By ADU Scope editorial team ยท Last verified: October 2026
A HELOC for an ADU works well when you have equity, a first mortgage worth keeping and a budget that survives a rate increase. You borrow as contractor invoices arrive and pay interest only on what you draw. The risks are a variable rate, a lender that can freeze the line, and a home that secures the debt. On a hypothetical $150,000 balance at 7.5%, your payment can jump by about $271 a month when repayment starts, even if rates never move.
This article is general information, not financial advice. Every payment example is hypothetical, and your actual rate, fees and limits will differ. Get written quotes before you borrow.
HELOC for an ADU: how the line works
The Consumer Financial Protection Bureau (CFPB) describes a home equity line of credit as a line you can borrow against repeatedly. It rests on your equity, meaning your home’s value minus what you owe. A HELOC has two phases:
- Draw period (typically about 10 years): you borrow up to your limit whenever you need to. Some lenders require interest-only payments here.
- Repayment period (typically 10 to 20 years): you stop borrowing and repay principal and interest. In some plans, you must repay the whole balance when repayment begins. That is a balloon payment.
A HELOC is a second mortgage if you already have a first one, so you pay it in addition to that loan. This structure suits construction because costs arrive in stages: design, permits, foundation, framing and finishes. For typical totals, see our ADU budget breakdown.
How the HELOC rate is set
Most HELOC rates are variable, so payments may change from month to month, per the CFPB. A variable rate combines an index with a margin. The index tracks market rates, and the U.S. prime rate is a common choice. The margin is the lender’s markup. Some lenders also offer a low introductory rate that later resets.
The Federal Reserve’s H.15 release put the bank prime rate at 7.00% for the week ending Oct 2, 2026. A prime-based HELOC therefore costs prime plus the lender’s margin. We have no survey of current HELOC offers, so this article does not list “current rates.” Your margin depends on your credit, equity and lender.
HELOC for an ADU: how payments work
The example below is hypothetical. It assumes you draw the full $150,000 and that a lender adds a 0.5-point margin to prime, for a 7.5% rate. Replace these inputs with your own quote.
| Payment type | At 7.5% | If the rate rises 1 point to 8.5% |
|---|---|---|
| Interest-only, draw period | $937.50 per month | $1,062.50 per month |
| Fully amortizing over 20 years, repayment period | $1,208.39 per month | $1,301.73 per month |
The interest-only payment is $150,000 times 7.5% divided by 12, or $937.50. The 20-year figures use the standard loan formula. A one-point rise adds $125 a month during the draw period. It adds about $93 a month once you repay principal.
The bigger shock is the phase change. Moving from interest-only to a 20-year repayment adds $270.89 a month at the same 7.5% rate. In short, plan for the repayment payment, not the draw payment. If you cannot afford the higher number, borrow less.
Staged draws lower the early payment
You rarely owe the full line on day one. For example, if you draw only $50,000 for design, permits and site work, interest-only costs $312.50 a month at 7.5%. Your payment then climbs with each draw. Ask the lender how it calculates the minimum payment during construction.
Pros of a HELOC for an ADU
- You keep your first mortgage. The CFPB notes that home equity lending lets you use your equity without replacing a low-rate mortgage with a higher-rate cash-out refinance. Our cash-out refinance vs HELOC comparison puts numbers on that.
- You borrow as you build. Interest accrues only on the amount drawn.
- One line covers many costs. Design, permits, utilities and contingencies can all come from the same line. Utility costs surprise many owners, as our hidden ADU costs guide explains.
- The line is reusable. During the draw period, repaid amounts become available again.
Cons and risks of a HELOC for an ADU
- Variable payments. Your payment can rise with the index.
- Your home is at risk. If you fall behind or cannot repay on schedule, you could lose your home.
- Freezes and reductions. The CFPB says a lender may stop further draws if your home value drops significantly or your finances worsen. Do not start construction counting on money you have not drawn.
- Balloon risk. Some plans demand the full balance when repayment starts.
- Fees. Ask about appraisal, application, closing and any annual or inactivity fees.
- Temptation. The CFPB warns against using home equity to pay other debts or living costs. Keep the line tied to the project.
When a HELOC for an ADU is the right choice
Choose a HELOC for an ADU if three things hold. First, your first-mortgage rate sits below today’s market. Second, you will pay the build in stages. Third, your budget still works at the amortizing payment, not just the interest-only one.
Choose a home equity loan instead if you want a fixed payment and your contractor needs the money in one lump sum. Consider a cash-out refinance only if your current rate is close to market. If you lack equity, price a renovation loan first. Because a lender can freeze the line, also keep a cash contingency of your own rather than counting on undrawn credit.
In short, the HELOC wins on flexibility and on protecting a cheap mortgage. It loses on payment certainty. Run the 8.5% column of the table above against your income before you sign, so a rate rise does not force a sale or a missed payment.
Qualifying for a HELOC for an ADU
A HELOC is based on your current home’s equity and your finances. Most lenders look at today’s value, not the value after the ADU is built, and we have not verified whether any lender counts projected ADU rent for a HELOC. Our guide to rental income and loan qualification covers the loan types where rent can count. If you want the lender to use the as-completed value, an ADU renovation loan may fit better.
How much equity you may need
Lenders cap combined borrowing as a percentage of your home’s value. That limit varies, and we found no official ADU-specific figure. Here is the arithmetic with a hypothetical 80% limit. A home worth $800,000 with a $300,000 mortgage allows $640,000 in total debt. That leaves $340,000 of room for a line. Ask each lender for its limit, then read our equity guide.
How to shop for a HELOC
- Request three estimates and compare limits, fees, rate structures and payment terms, as the CFPB advises.
- Ask for the index, margin, rate cap, rate floor and the date any introductory rate ends.
- Ask what happens when the draw period ends: amortization or balloon.
- Ask whether a fixed-rate conversion exists. The CFPB says the fixed rate is usually higher, but more predictable.
- Read the freeze and reduction terms, and check for fees if you leave the line unused.
- Calculate your budget at the amortizing payment, then add a cushion.
Finally, compare the HELOC with other routes in our guide on how to finance an ADU and see the ADU loan payment examples.
Frequently asked questions
Is a HELOC a good way to pay for an ADU?
It can be. You need equity, a low-rate first mortgage you want to keep and room in your budget for a higher payment. Compare it with a home equity loan, which pays out a lump sum at a fixed or adjustable rate. If you cannot qualify for debt at all, read our comparison of a home equity investment vs. HELOC for an ADU before you sign anything.
What rate will I get?
It depends on the index, the lender’s margin and your profile. Prime was 7.00% in the Fed’s latest weekly data for the week ending Oct 2, 2026.
Can the lender freeze my HELOC?
Yes, under the terms of your agreement. Per the CFPB, a lender may do so if your home value drops significantly or your financial situation worsens.
Can I fix the rate?
Some HELOCs let you convert part of the balance to a fixed rate. That rate is usually higher than the variable rate.