Mortgage Affordability Basics in Japan

Mortgage affordability is arithmetic before it is emotion: a payment, built from a loan, judged against an income. Japan adds its own screening conventions, age limits and insurance requirements to that arithmetic. This guide runs through the mechanics — how the payment is built, how banks judge it, and which levers actually move it.

How the payment is built

A Japanese housing loan is an annuity: the instalment equals the loan times the monthly rate, divided by the number that amortises it to zero over the term. Three levers move it. The loan amount scales it linearly — borrow 10% more, pay 10% more every month. The rate scales it too, but non-linearly at long terms: on a 35-year loan, moving the rate from 2% to 3% raises the instalment on ¥30,000,000 from about ¥99,400 to about ¥111,300 — roughly a 12% payment increase from a one-point rate move. The term trades payment for total cost: the same ¥30,000,000 at 2% costs about ¥99,400 a month over 35 years (roughly ¥11,700,000 of total interest) but about ¥127,200 over 25 years (roughly ¥8,100,000 of interest). Shortening the decade saves about ¥3,600,000 — if the higher payment fits. The mortgage calculator moves all three levers live.

How banks judge the payment

• Repayment ratio (返済比率): annual payments including bonus repayments over gross annual income; commonly cited screening benchmarks sit around 25–35%, with each bank applying its own rule.
• Age limit: banks commonly want the loan repaid before about age 80, which caps the available term — the reason a 50-year-old borrower cannot stretch to 35 years.
• Other debts: car loans and card balances reduce the capacity the bank will assign to the mortgage; clearing them before applying often improves the offer more than a rate negotiation does.
• Income stability: employment type, length of service and bonus structure feed internal scoring, as does the credit record held at bureaus such as JICC, CIC and KSC.
• Group credit life insurance (団信): effectively required at most banks, with premium folded into the rate or payment; health disclosures can change terms.

The down payment and the 90% threshold

Flat 35's published pricing distinguishes loans at or below 90% loan-to-value from loans above 90%, which can face different conditions and pricing; many banks apply similar tiers, so 10% in cash is a common reference point for the down payment — on top of the one-off buying costs of roughly 3–7%. A larger deposit does three jobs at once — it lowers the payment for a given price, it widens the pool of lenders and better rates, and it cushions against price declines, which matter in a market where buildings depreciate. The home affordability calculator works from your payment capacity and cash, then reports the implied loan-to-value and flags it if it passes the 90% threshold — knowing that is knowing which lever to pull.

Joint applications and bonus repayments

Two incomes are assessed together on a joint application, and the combined repayment ratio screens the combined payments. The bonus-repayment split (ボーナス返済) lowers the monthly instalment by routing part of the loan through two larger payments a year — banks count those in the ratio, so they genuinely expand borrowing capacity on paper. The honest question is whether your bonus is as reliable as your salary: a bonus-dependent repayment schedule converts employment risk into default risk. A moderate split, or none, is the conservative setting; the mortgage calculator prices the split's interest cost explicitly.

What improves affordability — in order of effect

Borrowing less outranks everything. Then, roughly in order: clearing other debts before applying (improves the ratio directly); a larger down payment (payment, pricing and approval at once); a shorter term you can actually carry (total cost, not payment); and only then rate shopping, whose one-point moves matter less than any of the above on long terms. One offset worth knowing: the mortgage tax credit (住宅借入金等特別控除) can reduce income tax for owner-occupiers meeting the property and income conditions, in the years after purchase — its value depends on your circumstances, and it belongs with a licensed tax adviser (税理士) rather than a calculator. Read the companion guides on the full cash picture and rate types before you commit a number.