The tempting 49.5% calculation is wrong
Jeroen earns €100,000 of 2026 Box 1 salary before home-related entries, is below AOW age, and pays €10,000 qualifying mortgage interest on his main residence. He sees the top income-tax rate of 49.5% and estimates €4,950 of mortgage-interest tax relief. The 2026 Dutch Tax Administration rule limits the top-band deduction rate to 37.56%, however. Under a deliberately simple top-band illustration, the interest relief is €3,756, or €1,194 less than his first guess. The top-bracket adjustment applies to home-related deductible costs as well as interest.
His house also produces an eigenwoningforfait, an imputed rental amount added to Box 1. The deduction and the addition belong in the same calculation, but they are different lines. A lender’s “after-tax monthly payment” that only multiplies interest by 49.5% leaves out both the deduction cap and the home-value addition.
Check whether the loan is eligible first
The Dutch Tax Administration’s loan guide says a first loan made on or after 1 January 2013 generally needs two things for mortgage-interest relief: borrowing for the owner-occupied home or qualifying improvement, and repayment at least on a linear or annuity schedule within 30 years. The ordinary interest deduction period is at most 360 months. Interest on an amount borrowed for spending unrelated to the home is not converted into deductible home debt merely because the house secures it.
Pre-2013 loans can retain transitional conditions, including existing interest-only arrangements. If an owner refinances without increasing the principal, the tax authority’s refinancing guide says that pre-2013 deductibility generally continues under its original 30-year period. That is not a fresh 30-year clock. If the owner borrows an additional amount, the extra slice must be tested under the later loan rules: use for the home and repayment within 30 years. Split the annual statement into old and new slices before estimating relief.
The government eligibility summary likewise identifies annuity and linear new mortgages that are fully repaid in 30 years. These rules say whether interest may enter the deduction at all. The 37.56% cap only controls the value of deductible costs for a taxpayer in the top band; it cannot rescue interest on an ineligible loan.
Separate the rate cap from the Box 1 rate
For people who have not reached AOW age, the 2026 Box 1 bands are 35.75% up to €38,883, 37.56% from €38,883 to €78,426, and 49.5% above €78,426. The mortgage-interest deduction is subject to a 11.94-percentage-point adjustment in the highest band: 49.5% minus 11.94% equals 37.56%. Jeroen’s high salary means our €10,000 interest illustration stays within that adjustment’s relevant income range.
Top Box 1 rate versus mortgage deduction value in 2026
If Jeroen’s full eligible interest is €10,000, the isolated interest-deduction effect at 37.56% is €3,756. At 49.5% it would be €4,950; the €1,194 difference is the overstatement. Do not apply 37.56% mechanically to every borrower: taxpayers below the top band, AOW-age taxpayers, fiscal partners and deductions crossing a band boundary can have different actual tax effects. General tax credits can also alter the final cash difference. Belastingdienst calculates the tariff adjustment automatically on the return.
Add the 2026 eigenwoningforfait
The official 2026 eigenwoningforfait table uses 0.35% for a home with a WOZ value above €75,000 and no more than €1,350,000. Assume Jeroen’s home has a €500,000 WOZ value and has been his main residence for the entire year. His Box 1 addition is €500,000 × 0.35% = €1,750. The simple net home entry before other allowable costs is €1,750 − €10,000 = −€8,250.
| Component in Jeroen’s assumed 2026 case | Amount or rate | Simple tax effect |
|---|---|---|
| Eligible interest paid | €10,000 | About €3,756 relief at 37.56% |
| Eigenwoningforfait on €500,000 WOZ | €1,750 | About €866.25 addition at assumed 49.5% |
| Combined comparison with neither home entry | — | About €2,889.75 lower Box 1 tax |
Jeroen’s simplified home-related Box 1 effects
The €2,889.75 is not a mortgage subsidy and is not guaranteed cash back. It is a simplified difference for this assumed salary, WOZ value and eligibility, holding everything else fixed and ignoring effects through tax credits or other deductions. Principal repayments are not an income-tax deduction. The annual mortgage cash cost also includes principal and possibly insurance or maintenance, so subtracting €2,889.75 from the interest bill does not give a complete housing budget.
The 2026 forfait rate changes by WOZ band, especially for high-value homes. Get the actual WOZ notice and the property’s main-residence dates. Do not reuse someone else’s 0.35% example if your value or ownership period differs. If deductible interest is small relative to the forfait, the declining Hillen relief may also affect the final addition; the 2026 relief is only 71.867% of the qualifying positive difference, subject to conditions.
Which costs belong in the estimate?
The Tax Administration’s cost list includes, where relevant, mortgage-advice and mediation fees, notary charges for the mortgage deed, mortgage valuation costs, NHG application costs and some early-repayment compensation. It expressly excludes the house purchase deed’s notary cost, property-transfer tax and mortgage principal. Paid home improvements themselves are not an immediate mortgage-interest deduction, even when interest on a properly qualifying improvement loan may be.
Keep dated invoices. A refinancing fee can be deductible only to the extent it relates to the eligible owner-occupied-home debt; the specific refinancing-cost guide says the mixed-use share has to be separated. An advertised all-in refinancing saving should account for the fee you actually pay, any deductible portion and the remaining loan term, not just the lower nominal interest rate.
If a rate reset or refinancing lowers Jeroen’s interest from €10,000 to €8,000, his gross interest cost falls by €2,000 while the isolated 37.56% deduction also falls by €751.20, assuming the same top-band conditions. His approximate after-tax interest saving is therefore €1,248.80, before fees or changes in the forfait. Less tax relief is not a reason to keep paying avoidable interest. Update a voorlopige aanslag if the new interest materially changes the estimate, as the tax authority’s refinancing guidance advises.
The practical order is: verify each loan slice and its original date, identify interest and qualifying costs actually paid in 2026, find the correct WOZ value and forfait, then apply the 2026 rate restriction to the relevant high-income deduction. The tax return performs the final calculation; any single advertised percentage is only a partial input.
