, because Seattle equity gains since 2015 commonly exceed $300,000 — often more than the ADU itself costs. A construction-to-permanent loan makes more sense if your project exceeds roughly $350,000 or you have limited current equity. RenoFi-style renovation loans, which underwrite against the home's after-construction value, are the right fit if your equity alone won't cover the build.

Three things are Seattle-specific: (1) exceptionally high average home equity, since many Seattle homes purchased before 2018 have appreciated $300,000–$450,000+; (2) no Washington state income tax, which improves after-tax rental cash flow on an ADU compared to California or New York at the same build cost; (3) Seattle's SDCI pre-approved DADU plans lower soft costs, which indirectly reduces how much you need to borrow. The loan products themselves — HELOC, cash-out refi, construction loan — are the same products available nationally.

Most Seattle HELOC lenders cap borrowing at 80–90% combined loan-to-value (CLTV). On a home worth $850,000 with a $400,000 remaining mortgage, an 85% CLTV HELOC could unlock roughly $322,000 in a bank offering 85% CLTV ($850,000 × 0.85 − $400,000). Actual limits depend on your lender, credit score, and DTI — get a written pre-qualification before committing to a DADU budget.

A standard HELOC works for most Seattle ADU projects, especially attached AADUs and smaller DADUs under roughly $250,000, since funds draw as needed and you only pay interest on what you've drawn. A dedicated construction-to-permanent loan is usually the better fit for larger custom DADUs (typically above $300,000–$350,000) because it's underwritten against the completed project's appraised value, not just your current equity, and it converts to a standard mortgage automatically at completion.

Most Seattle HELOC and home-equity lenders require a minimum FICO of 660–680 and a debt-to-income ratio under 43%. Construction-to-permanent loans typically add a requirement for a licensed general contractor and a detailed, itemized construction budget. Requirements vary by lender — RenoFi and Figure publish slightly different minimums than a traditional credit union like BECU.

Seattle does not currently offer a citywide cash grant for market-rate DADU construction, but the city's pre-approved DADU plans reduce design and permitting soft costs, which functions as an indirect subsidy. King County Housing Authority programs may offer subsidized financing for homeowners who commit their ADU to income-restricted affordable rental use — eligibility and terms vary by program and change over time, so verify current availability directly with KCHA before budgeting around one.

Some lenders offer DSCR (debt service coverage ratio) loans that qualify based on the ADU's projected rental income rather than your personal income, which can help investors who don't want the new debt to affect their personal DTI. These products are less common for ground-up DADU construction than for purchase-and-convert scenarios — ask any lender you're considering whether they underwrite DSCR loans specifically for new-construction ADUs, not just existing rental units.