How to Finance an ADU in Washington (2026): Loans & Funding Options

Once you've settled on a design and a builder, the next question is almost always the same: how do I actually pay for this? An accessory dwelling unit is a six-figure investment, and the way you finance it shapes your monthly cost, your build timeline, and your long-term return. For a full picture of what you're funding, start with our breakdown of ADU costs in Washington and our guide on how to budget for an ADU project.

This guide walks through the main ways Puget Sound homeowners pay for an ADU, who each option fits best, and what lenders look for.

How Much Do You Need to Finance?

Most detached ADUs (DADUs) in the Seattle-Tacoma area run $200,000 to $400,000 all-in, including design, permits, utility connections, and construction. Attached ADUs and garage conversions can come in lower, often $120,000 to $250,000. Before you talk to a lender, get a detailed estimate from a builder so you're financing a real number, not a guess. You can request quotes from verified builders to anchor your budget.

Two facts change the financing math in your favor:

• HB 1337 removed owner-occupancy requirements statewide. You can build an ADU purely as a rental and count the projected rent toward your plans.
• An ADU adds real appraised value to your property, which is exactly what equity-based lending is built around.

Option 1: Home Equity Line of Credit (HELOC)

A HELOC lets you borrow against the equity you already have in your home, drawing funds as you need them during construction and paying interest only on what you use.

• Best for: Homeowners with substantial existing equity who want flexibility to draw in stages as the builder invoices.
• Pros: Interest-only draw periods, you only pay on what you use, and you keep your existing low first-mortgage rate untouched.
• Cons: Variable rates, so payments can rise. Most lenders cap combined loan-to-value (CLTV) around 80 to 90 percent of current home value, which limits how much you can pull.
• Watch for: Because a HELOC is based on your home's current value, it may not cover the full cost of a large DADU if your equity is thin.

Option 2: Cash-Out Refinance

A cash-out refinance replaces your existing mortgage with a larger one and gives you the difference in cash to fund the build.

• Best for: Homeowners with a lot of equity whose current mortgage rate is at or above today's rates (so refinancing doesn't cost them a good rate).
• Pros: One fixed-rate loan, predictable payments, and typically lower rates than a HELOC or personal loan.
• Cons: If you're sitting on a low pandemic-era mortgage rate, refinancing the whole balance to a higher rate is usually a bad trade. You also pay closing costs on the full new loan.

Option 3: Renovation Loans (RenoFi, FHA 203k, Fannie Mae HomeStyle)

Renovation loans are designed for exactly this situation: they let you borrow against your home's future, after-completion value rather than its current value. That's the key difference from a HELOC.

• RenoFi loans are structured as a HELOC or second mortgage but underwritten on the as-completed appraisal, so you can often access far more than a standard HELOC without touching your first mortgage.
• Fannie Mae HomeStyle and FHA 203(k) loans roll the purchase or refinance and the renovation into a single mortgage, also based on after-renovation value.
• Best for: Homeowners who don't yet have enough equity to cover a full DADU but whose finished ADU will add substantial value.
• Cons: More paperwork, appraisals, and draw inspections. FHA 203(k) requires owner-occupancy of the primary home and has loan limits.

Option 4: Construction Loan

A construction loan is a short-term loan that funds the build in phases, then converts to (or is refinanced into) a permanent mortgage once the ADU is complete.

• Best for: Larger ground-up DADUs, or homeowners without enough equity for an equity-based product.
• Pros: Sized to the total project cost, releases funds as construction milestones are hit, and interest is often paid only on drawn amounts during the build.
• Cons: Higher rates, more scrutiny of the builder and plans, and required inspections at each draw. Expect a larger down payment and a detailed, fixed-bid contract from your builder.

Option 5: Cash or Retirement Funds

Paying cash avoids interest entirely and makes you the strongest, most flexible client for a builder. Some homeowners also tap retirement accounts, but early withdrawals can trigger taxes and penalties, so run the numbers with a tax professional before pulling from a 401(k) or IRA.

What Lenders Look For

Whichever route you choose, lenders generally evaluate:

• Equity / loan-to-value — how much cushion exists between what you owe and what the home is (or will be) worth.
• Credit score and debt-to-income ratio — the usual mortgage underwriting factors.
• A detailed, fixed-bid construction contract — vague estimates slow approval. A licensed builder's itemized bid speeds it up.
• Projected rental income — for renovation and construction loans, some lenders will count expected ADU rent toward your qualifying income, which HB 1337 makes easier since you no longer have to live on-site.

Which Option Is Right for You?

Next Steps

Financing an ADU is far easier once you have a real number to work with. Get an itemized bid first, then match the loan to your equity position and rate situation. Browse verified ADU builders serving Seattle, Tacoma, and the wider Puget Sound, or request quotes to compare bids before you approach a lender.

This article is general information, not financial advice. Loan terms, rates, and program rules change frequently — confirm current details with a licensed lender or mortgage broker before making decisions.