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We're borrowing about $60,000 to renovate our home. If we add it to the mortgage, how can we compare paying it off quickly with spreading it over the remaining loan term?
How to compare paying off your $60,000 renovation loan quickly versus over your mortgage term
If you're borrowing $60,000 to renovate your home in New Zealand, adding it to your mortgage can make the repayments look much more manageable. The catch is that if you spread the renovation cost over the remaining 20 or 25 years of your mortgage, you could pay a substantial amount in interest—even if the mortgage interest rate is relatively low.
The best way to compare your options is to look at three things:
- Your monthly repayments: How much will each option cost your household budget?
- The total interest paid: How much extra will the renovation actually cost you?
- Your flexibility: Can you make extra repayments when you have spare cash without incurring fees?
My starting recommendation would be to consider adding the $60,000 to your mortgage but repaying that portion over a much shorter period, such as 5 or 10 years, rather than automatically stretching it across the full remaining mortgage term.
You can compare the numbers to see whether that approach works for your budget.
1. Compare the numbers for a $60,000 loan
Let's use an illustrative mortgage interest rate of 6% per year, with monthly repayments. This is an example for comparison, not a quote for current New Zealand mortgage rates.
Here is what it could look like if you borrowed $60,000 and repaid that amount over different periods.
| Repayment period | Approx. monthly repayment | Total interest | Total repaid |
|---|---|---|---|
| 5 years | $1,160 | $9,600 | $69,600 |
| 10 years | $665 | $19,800 | $79,800 |
| 15 years | $506 | $31,000 | $91,000 |
| 20 years | $430 | $43,200 | $103,200 |
| 25 years | $387 | $56,000 | $116,000 |
Figures are approximate and assume a constant 6% interest rate throughout the entire repayment period, with principal and interest repaid monthly. They exclude fees and changes in interest rates.
What does this tell you?
Consider three possible approaches:
- Pay it off over 5 years: You would pay approximately $1,160 per month, but the total interest would be only $9,600.
- Pay it off over 10 years: You would pay approximately $665 per month and $19,800 in interest.
- Spread it over 25 years: You would pay approximately $387 per month, but the interest would total around $56,000.
The difference between the 5-year and 25-year options is striking: the longer option saves you about $773 per month compared with the shorter option, but costs approximately $46,400 more in interest.
That is the central trade-off: lower monthly repayments today versus a lower overall cost.
2. You don't necessarily have to choose between the two extremes
One option worth investigating is to add the $60,000 to your mortgage but make extra repayments specifically to pay off the renovation borrowing sooner.
For example, suppose your lender allows you to structure the borrowing as a separate loan portion.
You might arrange it like this:
- Your existing mortgage continues under its current repayment arrangements.
- The additional $60,000 sits in a separate loan portion.
- You set repayments on that portion to pay it off over 5 or 10 years.
- Once the renovation portion is repaid, you can redirect that money towards your existing mortgage or other financial goals.
This can make budgeting easier because you can see how much of your debt relates to the renovations and when you expect to clear it.
It also avoids treating the renovation borrowing as if it needs to last as long as the house loan itself.
However, the right structure depends on your lender, interest rates, and whether your existing mortgage is fixed or floating. New Zealand's Sorted guidance explains how making additional mortgage repayments can reduce both interest and the time it takes to clear your debt. Sorted: Managing a mortgage.
3. The strategy I would compare first
Rather than simply choosing the shortest term you can afford, I'd compare these three strategies.
| Strategy | How it works | Main advantage |
|---|---|---|
| A. Full mortgage term | Add $60,000 and repay it over the remaining 20–25 years | Lowest required repayments |
| B. Shorter repayment term | Put the $60,000 in a separate loan portion with a 5–10-year repayment target | Less interest and a clear payoff date |
| C. Flexible repayment | Keep the required repayments manageable, but make extra repayments whenever possible | Flexibility if your income or expenses change |
Strategy B is a useful benchmark, while Strategy C may offer the best balance of flexibility and interest savings. Whether either is best for you depends on the rates and repayment conditions your lender offers.
For Strategy C, the important question is whether your mortgage structure allows extra repayments without penalties.
If your mortgage is fixed, your lender may limit how much extra you can repay during the fixed-rate period. Exceeding those limits could trigger an early repayment charge. Check your particular loan conditions before committing to a faster repayment plan. New Zealand Consumer Protection: Mortgages and home loans.
4. How to get an accurate comparison for your household
I'd use the Sorted mortgage calculator to compare the repayment amounts, total interest and payoff dates.
For the most useful comparison, keep the interest rate the same across your scenarios initially. Then test what happens if rates rise or fall.
You should also account for any fees and the possibility that your mortgage rate will change when your current fixed term ends.
Most importantly, compare the additional cost of the renovation borrowing, rather than just looking at your new combined mortgage repayment.
For example, if your existing mortgage repayment is $3,000 a month and the renovation loan adds another $665, you would want to know whether your household can comfortably manage the resulting $3,665 monthly payment, while still covering other expenses and maintaining an emergency fund.
5. Let's calculate your actual options
I can put together a more precise comparison for your $60,000 renovation borrowing, including monthly repayments, total interest and the difference between paying it off quickly and spreading it over the remaining mortgage term.
I just need three details:
- Your mortgage interest rate: What rate are you currently paying, or what rate has your bank offered for the additional $60,000?
- Your remaining mortgage term: For example, 18, 22 or 25 years.
- Your preferred repayment target: Would you ideally pay off the renovation borrowing in 5 years, 10 years, or simply as quickly as your budget comfortably allows?
With those details, I can compare the full-term option against the shorter-term alternatives and show you how much interest you could save by paying the renovation borrowing off sooner.
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Captured 11 October 2026