A Japanese variable-rate home loan may keep its scheduled monthly payment unchanged for a time after the rate rises. That can make budgeting easier, but it does not freeze interest costs. Before buying a home, track both the amount debited each month and the balance you will still owe.
What the five-year and 125% rules actually limit
Some Japanese variable-rate loans with equal monthly payments of principal and interest apply a five-year rule: the scheduled payment is reviewed at specified intervals, rather than every time the interest rate changes. Under a separate 125% rule, an increase at a scheduled review is capped at 125% of the preceding payment. Neither rule caps the interest rate or cancels interest. Not every lender or product adopts them, and review dates and treatment after a prepayment vary by contract.
For a concrete example of that variation, Docomo SMTB Net Bank explains its payment review timing and states that its five-year and 125% rules do not apply when recalculating monthly payments after a partial prepayment. This is an example of one lender’s terms, not a standard clause in every Japanese mortgage. Ask for the rules governing the specific loan you are considering.
An unchanged payment can hide a slower decline in debt
Each equal monthly payment contains principal and interest. If the rate increases while the scheduled payment stays fixed, a larger share pays interest and less pays down principal. Depending on the rise and the contract, unpaid interest may arise. The 125% ceiling on a future payment increase does not erase the interest that accrued.
For a simple sensitivity check, a one-percentage-point increase applied to an outstanding ¥100 million balance corresponds to about ¥1 million more interest over a year at that starting balance, or roughly ¥83,000 per month. Actual interest changes with the outstanding balance and the effective date. This is an illustration of interest exposure, not a forecast of the contractual monthly payment.
If you plan to refinance or sell, the loan balance at that point matters as much as the payment shown on today’s statement. Two paths with similar monthly debits can leave different amounts to repay when the property is sold.
Put three figures side by side before choosing a budget
- Initial monthly payment. Check that it fits recurring take-home pay at the initial interest rate.
- Payment and loan balance after a rate increase. Run more than one rate scenario. Compare the balance at the same future date, including periods before and after a payment review.
- Other ownership costs. Add condominium management fees and repair reserve contributions, property taxes, insurance and likely repairs. Include acquisition costs and a possible future sale.
Where compensation includes a bonus or stock awards, distinguish dependable monthly cash from variable proceeds. Our RSU and bonus guide explains why the amount a lender might accept is not necessarily a prudent household budget.
Questions for the lender before signing
Ask when the interest rate resets, when the scheduled payment is reviewed, whether a 125% ceiling applies, what happens after a partial prepayment, how unpaid interest is handled and what happens at loan maturity. Compare the answers with the contract and repayment schedule. For purchase planning, use the English mortgage planning tool to test alternative rates, then check its assumptions against the lender’s actual terms.
Source and scope
Docomo SMTB Net Bank: payment reviews, five-year and 125% rules (Japanese). Guidance checked 26 September 2026. Product rules and future rates must be confirmed with the lender. Read the Japanese edition.