Can I Finance an ADU Without Refinancing My Low-Rate Mortgage?

By Buz Artiano, BuildX | March 2026

Watch: How to Build an ADU Without Touching Your First Mortgage

You locked in a mortgage rate around 2 or 3 percent and it still saves you hundreds of dollars every month compared to what the banks are currently offering. Now you want to build an ADU for a parent, an adult child, or for rental income, and the banks all seem to have the same suggestion, which is to refinance your first mortgage into a construction loan. But that means you'd have to give up the low interest rate you already have locked in. For most homeowners in this position, the numbers stop adding up as soon as the subject of refinancing comes up.

My name is Buz Artiano, and I am the founder of BuildX. We've built dozens of ADU projects across Massachusetts, mostly around the South Shore and Plymouth County areas. We are the only team you need, we handle design, permitting, and construction. So we see every phase of a project from the first site visit up until you move in. We deal with septic constraints, zoning issues, and the state's new ADU laws. The number one concern people have when it comes to financing an ADU is how they can pay for it without losing their current mortgage that has a great low rate. This subject comes up in just about every consultation we do.

The answer is yes, you can finance your ADU and keep your existing mortgage at exactly the rate and terms it's at now. There is something called a Renovation HELOC that makes this possible by basically just taking a back seat to your current loan. In this article we're going to walk through how that type of loan works, what it costs, and who qualifies, so you can decide whether it's a good financing option for your family and your ADU project.

Can I Really Keep My Low Mortgage Rate and Still Build an ADU?

Yes. There's a type of loan that actually makes this possible, it's called a Renovation HELOC, and it's different than a traditional home equity line in one major way. It uses the after-repair market value of your property instead of what the property is valued at now. That means the appraised value of your home with the future finished ADU factors into how much you can borrow, even before we've started building it.

The Renovation HELOC sits in second position behind your existing first mortgage. Your first mortgage stays exactly how it is with the same interest rate and monthly payments you've always had. The HELOC is a completely separate loan that is secured by the equity in your property and the future value of the ADU combined.

Here's how the numbers work in real life. Say you own a home worth $600,000 and you owe $200,000 on your first mortgage at 2.85 percent and you want to build a $400,000 ADU. The lender takes your current home value of $600,000, multiplies it by 1.25 (the 125 percent of as-is value cap), and gets a maximum loan amount of $750,000. Then you subtract the $200,000 you owe, and you have $550,000 in available borrowing power when you only need $400,000 for the ADU. And your 2.85 percent first mortgage stays exactly the same.

The alternative (a 95 percent of after-repair value formula) could apply just depending on how much equity you have. The lender uses whichever formula gives you the smaller amount. Either way, you never have to refinance anything.

How Does a Renovation HELOC Differ from a Regular HELOC or a 203K Loan?

A standard HELOC only lends you money against whatever equity you currently have. It doesn't account for any improvements you're going to make. If your home is worth $600,000 and you owe $200,000, a traditional HELOC could work, they might let you borrow against part of that $400,000 in equity, but it's not going to account for the $400,000 ADU you are about to build. That puts limits on how much money you can borrow.

A Renovation HELOC makes sure you have plenty of borrowing power because it uses the after-repair value. The appraiser decides what your property will be worth once the ADU is done, and the lender uses that higher number as the basis for the loan.

A 203K loan (the FHA renovation product) and other similar loans like HomeStyle also use after-repair value. But there's a big difference between these and a reno HELOC: they replace your first mortgage. If you were able to secure a 2.85 percent interest rate, a 203K forces you to refinance with current rates which are much higher. That's the whole issue that homeowners are trying to avoid.

The 203K also requires a HUD inspector. Every time your contractor needs to pull money for the next phase of construction, the lender has to send an inspector to the site, the inspector then files a report, then the underwriter reviews it, and then a check gets issued. That whole thing can add weeks to your timeline. The Renovation HELOC eliminates all that nonsense. When the contractor needs a draw, the homeowner writes a check directly from the HELOC checkbook. No need for inspectors or long delays.

The HUD inspector on a 203K costs between $1,000 and $2,000. More importantly, the delays when drawing money for the build can add up significantly over the course of the project. On a Renovation HELOC, there's only one inspection, and it's at the end of construction to confirm the work is all done. During construction, there's a $150 monthly monitoring fee, that's it.

We've managed construction draws on both of these types of loans. On a 203K, every draw turns into a whole scheduling song and dance. The inspector has to come out, take photos, send them to the underwriter, wait for approval, and then they will finally cut a check. That can take two weeks every time you need to draw money, and on a typical ADU that has five or six draws, those delays can add months to your timeline. On a Renovation HELOC, our clients write a check the same day they approve the work. Managing the loan that way is the main advantage of the HELOC, and it's something most homeowners don't think about until they're in the middle of the project.

What Does a Renovation HELOC Actually Cost?

The interest rate on a Renovation HELOC depends on the current prime rate. As of November 2025, the prime rate is 7 percent. Your actual interest rate depends on your credit score and loan-to-value ratio though.

A borrower with good credit (780 or above) and a good loan-to-value ratio, could get a rate as low as prime plus 0.5 percent, or about 7.5 percent. For a borrower at 90 percent LTV or with a lower credit score, their rate might come in at prime plus 2 to 3 percent, which means they'd be paying 9 and 10 percent interest.

Unlike a 203K or a traditional construction loan, you only pay interest on the money you've actually used. If you get a HELOC for $400,000 to build an ADU but have only drawn $100,000 so far, you're only paying interest on $100,000. On a renovation loan, you pay interest on the full amount from day one.

Here's what the monthly cost looks like if you use the full amount you are approved for. If you borrow $400,000 at 10 percent, your annual interest is $40,000, which works out to be about $3,300 per month in interest-only payments. At 7.5 percent with the same $400,000 balance, the monthly interest payment goes down to $2,500 per month.

Closing costs are about the same as a standard mortgage. Expect $12,000 to $15,000 total, that's usually $6,000 for the closing costs and prepaids of another $5,000 to $6,000. There could also be points depending on your situation.

The first out-of-pocket cost for you would be the appraisal, which runs $575 to $675 for a conventional appraisal or $800 to $900 for an investment property appraisal. Before the appraisal, there's no out of pocket expenses. The lender covers the credit report fee and all pre-approval costs until closing.

There's no prepayment penalty. You can pay down the balance at any time, and the draw period lasts 10 years. After 10 years, the HELOC turns into a regular loan with a 20-year repayment period.

How Much Equity Do I Need to Qualify?

The Renovation HELOC uses two formulas to determine how much you can borrow, and the lender uses whichever has the lower max amount.

Formula 1: 95 percent of the after-repair appraised value (the value of your property with the ADU completed).

Formula 2: 125 percent of the current as-is value.

For both of these options the lender subtracts your existing mortgage balance from the maximum approved amount to decide how much you can actually borrow.

Using the scenario above: a $600,000 home with a $200,000 mortgage. Under the 125 percent formula, your maximum borrowing amount is $750,000 minus $200,000 you still owe on the loan. That leaves $550,000 in available borrowing, which is plenty for a $400,000 ADU.

But change the mortgage balance to $500,000, and numbers get a little tighter. $750,000 minus $500,000 leaves only $250,000 in borrowing power, which won't cover a $400,000 ADU.

The loan cap is usually $500,000 to $750,000 depending on your situation. In some cases we've seen it be higher than that. A minimum credit score of 660 to 680 is required to qualify.

There's one other scenario that some people come across as well. If the homeowner's parent sells their existing home and then uses that money to pay down the HELOC after the ADU is built, the balance could go to zero on the first day. The HELOC line stays open for the rest of the draw period, and the homeowner pays no interest because the HELOC has a zero balance. That's a pretty common method that families will use when the parent is moving out of a home they own.

What Are the Real Downsides of This Product?

The interest rate is not locked in, it changes. If the prime rate rises, your payment will also go up. The rate adjusts every so often, which means your monthly cost is not set in stone. For homeowners who are used to the stability of a fixed-rate mortgage, this can be a hard adjustment.

The rate is also going to be much higher than your first mortgage. A 7.5 to 10 percent rate on $400,000 is a significant monthly cost, even if it's interest-only. You're trading that stability of a locked-in payment for the flexibility to keep that payment the same.

It's also harder to qualify for this type of loan. When you're looking at the whole mortgage market, around 70 to 80 percent of people who apply for this type of loan don't qualify. Likely because of bad credit, not enough income, or not enough assets. It's not the type of loan where everyone who calls gets approved.

These loans require your contractor to be pre-approved by the lender, which adds yet another step to the process. But once a contractor is approved with a given lender, they stay approved for all future projects. The lender also needs a detailed breakdown of materials, labor, and project scope. A lump-sum contract is not good enough.

Is a Renovation HELOC the Right Financing Path for Every ADU Project?

No. This product works well for homeowners who have significant equity, good credit, and a low first mortgage rate they don't want to give up. But it's not the only path, and it's definitely not always the best one.

If you have enough cash or family resources to pay for your ADU without borrowing money from a bank, you save all that money in interest payments. If your existing mortgage rate is already close to today's market rates, refinancing into a construction loan could make things easier because you'd only have one payment and a fixed rate. And if you need the oversight that comes with a HUD inspector reviewing every draw, a 203K loan provides a built-in layer of third-party verification that a Renovation HELOC doesn't.

We work with families using all of these different financing plans. Our job is to help you understand which one fits your situation. It doesn't matter to us which loan product you choose, we just want to make sure it's the best option for your situation.

Your Rate Is Not the Obstacle You Think It Is

The homeowners we talk to every week assume they have to choose between keeping their low mortgage rate and building an ADU for their family, but fortunately that's not true. A Renovation HELOC lets you do both because it sits behind your existing mortgage, uses the future value of your property to let you borrow more money, and gives you direct control over construction draws without the delays and costs of a HUD inspector.

The next step is finding out whether your equity position and credit profile qualify. That conversation costs nothing and takes about 30 minutes.