Mortgage overpayments are one of the highest-return moves available to a UK homeowner. They're also one of the most paperwork-bound. Most fixed-rate mortgages allow up to 10% of the balance per year before triggering an Early Repayment Charge (ERC) — and the ERC, when triggered, is large enough to wipe out the savings the overpayment was meant to generate. Here's how the rules actually work, and how to time overpayments around them.
Most UK fixed-rate mortgages from major lenders allow you to overpay up to 10% of the outstanding balance, per calendar year, without an ERC. Some lenders measure 10% from the original advance; others from the balance at the start of the year. Some reset on 1 January; others on the anniversary of the mortgage start date. Read your offer document — these details vary.
For a £200,000 mortgage, the 10% allowance is £20,000 per year — comfortably more than most overpayments. For a smaller balance, say £80,000, the £8,000 allowance can be tight if you're trying to clear quickly with a windfall.
ERCs are usually quoted as a sliding percentage tied to how much of the fixed deal remains. A typical 5-year fix might charge:
So if you overpaid £30,000 in year 2 of a 5-year fix on a £200,000 mortgage (allowance £20,000, excess £10,000), the ERC would be 4% × £10,000 = £400. On a £100,000 excess overpayment, it'd be £4,000. The maths gets ugly fast.
Between the end of one fixed deal and the start of the next, most lenders allow unlimited overpayment without ERC. This window is usually 30–90 days long.
If you've come into a sizeable lump sum — bonus, inheritance, sale of an investment — and you're 6 months from the end of your fix, parking it in a high-interest account until the fix ends, then dumping it into the mortgage, can save you thousands in ERCs. The lost interest from waiting is almost always less than the ERC saved.
An example. £40,000 lump sum, mortgage at 4.75%, ERC of 3% on excess overpayments above £15,000 allowance. Overpaying immediately: ERC of 3% × £25,000 = £750. Waiting 6 months in a 4.5% easy-access savings account, then overpaying: lost interest on the future overpayment of about £950, but you also earn £900 of interest on the £40k while it's parked. Net cost of waiting: about £50. Net cost of overpaying now: £750. Wait wins by £700.
If your lender's 10% allowance resets on 1 January (it varies — check), an overpayment of £25,000 on a £200,000 mortgage made entirely in October would exceed the £20,000 limit. Splitting it as £15,000 in October and £10,000 in February keeps both inside their respective annual limits. The lost interest from the three-month delay is small compared to the saved ERC.
When you make an overpayment, your lender will ask whether to apply it as "reduce the term" or "reduce the payment." Both shrink the balance and save interest, but in different ways:
Reducing the term saves the most interest but commits you to the same payment. Reducing the payment is more flexible — you can re-overpay later by the freed-up amount and effectively achieve the term-reduction outcome anyway. Many borrowers prefer "reduce the payment" specifically for that flexibility.
If your monthly payment is £1,128 and you pay £1,300, the £172 excess counts as an overpayment toward your 10% limit. Other lenders only treat lump-sum payments separate from the monthly direct debit as overpayments. The treatment matters for tracking.
You can't roll unused allowance into next year. A year where you didn't overpay doesn't earn you 20% allowance the following year.
If you have an offset mortgage, the savings linked to it reduce the interest charged without being treated as overpayments. Your money stays accessible. The trade-off is usually a slightly higher headline rate. For people sitting on lump sums, offset is sometimes the better wrapper than overpayment-with-ERCs.
Want to see how staying within ERC limits affects your savings? Open the mortgage overpayment tool.
No — the contractual monthly payment is the baseline. "Overpayment" specifically means money paid above the contractual payment. Setting your direct debit to £100/month higher creates an overpayment of £100/month.
Most modern UK mortgages are "portable," meaning you can take the deal with you to a new property without triggering ERCs. The new property must meet the lender's criteria. If the new mortgage is larger, the additional borrowing is usually at current rates, not the original deal.
Trackers and SVRs typically have no ERCs at all — overpay freely. The 10% rule mostly applies to fixed-rate deals where the lender wants to recoup the cost of locking in the rate.
Both reduce the interest you pay. Offset keeps the cash accessible (better for emergencies); overpayment is more permanent (better for resisting temptation). The maths is roughly equivalent at the same rate. Offset mortgages tend to charge 0.2–0.5% more, so the choice usually depends on whether you'd benefit from the access.
This article is general information about UK personal finance. It is not regulated financial advice and Pennywise Finance is not authorised by the Financial Conduct Authority. For decisions involving large sums or complex situations, please consult an FCA-authorised adviser.