Accessory dwelling units and in-law suites have moved from zoning curiosity to serious construction project for a lot of homeowners, particularly as multigenerational living arrangements become more practical and local permit rules loosen up. The financing decision that comes before the first shovel of dirt, though, gets far less attention than the floor plan. Choose the wrong vehicle and you could end up with a draw schedule that doesn’t match your contractor’s billing cycle, a rate reset that arrives before the unit is earning rent, or closing costs that quietly add several thousand dollars to a project that already stretched the budget.
This article walks through the three most common paths — HELOC, cash-out refinance, and construction loan — with enough structural detail to help you ask sharper questions before you sit down with a lender.
What You’re Actually Comparing
Before running any numbers, it helps to understand what each instrument is doing mechanically, not just what the marketing language says about it.
A HELOC (home equity line of credit) works like a credit card secured against your home’s equity. You draw funds as needed during a draw period — typically five to ten years — paying interest only on what you’ve pulled. After that, the line closes and you repay principal plus interest.
A cash-out refinance replaces your existing mortgage with a new, larger one. The difference between your old balance and the new loan amount comes to you in cash. You now have one loan, one payment, and a rate that reflects current market conditions — which is a benefit or a liability depending on what your original rate looks like.
A construction loan is a short-term product designed specifically for building. Lenders release funds in stages tied to completed milestones (foundation poured, framing complete, rough plumbing in, and so on). Once construction finishes, you either refinance into a permanent mortgage or roll into a construction-to-permanent loan automatically. These are harder to qualify for and carry more administrative overhead, but they exist for exactly this kind of project.
The Case for a HELOC
For an ADU project where the scope is reasonably firm but the timeline is messy — as most construction timelines are — a HELOC offers flexibility that a lump-sum product can’t match. You borrow what you need when you need it, which means you’re not paying interest on $180,000 the week your contractor is still waiting on a permit.
The catch: HELOCs almost always carry variable rates, typically indexed to the prime rate. If you draw heavily and rates move during your build, your carrying cost changes mid-project. That’s manageable if you plan to pay down the balance quickly or convert to a fixed product, but it’s worth stress-testing with a range of rate scenarios before you commit.
Lenders generally want to see combined loan-to-value (CLTV) stay below 80–85% after the line is opened, though that threshold varies — confirm current underwriting criteria with your specific lender. The Credit Card vs. HELOC for Home Improvement piece on this site goes deeper on when a revolving credit structure makes sense for renovation spending generally.
The Case for a Cash-Out Refinance
If your existing mortgage rate is already above current market rates, a cash-out refi can let you accomplish two things at once: pull equity for the ADU and improve your underlying rate. That double-win scenario was common before 2022. In a period where rates have climbed and stayed elevated, the math flips — you’d be trading a lower locked rate for a higher one, and paying closing costs (typically $3,000–$6,000, though this range varies by loan size and lender) to do it. Verify current closing cost estimates with lenders in your area before planning around any number.
Where cash-out refis still make sense: projects with a clearly defined total cost, homeowners who want the simplicity of one loan and one payment, and situations where the original mortgage rate was already uncomfortably high. The lump-sum structure also removes the temptation to keep drawing — which sounds like a small thing, but renovation budgets have a tendency to expand when money remains accessible.
One mechanical point worth confirming with your lender: most conventional cash-out refis cap at 80% LTV on the post-refi value, meaning your home needs to appraise high enough to support the new loan balance. If you’re in a market where values have been flat, run those numbers carefully before assuming you have enough equity to work with.
The Case for a Construction Loan
Construction loans are the least convenient option and, for a ground-up ADU or a significant structural addition, often the most appropriate one. The staged draw structure — where the lender sends an inspector to verify milestone completion before releasing each tranche of funds — creates a natural checkpoint system that can actually reduce the risk of paying for work that hasn’t happened yet.
The administrative friction is real. Expect a more intensive underwriting process, a higher interest rate during the construction phase (as of our last check, construction loan rates typically run a percentage point or more above conventional mortgage rates, though spreads shift — confirm with current quotes), and a requirement to submit detailed plans and a contractor budget before closing. Some lenders require a licensed general contractor; bringing in a series of subs on your own can disqualify you from certain products.
The most expensive renovation mistakes happen before construction starts — and that observation applies directly here. An underbaked budget that can’t withstand lender scrutiny is a sign that the project needs more planning, not a faster financing path.
For a full ADU build — detached structure, new foundation, separate utility connections — a construction-to-permanent loan that converts automatically at the end of the build period can simplify the end-of-project transition and lock in a permanent rate before you’ve exhausted your contingency budget.
Choosing: A Quick Comparison
| Factor | HELOC | Cash-Out Refi | Construction Loan |
|---|---|---|---|
| Rate type | Usually variable | Fixed (new rate) | Variable during build, fixed after |
| Draw structure | As-needed | Lump sum at close | Staged milestones |
| Best for | Phased or uncertain scope | Known cost, rate improvement | Ground-up or major structural work |
| Closing costs | Lower | Moderate to high | Moderate to high |
| Qualification complexity | Moderate | Moderate | Higher |
| Risk to existing rate | None | Replaces existing rate | None (separate product) |
A few other factors to weigh before deciding:
- Rental income projections. If the ADU will generate rent, a lender may factor that into qualification — ask specifically how they treat projected ADU income, since underwriting varies considerably.
- Timeline to occupancy. A HELOC draw period gives you years of flexibility; a construction loan has a defined term, often 12–18 months, after which you must convert or pay off.
- Your contractor’s billing rhythm. If your GC expects large deposits and milestone payments on their schedule, not yours, a HELOC’s on-demand structure is often more practical than waiting for a lender’s draw inspection.
The backyard ADU boom in states like California — L.A. has seen record ADU construction following zoning reform — has pushed lenders to develop more ADU-specific products in high-activity markets. If you’re building in a region with significant ADU activity, ask lenders directly whether they offer products designed for this project type, since the terms can be more favorable than a generic HELOC or construction product.
Before You Call a Lender
Pull your most recent mortgage statement and note your current interest rate, remaining balance, and estimated home value. Run a rough LTV calculation to see what equity you’re working with. Then get at least three quotes across all three product types — not just from your current lender — so you’re comparing real numbers rather than assumptions.
If you haven’t nailed down the project’s total cost yet, that step comes first. A general contractor walkthrough with a written estimate, even a preliminary one, is worth more than any financing conversation you’ll have without it.