What happens to your mortgage when interest rates rise? Things to check before your repayment amount changes

When I hear the term “interest rate hike,” I worry that my mortgage payment might suddenly increase starting next month.

However, it is not just the interest rate news that I need to check. I need to know when the interest rate changes for my specific contract and when the repayment amount will change. Separating these two factors makes it easier to see the impact on household finances.

In this article, I will organize information ranging from the differences between variable and fixed interest rates to considerations for refinancing and available cash. This is not an article recommending any specific financial institution, but a guide to reading your repayment schedule.

*Information verification date: October 1, 2026. The calculations are hypothetical examples; please check your individual repayment amount through your lender’s guidance.

1. The Bank of Japan’s interest rate and your mortgage interest rate are different

At its meeting on September 18, 2026, the Bank of Japan decided on a policy to set the target for short-term market interest rates at around 1.25%. This will be applied starting September 24.Bank of Japan published materials

This “1.25%” is not automatically applied to everyone’s mortgage.

The interest rate applied to you depends on the bank’s base rate, the reduction margin in your contract, and the review date. It is also important to distinguish between the advertised rates for new loans and the rates for those who have already borrowed.

After checking the news, first go to the “For Existing Customers” section of your lender’s website. This is the shortcut to confirming when it will be reflected for you.Regarding the review of base rates for variable interest rates at MUFG Bank

2. Where to check differs between variable and fixed rates

For variable interest rates

The applicable interest rate is reviewed at the times stipulated in the contract. However, the interest rate review and the monthly repayment amount review do not necessarily happen at the same time.

If you have a fixed rate for a certain period

Consider the fixed period and the period after it separately. If you have a “10-year fixed” rate, it does not necessarily mean the entire remaining repayment period is fixed. The end date and the conditions applied thereafter are key points to check.

Review periods and available contracts vary by bank.Sumitomo Mitsui Trust Bank: Explanation of interest rate plans and repayment amount changes

For fixed-term loans for the entire period

For types where the interest rate is determined until the end of repayment at the time of borrowing, the contracted interest rate does not change due to subsequent market interest rate increases. Check separately for the end of any pre-determined preferential periods.Japan Housing Finance Agency: Flat 35

The situation for those who are about to borrow is different from those who have already signed a contract. Make sure to check whether the interest rate at the time of application or the interest rate at the time the loan is actually executed will be applied.

3. Even if the “repayment amount is the same,” interest may increase

The 5-year rule and the 125% rule are often heard in explanations of variable interest rates.

• 5-year rule: A mechanism where the monthly repayment amount remains fixed for a certain period and is reviewed at a specified time.

• 125% rule: A mechanism that limits the increased repayment amount to 125% of the previous amount when the repayment amount is reviewed.

Even in contracts that include these, if the applicable interest rate rises, the portion of the fixed repayment amount allocated to interest increases, while the portion allocated to principal repayment decreases.

For example, even if you pay 100,000 yen every month, if the breakdown changes, the way your loan balance decreases will also change. The 125% rule is not a system that sets a cap on interest or the total repayment amount.MUFG Bank: 5-year rule and 125% rule

Also, not all variable interest rate contracts have this rule. There are contracts that do not apply it even with principal and interest equal repayment. Do not assume that “it’s variable, so it will be fine for 5 years”; check your contract.Sumitomo Mitsui Trust Bank: Differences by contract

If the interest rate increase is significant and the interest alone exceeds the monthly repayment amount, unpaid interest may arise. This must be distinguished from a state where the principal is simply decreasing more slowly. Since it may be necessary to settle the remaining balance on the final repayment date, you should also check how that is handled.MUFG Bank: Explanation of unpaid interest and final repayment date

*The pie charts in the figure are conceptual images of the mechanism. They do not show the actual ratio of principal to interest.

4. If the balance is 30 million yen, what is the monthly difference?

To consider the burden, I calculated it independently under the following conditions.

Loan balance 30 million yen / 30 years remaining / principal and interest equal repayment / no bonus repayments. It is assumed that each interest rate remains constant throughout the remaining period, and the 5-year/125% rule and various fees are not considered.

• 1.00% per year: 96,492 yen per month

• 1.25% per year: 99,976 yen per month

• 1.50% per year: 103,536 yen per month

• 2.00% per year: 110,886 yen per month

Compared to 1.00% per year, the difference is about 7,000 yen per month at 1.50% per year, and about 14,400 yen per month at 2.00% per year. The figures are rounded to the nearest yen.

This is not a prediction that “the next withdrawal will definitely be this amount.” In reality, it changes depending on the balance, remaining years, and review mechanism. Also, this interest rate is a hypothetical applicable interest rate for a home loan and is separate from the Bank of Japan’s policy interest rate.

What I want to see in the trial calculation is whether you can comfortably pay the increased amount. It is more realistic to consider not only the monthly surplus but also money that goes out in lump sums from time to time, such as fixed asset taxes, vehicle inspections, and educational expenses.

5. Do not decide on refinancing based solely on the “interest rate difference”

Even if you find a product with a low interest rate, you should first get a quote that includes various fees.

Refinancing may involve loan processing fees, registration-related costs, and fees for paying off your current loan. The handling of guarantee fees and stamp duty also varies depending on the product and procedures.Bank official: Explanation of costs for refinancing

When comparing, align the following three points.

• The total payment amount including fees when the remaining repayment period is aligned

• The coverage details and enrollment conditions of group credit life insurance

• The conditions for changing interest rate types at your current bank

Even if extending the period reduces your monthly burden, it does not necessarily mean the total payment amount will decrease. If comparing variable interest rates, you need to estimate based on multiple cases rather than assuming a single future interest rate.

Changing to a fixed rate also helps make future payments easier to predict. You should use not only “whether it will be profitable” but also “whether you can maintain your household budget with this payment amount” as criteria for your decision.

6. Consider early repayment and investments only after checking your available funds

Reducing your loan balance is one way to curb future interest costs. However, money used for early repayment cannot be withdrawn as easily as savings.

What you should sort out first is the money you will need in the near future and the money you will need if your income decreases. The required amount varies depending on your monthly living expenses, work style, dependents, and planned expenditures.

It cannot be definitively said that “you should prioritize investment because the expected return on investment is higher than the loan interest rate.” There is no guarantee that the expected investment performance will be realized, and there is a possibility that the market may be down when you need to make payments.

In this article, we propose the order of considering early repayment and savings amounts after securing funds for repayment and living expenses.

If you are already finding it difficult to make repayments, please consult with your lender before you miss a withdrawal date. It is more important to first confirm the available options than to rush into refinancing or early repayment.

7. These are the 6 items to check today

Once you open your repayment schedule or bank app, try noting down the following items.

1. Current applicable interest rate: __% per annum

2. Loan balance: __0,000 yen / Remaining period: __ years

3. Next interest rate review date / Date new rate applies: __

4. Next repayment amount review date: __

5. Presence of 5-year/125% rule: __

6. Household budget flexibility if repayments increase by 5,000 yen, 10,000 yen, or 15,000 yen per month: __

The final amount is a hypothetical stress test to check your household budget. It is not a prediction of future increases.

Even if it is difficult to accurately predict the future of interest rates, you can check your contract terms and the flexibility of your household budget.

Enjoy your hobbies and daily life today while preparing for the future. To do that, why not start by opening your repayment schedule?

*This is general information and does not recommend any specific refinancing or investment decisions. Please check with your financial institution regarding the handling of your contract.