Can You Use a HELOC to Build an ADU? The Phase-by-Phase Cost

Can you use a HELOC to build an ADU? Yes, if you have the equity today. A HELOC lets you draw construction costs in stages and pay interest only on what you’ve spent, while leaving your first mortgage and its rate alone. The catch is the payment jump when the draw period ends.

• Borrowable at 80% CLTV: $900k home − $400k owed → roughly a $320,000 line
• While you build (interest-only): $300,000 drawn ≈ $1,868/month
• After the draw period (P&I, 20-year payback): ≈ $2,411/month — budget for this number, not the cheap one
• Not enough current equity? A HELOC alone won’t reach the build cost. That’s a construction-loan job, not a HELOC one.

TL;DR: A HELOC fits an ADU well because its draw period matches construction billing: you borrow in stages and pay interest only on what you’ve drawn, while your first mortgage stays put. Borrowing power is capped by your current equity, typically 80–85% combined loan-to-value, so on a $900,000 California home with $400,000 owed you could open roughly a $320,000 line. The honest catch: when the interest-only draw period ends, a $300,000 balance jumps from about $1,868/month to about $2,411/month at the 7.47% average rate. If you lack the equity today, a HELOC alone can’t fund the build.

California is where this question gets asked most, and for good reason. The state has spent years legalizing accessory dwelling units statewide and forcing cities to approve them ministerially, on a permit-counter basis rather than a discretionary hearing, per the California Department of Housing and Community Development. Meanwhile, most owners with enough equity to build also hold a first mortgage in the 3% range they refinanced into years ago and have no intention of giving up. A HELOC threads that needle: it taps the equity without touching the cheap first mortgage, which is why building an ADU ranks among the smartest ways to use a HELOC. This guide works through exactly how it funds the build and what it actually costs across both payment phases, then turns to the question that matters just as much: when it’s the wrong tool. If you need a refresher on the product itself, start with how a HELOC works; for every financing route compared side by side, see how to finance an ADU; for the broader state picture, see our California HELOC guide.

How does a HELOC actually fund an ADU build?

The mechanical fit is the whole reason a HELOC is the default ADU instrument: a construction project bills in stages, and a HELOC funds in stages.

You don’t pay for an ADU all at once. The money goes out over months: design and permits, then foundation, framing, rough-ins, finishes, final inspection. A HELOC mirrors that. You’re approved for a maximum line, but you only draw against it as bills come due, and you pay interest only on the amount you’ve actually drawn. Draw $40,000 for the foundation pour and you pay interest on $40,000, not on the full $300,000 you were approved for. Credit unions and lenders that market ADU lines, like Foothill Credit Union, lean on exactly this draw-as-you-go structure.

The HELOC sits in second-lien position behind your existing mortgage, and in 2026 that position is the whole point. The real decision usually isn’t the HELOC’s rate. It’s whether tapping your equity is worth disturbing the sub-4% first mortgage you’re already sitting on. A cash-out refinance replaces that entire loan at today’s rate; a second-lien HELOC borrows only against your equity layer and leaves the first mortgage untouched. That trade is the part most equity comparisons underweight.

The draw period typically runs the first 10 years, and during it most lenders require interest-only payments. That low payment is a real advantage while you’re building and the unit isn’t earning rent yet. It’s also where ADU borrowers most often trip themselves up. The next section is where that mistake gets expensive.

How much can you actually borrow against your equity?

One limit decides whether a HELOC is even on the table: a HELOC is underwritten against your home’s value as it stands today, not the value the finished ADU will add.

The formula every lender runs is the same one behind our home equity calculator:

(current home value × CLTV cap) − first-mortgage balance = your maximum line

Most lenders cap combined loan-to-value (CLTV) at 80% to 85%; some go to 90% for strong borrowers. Run it on a realistic California example — a $900,000 California home with $400,000 still owed:

• At an 80% cap: $900,000 × 0.80 = $720,000, minus $400,000 = $320,000 available
• At an 85% cap: $900,000 × 0.85 = $765,000, minus $400,000 = $365,000 available

A $320,000 line comfortably covers a typical $300,000 detached ADU. In California, detached new-construction units generally run $225,000 to $400,000-plus and garage conversions land lower, often well under $200,000, per California HCD cost data (corroborated by 2026 builder figures). So for an owner with real equity, the math works.

For an owner without it, it doesn’t, and no amount of projected rent changes that. This is the most common misread in ADU financing content: pages that tout borrowing “up to 125% of your home’s value” are describing renovation or construction loans that lend against the after-completion value, not a HELOC. A HELOC will never reach past your current equity. If 80–90% of today’s value minus your balance doesn’t cover the build, the HELOC stops there. Whether you qualify for the full line is a separate question your income and credit decide. See what lenders check.

What will the payments actually be?

If homeowners get burned by an ADU HELOC, this is usually where it happens. The lender shows the interest-only payment. The borrower builds a budget around it. Ten years later the balance starts amortizing and the payment jumps. Don’t budget around the first number. Budget around the second.

During the draw period you pay interest only. After it ends, the outstanding balance converts to principal plus interest, amortized over the repayment term, and the payment jumps. Run the same $300,000 ADU balance at the 7.47% national average rate (Bankrate, as of June 17, 2026), with a 10-year draw and 20-year repayment:

	Draw period (interest-only)	Repayment period (principal + interest)
What you’re paying	Interest on the drawn balance only	Full amortization of the balance
Monthly payment on $300,000 @ 7.47%	~$1,868	~$2,411
Typical length	First 10 years	Following 20 years
The catch	Feels affordable; principal never drops	A +$543/month (+29%) jump — budget for this

The interest-only payment isn’t the cost of the ADU. It’s the cost of holding the ADU debt during construction. The real recurring number is the ~$2,411 repayment figure, the one your household budget has to absorb for two decades. Plan around that from day one, and model it yourself with our HELOC payment calculator before you commit to a build budget.

One more number that doesn’t sit still: the rate is variable. A HELOC prices at the prime rate (6.75% as of June 2026, per JPMorganChase) plus a lender margin. Every quarter-point move in prime changes the payment on a $300,000 balance by about $63 a month, in either direction. Because the rate floats with prime, that’s not a footnote; stress-test the payment a point or two higher before you draw. And if you later want to retire the line before that repayment jump hits, refinancing after the ADU is built covers the options.

When is a HELOC the wrong tool for an ADU?

A HELOC is the default, not the universal answer. Lender-owned content rarely says so, but there are four situations where something else fits better, and recognizing yours saves real money.

You don’t have the current equity. Covered above, and it’s the big one. If your equity today won’t reach the build cost, a HELOC can’t stretch to cover it. A construction loan or a renovation loan underwritten against the home’s projected finished value is the structure designed for that gap. We compare a HELOC against a construction loan, dollar for dollar, in a dedicated guide; for the broader menu of equity products, see our comparison hub.

You need a fixed, predictable payment. A HELOC’s variable rate is fine if you can absorb movement, but if a stable payment over a 20-year-plus payoff matters more than flexibility, a fixed-rate home equity loan (or a construction-to-permanent loan that locks a rate) removes the budget risk a HELOC carries.

You’re counting on the ADU’s rent to make the payment. Be careful here. A HELOC qualifies on your current income; the unit’s future rent doesn’t help you get approved, and as the table shows, rent that comfortably covers the interest-only draw payment may not cover the repayment-phase payment. Run the rent against the repayment number, not the cheap one. (How lenders treat ADU rental income for qualifying is its own topic, and one we cover separately.)

The build is small and you’d rather not put a variable lien on the house for it. For a sub-$150,000 garage or junior ADU conversion, a smaller fixed product may be a cleaner fit than a six-figure revolving line. The cheapest build doesn’t always call for the most flexible financing.

The honest test is the same one that governs every smart HELOC use: the math has to work on the repayment payment, at a stressed rate, against income you actually have, not the draw-period teaser against rent you hope to collect.

Is HELOC interest for an ADU tax-deductible?

Often yes, and an ADU is one of the cleaner cases. Under IRS Publication 936, interest on home equity debt is deductible when the money is used to buy, build, or substantially improve the home that secures the loan. Building an ADU on the property securing the HELOC is a textbook substantial improvement, so the interest generally may qualify, depending on how the property is used and how the proceeds are traced, unlike a HELOC used for debt consolidation or a car, which doesn’t.

Before you count the deduction as savings, two conditions decide whether you actually get it. Your combined acquisition debt (first mortgage plus the improvement-related home equity debt) has to fall within the $750,000 cap, and you have to itemize rather than take the standard deduction. Given how high the standard deduction is today, many households won’t. These rules were made permanent by the One Big Beautiful Bill Act. This is general information, not tax advice; the substantial-improvement test and mixed-use draws have edge cases, so confirm your situation with a tax professional before you draw.

What makes ADU financing in California different?

Three things, and together they’re why this is the signature California home-equity move.

First, the law cleared the runway. California requires cities and counties to permit ADUs and to approve qualifying applications ministerially, without discretionary review, per HCD. The state also runs and tracks ADU financing programs for eligible owners. Building is more feasible here than almost anywhere else.

Second, the equity is the deepest in the country. California’s high values mean even a moderate ownership position throws off six figures of borrowable equity, the raw material a HELOC runs on.

Third, Prop 13 makes borrowing structurally cheaper than moving. This is the part California owners most often get backwards: the worry is that building will reassess the whole house, and it won’t. Borrowing against your home isn’t a change of ownership, so a HELOC never resets your assessed value. Building the ADU does add some assessed value (the county reassesses the new construction itself), but the rest of your home keeps its protected Prop 13 basis, and you avoid the full step-up that selling and rebuying would trigger. The details of that interaction live in the California HELOC guide.

This is the first piece in a California ADU financing cluster. Its companion guides pit a HELOC against a construction loan, break down garage and junior-ADU conversions, walk through how lenders treat ADU rental income, and cover the paths available when you’re short on equity. Start with your own numbers in the equity calculator and the payment calculator, check whether the whole build pencils in the ADU Feasibility Analyzer, and price the line on our rates page before you talk to a lender.

Rates cited are national averages as of June 2026 and change frequently; your offered rate depends on your credit, equity, and lender. This article is general information, not legal, tax, or financial advice, and not a recommendation of any specific transaction. Consult a California-licensed professional about your situation.