Overpaying a UK mortgage can save a substantial amount of interest — but unlike the US, most UK fixed-rate deals cap how much you can overpay penalty-free. Here's exactly how the common "10% rule" works, what an Early Repayment Charge actually costs, and how to plan overpayments around it.
The 10% annual overpayment allowance
Most UK fixed-rate and tracker mortgage deals let you overpay up to 10% of your outstanding mortgage balance each year without any penalty. This allowance:
- Usually resets each year on your mortgage anniversary date (not the calendar year) — check your specific offer document.
- Is typically calculated on the balance at the start of that year, not the original loan amount.
- Applies during your fixed or tracker deal period — once you roll onto your lender's Standard Variable Rate (SVR), overpayment limits are usually removed entirely.
Exact allowances vary by lender and product — some offer more than 10%, a few offer less, and some structure it as a fixed annual cash amount instead of a percentage. Always check your mortgage offer document or ask your lender directly.
What happens if you overpay beyond the allowance?
Exceeding your allowance during a fixed or tracker deal typically triggers an Early Repayment Charge (ERC), usually calculated as a percentage of the amount that exceeds your allowance — commonly in the range of 1% to 5%, often tiered to be higher earlier in the deal and lower as you approach the end of the fixed period. On a large excess overpayment, an ERC can easily outweigh the interest you'd save, so it's worth checking your specific ERC schedule before making a large overpayment.
How UK mortgage interest is calculated
UK residential mortgages generally accrue interest daily on the outstanding balance, with that daily interest totalled and applied to your account monthly. For standard payment-calculation purposes — working out your fixed monthly instalment — lenders and calculators, including this one, use monthly compounding of the nominal annual rate (rate ÷ 12), which closely approximates the daily-accrual outcome for typical UK mortgage products.
Worked example
A £250,000 repayment mortgage at 4.5% over 25 years has a monthly payment of about £1,389.58. Overpaying by £300/month — comfortably within a typical 10% annual allowance on most balances — is projected to save roughly £51,500 in interest and pay the mortgage off around 6-7 years early. Try your own figures in our extra payment calculator (select "United Kingdom" for GBP and the correct assumptions).
Staying within your allowance: a practical approach
- Check your mortgage offer document (or ask your lender) for your exact overpayment percentage and how it's calculated.
- Convert that percentage to a cash figure based on your current balance, then divide by 12 for a safe monthly overpayment amount, or plan a single annual lump sum within the limit.
- If you have more to overpay than your allowance permits, consider spreading the excess into next year's allowance, or ask your lender whether a further advance or product transfer offers more flexibility.
- Once you move to your lender's SVR (typically at the end of a fixed deal), overpayment limits usually disappear — a good time to overpay more aggressively if you're planning to remortgage anyway.
Key takeaways
- Most UK fixed/tracker deals cap penalty-free overpayments at around 10% of the balance per year.
- Exceeding the allowance can trigger an Early Repayment Charge of roughly 1-5% of the excess.
- Limits typically disappear once you're on your lender's Standard Variable Rate.
- Always confirm your specific allowance and ERC schedule with your lender before a large overpayment.