Mortgage guide · worked examples in GBP
Lump-sum mortgage overpayments: payment or term?
A lump sum reduces the debt immediately, but its effect on future payments depends on how the lender applies it. Separate the balance reduction from the choice of payment or term.
Start with the new balance
If you owe £200,000 and pay £10,000 directly toward principal now, the balance falls to £190,000 before any charges. At 5% annual interest, the first full month’s interest in a monthly model falls from £833.33 to £791.67. That £41.67 difference describes one month, not the total lifetime saving.
If the lender recalculates the payment
If the balance is re-amortized over 25 years at the same 5% rate, the payment falls from £1,169.18 to £1,110.72. This example assumes the lender agrees to keep the remaining term unchanged and lower the payment. It does not model a term reduction.
If you keep the payment unchanged
Keeping the previous payment after reducing the principal generally pays off a positive-rate repayment loan sooner. To compare that approach, the model must preserve the old payment and calculate a new payoff date. Simply entering the lower balance into a normal repayment calculator recalculates the payment and answers a different question.
Check charges and timing
Ask your lender when the payment is credited, whether an early repayment charge applies and whether the regular payment or remaining term changes. Overpayment allowances differ by agreement; do not assume a universal percentage. The cash used for an overpayment may no longer be available for other expenses.
Monthly extras are a separate scenario
Our overpayment calculator models an extra amount every month, keeping the scheduled base payment and shortening the payoff time. Do not enter a one-off lump sum as a recurring monthly extra: that would substantially overstate what you plan to pay. Its dollar labels do not convert currency; use it only with consistently denominated amounts.