How the Housing Loan Tax Deduction Works and Its Impact on Investment Decisions
The housing loan tax deduction (jutaku kariirekin-tou tokubetsu koujo) credits 0.7% of the year-end loan balance against income tax for up to 13 years. With a loan balance of 30 million yen, you receive a 210,000 yen annual tax credit. This system has a major impact on investment decisions because during the deduction period, the "effective borrowing rate" drops well below the nominal rate. For example, with a nominal rate of 0.5% and a 0.7% deduction, the effective rate becomes negative 0.2% - meaning you actually profit just by having the loan.
In this situation, rushing to prepay is not optimal from a purely economic standpoint. Rather than prepaying when the effective rate is negative or near zero, directing those funds into investments offers a higher expected return. However, this is an "expected value" argument, and investing carries risk. Mortgage prepayment delivers a guaranteed return (reduced interest expense), whereas investment returns are uncertain. How you weigh this certainty gap is where individual judgment diverges.
Simulating the Optimal Fund Allocation During the Deduction Period
Let's work through specific numbers. Assume a mortgage balance of 30 million yen (0.5% interest, 30 years remaining), 10 years left on the housing loan deduction, and 1 million yen in surplus funds. Directing it to prepayment saves approximately 150,000 yen in interest over 30 years. Investing the same 1 million yen at 5% annual return for 30 years yields an expected return of about 3.32 million yen. Even limited to the 10-year deduction period, the expected investment return (about 630,000 yen) far exceeds the guaranteed prepayment return (about 50,000 yen).
However, this calculation rests on important assumptions. Books on optimizing mortgages and asset management point out that the 5% expected investment return is a long-term average, and there will be years with negative returns. For variable-rate mortgages, the risk of future rate increases must also be considered. If rates exceed 2%, the guaranteed return from prepayment approaches the expected investment return, shifting the decision calculus.
Strategic Pivot Points After the Deduction Period Ends
When the housing loan deduction period expires, the effective rate reverts to the nominal rate. At this point, you should reassess the priority between prepayment and investing. If the nominal rate remains below 1%, investing still takes priority rationally, given the large gap with the long-term expected equity return of 5-7%. However, if rates exceed 2%, you need to carefully weigh the trade-off between the "guaranteed 2% return" of prepayment and the "uncertain 5-7% return" of investing.
The most rational approach is a "hybrid strategy" that prioritizes investing during the deduction period and redirects a portion to prepayment after the deduction ends. Books on mortgage repayment and investment strategy detail methods such as using assets accumulated in NISA during the deduction period as prepayment funds after the deduction ends, and optimal repayment-to-investment ratios under various interest rate scenarios. The key is not to lock in a single approach, but to flexibly revisit your strategy as interest rate conditions and household finances change.
Next Actions for Finding the Optimal Balance Between Mortgage and Investing
Start by organizing the current state of your mortgage. Write down your outstanding balance, applicable interest rate (fixed or variable), remaining years of the housing loan deduction, and annual deduction amount. Then calculate your effective rate (nominal rate minus deduction rate) and check whether it is positive or negative. If the effective rate is negative or near zero, there is a rational basis for prioritizing investing over prepayment. Use our simulator to compare the 20-year asset difference between prepaying and investing.
Next, decide your post-deduction strategy now. Set decision criteria in advance for whether to switch to prepayment or continue investing based on the interest rate level when the deduction expires. For example, a rule like "if rates exceed 1.5%, redirect 50% of surplus funds to prepayment." Since interest rate conditions change, make it a habit to review your mortgage terms and investment returns once a year and recalculate the optimal fund allocation - this is what leads to long-term asset maximization.