HELOC vs Construction Loan for an ADU: Which Costs Less Overall

The Real Cost of Financing Your Altadena ADU: Why the Cheapest Rate Isn't the Cheapest Loan

Most Altadena homeowners shopping for ADU financing make the same mistake: they compare the two headline rates and stop there. But the rate you're quoted tells you almost nothing about what you'll actually pay — because closing costs, draw mechanics, and rate behavior over the life of the loan quietly drive the real bill.

The numbers make this concrete. HELOC closing costs typically run $0–$500, while construction loans charge 2–5% of the loan amount — meaning $20,000 to $50,000 in fees on a $1M loan before a single beam goes up, according to one contractor comparison. Draw mechanics matter just as much: a HELOC charges interest only on funds you've actually pulled, so a homeowner with a $200,000 approved line who draws $170,000 in staged payments never pays interest on the unused $30,000.

Then there's rate risk. HELOCs stay variable for the life of the loan — cited ranges of 7–10% — while construction loans run 8–12% during the build but then convert to a fixed permanent mortgage at 6.5–8%, per ADU financing guidance. And the "cheapest" HELOC carries a trap: interest-only payments during the draw period don't reduce principal at all — a warning CFP® Kyle Ryan makes explicitly.

Strip away the noise, and the decision really comes down to two questions:

• How much equity do you have? HELOCs typically cap borrowing at 80–85% LTV minus your mortgage balance.
• How big is your ADU? HELOCs work best for projects under roughly $200K–$300K; construction loans can finance larger builds and up to 90% of project cost.
• Do you need to borrow against future value? Construction loans are underwritten on the completed property's projected worth, not current equity.

The research-backed answer that emerges is consistent: HELOCs usually cost less overall for equity-rich homeowners on ADUs under about $250K, thanks to minimal closing costs and pay-only-for-what-you-draw flexibility. Construction loans win when your ADU is larger, your equity is thin, or you need to borrow against what the property will be worth once the unit is finished.

Where you sit on that spectrum depends on numbers specific to your property — your equity, your lot, and your build budget. That's exactly what our free ADU feasibility consultation at Altadena ADU Builder is designed to surface, with a cost-quiz concept render tailored to your backyard.

Where the Money Actually Goes: Closing Costs and Draw Mechanics Side by Side

Before a single board is ordered, the financing structure has already shaped your total cost. The clearest gap between a HELOC and a construction loan shows up at closing — and it's measured in thousands, not hundreds.

HELOC closing costs typically run just $0–$500, while construction loans charge 2–5% of the loan amount in closing costs and origination fees — roughly $20,000–$50,000 on a $1 million loan, before construction even begins. That upfront difference alone can offset years of rate advantage on either side, according to Bay Area contractor research.

Draw mechanics compound that advantage. Because ADU construction happens in phases, a HELOC lets you draw only what each phase requires and pay interest only on funds actually used. One worked example shows a borrower approved for a $200,000 line who drew just $170,000 through staged draws of $75K, $25K, $50K, and $20K — meaning interest never accrued on the unused $30,000. If your build comes in under budget, you don't pay interest on money you never borrowed.

Construction loans work differently. Funds release in lender-inspected milestone draws, with typical allocations of 15–20% at foundation, 20–25% at framing, 20–25% at rough-in, 15–20% at drywall and interior, and 10–20% at final inspection. That structure suits lender-managed builds, but it adds inspection friction and ties your cash flow to a fixed schedule rather than your project's actual pace.

The illustrative first-year totals make the gap vivid. A $200,000 HELOC draw costs roughly $14,560–$15,060 in interest plus closing costs, while a $750,000 construction loan runs approximately $64,750–$108,500 including interest, closing costs, appraisal, and inspections. These figures are illustrative — actual costs depend on your rates, timeline, and lender — but the directional gap is consistent across sources.

One warning belongs in every HELOC decision. "Making interest-only payments does not decrease your principal balance," cautions Kyle Ryan, CFP®. During a typical 5–10 year draw period, minimum payments cover interest only, so the balance you owe on day one is the balance you owe when the draw period ends — a hidden cost that can erode the HELOC's apparent advantage if you never amortize.

For Altadena homeowners weighing these trade-offs against a specific lot and budget, Altadena ADU Builder offers a free ADU feasibility consultation with a cost-quiz concept render — a practical way to size your project before you commit to either financing structure. As always, verify current terms directly with lenders; rates and fees vary and change.

Rate Risk and Borrowing Power: When a Construction Loan Beats a HELOC

HELOCs look cheap until you realize the rate never locks in. Across the sources we reviewed, HELOC rates run variable in the 7–10% range — one ADU financing guide cites a national average of 7.43% as of June 2026 — and that variability lasts for the life of the loan, not just the construction phase.

Construction loans flip that script. They carry higher variable rates during the build (8–12% per most sources) but convert to a fixed permanent mortgage at 6.5–8% once the ADU is complete, according to ADU zoning guidance. For a $150K balance, a 1% rate difference saves roughly $1,500 per year — meaningful over a 20-year permanent mortgage.

Borrowing power is where construction loans truly separate from HELOCs. HELOCs cap at 80–85% of current home value minus your mortgage balance, while construction loans underwrite against the completed property's future value (ARV). One worked example shows the gap: on a $700K home becoming a $900K after-ADU value with a $500K mortgage, a HELOC maxes at ~$60K while an ARV-based product reaches ~$220K.

For Altadena homeowners, equity preservation matters too. A HELOC acts as a second mortgage, leaving your existing first mortgage's rate and payment untouched — a real concern given FHFA data on 1.72 million prevented home sales among homeowners holding 3–4% mortgages from 2020–2021. If your current rate is under 4–5%, guidance is clear: don't refinance to fund the ADU.

Emerging ADU-specific HELOCs blur the traditional equity constraint. Patelco Credit Union's ADU line of credit, for example, allows borrowing up to 125% of current home value and counts projected ADU rental income toward qualification — with variable APRs starting at 8.50% as of October 2026.

When does each product make sense for an Altadena ADU?

• Thin equity or a $200K+ build: construction loans can finance up to 90% of project cost and cover the full build where a HELOC would max out early.
• Rate certainty desired: construction loans convert to fixed 6.5–8% permanent mortgages; HELOCs stay variable for the life of the loan.
• Low-rate first mortgage in place: a HELOC preserves it, while a cash-out refi would surrender it.
• Modest ADU under ~$250K with 20%+ equity: a HELOC typically costs less overall despite the variable-rate trade-off.

The honest takeaway: construction loans win when borrowing against future value is required or when rate certainty matters most, while HELOCs win on total cost for equity-rich homeowners building modest ADUs. Altadena ADU Builder's free feasibility consultation with a cost-quiz concept render can help you map your equity position against your ADU budget — and featured builder JMB Builders LLC (CSLB #1103479) can discuss financing-friendly project scoping during that conversation. Verify current rates directly with lenders, as figures here are directional ranges.

Your Financing Decision Tree: Matching the Loan to Your Equity and ADU Budget

Two questions determine which loan costs you less: how much equity you have, and how big your ADU budget is. Answer those honestly, and the financing choice largely makes itself.

Start with equity. If you hold at least 20% equity and your ADU budget sits under roughly $250K, guidance from ADU financing specialists points squarely toward a HELOC — the lowest-cost option with flexible draws. A typical Altadena scenario illustrates why: an $800K home with a $400K mortgage leaves about $240K available at 80% LTV, often more than enough for a garage conversion or detached unit.

If your build is larger or your equity is thin, the calculus shifts. Construction loans underwrite against the completed property's future value rather than current equity, and can finance up to 90% of project cost for $200K+ builds. One worked example shows the gap clearly: on a $700K home, a HELOC might cap at ~$60K while an ARV-based product reaches ~$220K, per ADU lender comparisons.

Be aware that sources disagree on where the crossover sits. Some put the HELOC sweet spot under $250K, while Bay Area contractor analysis reserves construction loans for projects over $500K. Treat these thresholds as directional, not precise — and remember that a 1% rate difference on a $150K loan swings roughly $1,500 per year.

A few practical checkpoints before you commit:

• Confirm your equity position — most HELOC lenders want 10–20% equity and lend up to 80–85% CLTV.
• Beware the interest-only trap: as one CFP® warns, HELOC draw-period payments don't reduce your principal balance.
• If your first mortgage rate is under 4–5%, a HELOC preserves it; a cash-out refi would forfeit it.
• Interest used to build or substantially improve the ADU may be tax-deductible, subject to the $750K mortgage debt limit — confirm with a tax advisor.
• Note that the CalHFA ADU grant (up to $40,000 forgivable) is paused, with funds fully allocated as of December 2023.

For Altadena homeowners, the smart sequence is to size the ADU budget first, then verify current rates with two or three lenders — published figures are dated estimates that vary by borrower. A free ADU feasibility consultation from Altadena ADU Builder, including a cost-quiz concept render, gives you the project scope and budget clarity lenders will ask for. A local licensed builder like JMB Builders LLC (CSLB #1103479) can supply the detailed construction cost estimates your lender requires, whether you pursue a HELOC or a construction loan.