HELOC or Construction Loan for an ADU? The Dollar-for-Dollar Math
A HELOC usually beats a construction loan for an ADU when you have the equity: it leaves your low first mortgage alone. The worked math, and when it flips.
HELOC or construction loan for your ADU? Two questions decide it: do you have the equity today, and are you protecting a cheap first mortgage?
- Have the equity and a sub-4% first mortgage? The HELOC almost always costs less, because it leaves that first mortgage alone. A construction loan that refinances it reprices your cheap money: on a $400,000 balance, moving from 3.25% to 7% costs about $15,000 a year.
- Don’t have the equity? Only a construction or renovation loan can reach the build, because it underwrites the finished value. A HELOC can’t stretch past your current equity.
- Want a fixed, predictable payment? A construction-to-permanent loan locks a rate; a HELOC is variable (prime + margin, 7.47% average today).
TL;DR: For most California homeowners who already have the equity, a HELOC beats a construction loan for an ADU, not because its rate is lower (it usually isn’t) but because it leaves your existing first mortgage untouched. A construction loan’s edge is reaching a build your current equity can’t cover, because it underwrites the home’s after-completion value. The cost most comparisons skip: a construction-to-permanent loan that refinances your first mortgage reprices it at today’s rate. On a $400,000 balance, moving from 3.25% to 7% runs about $15,000 a year, often more than the rate gap on the ADU money.
The default advice you’ll find is that a HELOC is for smaller projects and a construction loan is for bigger ones. That framing misses the number that actually decides it. The question isn’t project size. It’s whether you have the equity today and what rate your current mortgage carries. Get those two right and the choice usually makes itself. Building an ADU is among the highest-value ways to use a HELOC, so this is a choice worth getting right. It’s the comparison the main ADU financing guide points to, and here we run it dollar for dollar. For the product basics, start with how a HELOC works; for every ADU financing route compared, see how to finance an ADU; and for the broader menu of equity products see the comparison hub. This comparison assumes a full-size build; the math shifts for a small garage conversion.
What’s the real difference between a HELOC and a construction loan for an ADU?
The two products solve the build differently, and the difference is structural, not cosmetic.
A HELOC is a second lien against the equity you have right now. It sits behind your existing mortgage and leaves it completely alone. You draw funds as construction bills come due and pay interest only on what you’ve drawn during the draw period. The rate is variable: prime (6.75% as of June 2026, per JPMorganChase) plus a lender margin, averaging 7.47% nationally in June 2026, per Bankrate. The hard ceiling is your current equity.
A construction loan, most often a single-close construction-to-permanent loan or a renovation loan like FHA’s 203(k) or Fannie Mae HomeStyle, underwrites against the home’s value after the ADU is finished. The appraiser values the property “subject to completion,” as if the unit already exists. That’s what lets it fund a build your present equity can’t. The catch is what it does to your existing mortgage: most of these loans refinance or wrap your first mortgage into one new, larger loan at today’s rate.
That last point is the one that decides the money, and most comparisons gloss over it. So lead with it.
Which one actually costs less?
Run a realistic California scenario: a $900,000 home with a $400,000 first mortgage locked at 3.25% a few years ago, and an owner who wants to build a $300,000 detached ADU. They have the equity for either path. Here’s the first-year cost of each, with the existing mortgage shown explicitly because that’s where the real difference lives.
| HELOC path (keep first mortgage) | Construction-to-permanent (refinance everything) | |
|---|---|---|
| First mortgage | $400,000 stays at 3.25% | Paid off and rolled into the new loan |
| New loan | $300,000 HELOC at 7.47% (variable) | $700,000 at ~7.0% fixed |
| First-year interest, first mortgage | ~$13,000 | — (now part of the $700k) |
| First-year interest, ADU money | ~$22,290 (interest-only draw) | — (part of the $700k) |
| First-year interest, total | ~$35,290 | ~$49,000 |
| What moved | Nothing on the old loan | The $400k repriced from 3.25% to 7% |
The gap is about $13,700 in the first year, and almost all of it traces to one line: the $400,000 that got repriced. Borrowing against the home at 7% to build costs roughly the same either way. What differs is whether you also reprice the money you’re already borrowing cheaply. Repricing $400,000 from 3.25% to 7% costs $15,000 a year on its own, and you’d pay that every year the new loan is outstanding, not just the first.
So the decision hinge is the first mortgage, not the ADU. Protecting a sub-4% first mortgage is usually where the HELOC wins, and it wins by more than the rate sheet suggests. Model your own version in the HELOC payment calculator, check your borrowing room in the equity calculator, and test whether the build pencils in the ADU Feasibility Analyzer before you assume the construction loan’s single payment is the cheaper one.
(If your existing mortgage rate is already at or above today’s rates, say you bought recently at 6.5%, the repricing penalty mostly disappears, and the construction loan’s fixed single payment becomes far more competitive. The hinge swings on your first-mortgage rate.)
When is a construction loan the better tool for an ADU?
A HELOC is the default for an equity-rich owner, not the universal answer. Three situations flip it, and naming yours saves real money.
You don’t have the current equity. This is the big one, and it’s the construction loan’s reason to exist. A HELOC is capped at roughly 80% to 85% of today’s value minus your balance. If that doesn’t reach the build cost, no amount of projected rent changes it. A renovation or construction loan underwrites the finished value instead, so it can fund a project your present equity falls short of. We walk through that case in detail in financing an ADU when you’re short on equity.
You want a fixed, predictable payment. A HELOC’s rate moves with prime every time the Fed acts. A construction-to-permanent loan can lock a fixed rate for the life of the loan. If a stable payment over a 20-to-30-year payoff matters more to you than flexibility, that certainty has real value, especially given that your HELOC payment also jumps when the interest-only draw period ends.
Your first-mortgage rate is already high. If you bought or refinanced recently and your first mortgage is at today’s levels, refinancing it into a construction-to-permanent loan costs you nothing in repricing. At that point the construction loan’s single fixed payment and higher borrowing capacity often win outright. The HELOC’s edge is specifically about protecting cheap existing debt; with no cheap debt to protect, that edge is gone.
What’s the catch with each one?
Neither product is clean. Know the failure mode before you sign.
The HELOC’s catch is the payment shock and the variable rate. During the draw period you pay interest only, about $1,858 a month on a $300,000 balance at 7.47%. When the draw period ends, the balance amortizes and the payment jumps to roughly $2,404 a month, a 29% increase. And because the rate is variable, every quarter-point move in prime shifts the payment on $300,000 by about $62 a month in either direction. Budget around the repayment number at a stressed rate, never the interest-only teaser. Because the rate floats with prime, that’s not a footnote.
The construction loan’s catch is the repricing trap above, plus process friction. These loans run on inspections, draw schedules, and lender oversight of the build that a HELOC doesn’t impose. Two-close structures (a construction loan that converts to a separate permanent loan later) also carry the risk that rates move against you before the permanent loan locks. A single-close construction-to-permanent loan removes that specific risk but commits you to today’s rate on the whole balance.
Does anything change in California?
Three things, and they’re why this question gets asked here more than anywhere else.
California’s ADU laws require cities to approve qualifying units ministerially, so the build is more feasible here than in most states. The state’s high home values mean equity-rich owners are common, which tilts more Californians toward the HELOC path than you’d see nationally. And Prop 13 shapes the math underneath both options: borrowing against your home, whether by HELOC or construction loan, is not a change of ownership and never resets your assessed value. Building the ADU adds the new unit’s value to your assessment (a partial reassessment of just the new construction), but the rest of your home keeps its protected basis. The financing choice doesn’t change that; only the building does. The full interaction lives in the California HELOC guide.
The honest bottom line is the same one that governs every smart use of home equity: the cheapest path is rarely the lowest rate on the new money. It’s the one that doesn’t quietly reprice the cheap money you already have. Run your two numbers, current equity and current first-mortgage rate, before a lender frames the decision around project size.
Rates cited are national averages or representative ranges as of June 2026 and change frequently; construction loan pricing varies widely by lender and structure, and your offered rate depends on your credit, equity, and program. 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.
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Frequently asked questions
Is a HELOC or a construction loan better for building an ADU?
For a homeowner who already has the equity, a HELOC usually costs less, not because its rate is lower but because it leaves your existing first mortgage untouched. A construction loan's advantage is reaching a build your current equity can't cover, because it underwrites the home's after-completion value rather than today's value. The deciding question is your equity: enough today points to a HELOC; not enough points to a construction or renovation loan.
Why would a construction loan cost more than a HELOC if its rate is similar?
Because most construction and renovation loans for an already-mortgaged home refinance or wrap your first mortgage into a new, larger loan at today's rate. If you locked a sub-4% first mortgage years ago, that repricing is the real cost. On a $400,000 balance, moving from 3.25% to 7% costs about $15,000 a year, which can dwarf the rate difference on the ADU money itself. A second-lien HELOC borrows only against your equity and never touches the first mortgage.
When is a construction loan the better choice for an ADU?
Three situations. First, you don't have enough current equity, because a construction loan underwrites the finished value and a HELOC can't stretch past today's equity. Second, you want a fixed, predictable payment instead of a variable one. Third, the build is large enough or your existing mortgage rate is already high enough that refinancing everything into one fixed loan doesn't cost you a cheap first-mortgage rate you'd otherwise protect.
Does a construction loan let me borrow more than a HELOC for an ADU?
Often yes, and that's its core advantage. A HELOC is capped at your current equity, typically 80% to 85% of today's value minus your mortgage balance. A construction or renovation loan can lend against the home's projected value after the ADU is built, so it can reach a project your current equity falls short of. The trade is that it usually involves refinancing your first mortgage and carries inspections and draw oversight a HELOC doesn't.
Can I keep my low first mortgage and still get a construction loan for an ADU?
Sometimes, but it's the less common structure. Most construction-to-permanent and renovation loans (FHA 203(k), Fannie Mae HomeStyle) replace or wrap your existing first mortgage. A few lenders offer after-renovation-value second-lien products that sit behind your first mortgage and leave it alone; those are newer and offered by fewer lenders. If protecting a sub-4% first mortgage is the priority and you lack the equity for a HELOC, that second-lien path is the one to ask about.
Do construction loan rates run higher than HELOC rates in 2026?
It depends on structure. Single-close construction-to-permanent loans through banks and credit unions generally price in the high-6% to 9% range in 2026, overlapping with the 7.47% national average HELOC rate. Standalone construction loans tend to run higher. But the rate comparison on the ADU money is rarely the deciding number. Whether the loan reprices your existing first mortgage usually matters more to total cost than a point of rate on the new money.