Property investment
You know your current mortgage payment. The harder question comes when the fixed rate ends: you still owe money, but the rate on the next loan is unknown. I would test several rates now and check how much cash remains for repairs and bad months.
The risk
A fixed borrowing rate protects you only until the agreed period ends. If the mortgage is still outstanding, the remaining balance needs new terms. A higher rate can push up the payment. Keeping the payment nearly unchanged can slow repayment instead.
Start with the expected balance when the fixed period ends. Take it from your repayment schedule and adjust it only for extra payments you can realistically make. A hoped-for pay rise or future sale is too uncertain to use as the base case.
If you are still planning the purchase, read how purchase costs and the mortgage fit together. This stress test starts with the balance at refinancing, not the original property price.
The inputs
Write down the remaining balance, several possible new borrowing rates, an initial repayment rate for the new loan and the amount you can reliably set aside each month for the mortgage and reserves. Your current payment alone cannot tell you whether the next deal is affordable.
Run a favourable, a middle and a difficult rate scenario. These are tests, not forecasts. The useful result is the rate at which your household or rental property no longer has enough room for setbacks.
Worked example
Suppose €220,000 remains after ten years. In both refinancing scenarios, initial repayment is 2% of that balance a year. At 2.5% interest, the starting payment covers 4.5% in interest and principal annually. At 5% interest, it covers 7%.
The extra €458 is the point of the exercise. Can you pay it without using the money reserved for maintenance and unexpected costs? If it works only while income stays perfect and nothing breaks, the budget is too tight.
Same payment
At 5% interest, the interest alone on €220,000 starts at about €917 a month. The old €825 payment does not even cover that. With the same balance and rate, an unchanged payment would not reduce the loan. The initial shortfall is about €92 a month.
You may be able to agree a lower initial repayment rate if the lender accepts it and the payment covers interest. That eases the monthly burden but lengthens repayment and leaves more debt for later. A smaller payment has a cost.
Also reverse the question: how far would the balance have to fall to keep the old payment? Extra repayments help only if the contract allows them and they do not consume the cash reserve you need.
Affordability
Assume you have €1,500 a month left after normal living costs for the mortgage and reserves. You want to keep €300 of that for repairs and unexpected bills. Your payment ceiling is €1,200. At 5% interest and 2% initial repayment, the roughly €1,283 payment exceeds it by about €83.
Reducing initial repayment to 1.5% brings the payment to about €1,192. The monthly budget just about works, but the debt falls more slowly. Whether that is acceptable depends in part on your age, future income and how long you have left to repay. One positive monthly balance is not enough evidence.
For a rental property, also test vacancies, costs you cannot pass on to tenants and maintenance. Rent that just covers today’s payment is no protection against a later rate rise. The 30-year property projection shows why cash flow and remaining debt need to be read together.
Preparation
Update your repayment schedule each year with the actual outstanding balance. Keep three refinancing rates and the monthly cash buffer beside it. You will see early if you need faster repayment, a less expensive property or a different financing plan.
As the end date approaches, compare your current lender’s offer with other lenders. A German forward loan can fix future terms in advance. It gives certainty, but you remain bound to the agreed terms even if rates later fall.
The stress test cannot tell you where rates will be in ten years. It tells you which rates and setbacks your financing can still absorb. That boundary is more useful for a decision than one optimistic payment figure.
The longer property projection connects debt, cash flow and reserves.
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