UK Mortgage Overpayment Calculator

About UK Mortgage Overpayment Calculator

How Mortgage Overpayments Save Money

Overpayments go directly to reducing the principal balance. Since interest is calculated on the outstanding balance each month, a lower balance means less interest charged, which means more of your next regular payment goes to principal. This creates a compounding saving effect.

Worked example: £200,000 mortgage at 4.5% over 25 years (monthly payment: £1,112):

Overpayment	Total Interest	Interest Saved	Term Reduction	New Term
None	£133,539	-	-	25 years
£100/month	£107,382	£26,157	4 years 2 months	20 years 10 months
£200/month	£87,598	£45,941	7 years 2 months	17 years 10 months
£300/month	£72,427	£61,112	9 years 6 months	15 years 6 months
£500/month	£51,277	£82,262	12 years 8 months	12 years 4 months

£100/month extra saves over £26,000 in interest and takes over 4 years off the mortgage. That is £1,200/year in overpayments saving £26,000 total - a return of over 20x.

Monthly Overpayment vs Lump Sum

Both reduce your balance and save interest, but they work differently:

	Monthly Overpayment	Lump Sum
How it works	Extra amount added to each monthly payment	One-off additional payment
Cash flow impact	Ongoing commitment, reduces disposable income	One-time hit, no ongoing commitment
When to use	Steady extra income, pay rises	Bonus, inheritance, savings reached
Flexibility	Can stop any time if finances change	Done in one go

Lump sum example: A £10,000 lump sum on the same £200,000 mortgage at 4.5% over 25 years (applied at year 5) saves roughly £16,000 in interest and reduces the term by about 18 months.

The 10% Overpayment Rule

Most UK fixed-rate mortgages allow overpayments of up to 10% of the outstanding balance per year without incurring an Early Repayment Charge (ERC). On a £200,000 mortgage, that is up to £20,000 per year (£1,667/month).

ERCs typically range from 1-5% of the overpaid amount, decreasing each year of the fixed term. Always check your specific mortgage terms. Some lenders:

• Allow unlimited overpayments (common on tracker and variable rate mortgages)
• Calculate the 10% allowance from the start of the mortgage year, not the calendar year
• Carry forward unused allowance (rare, but some do)

Should You Overpay or Invest?

This is one of the most debated personal finance questions. The basic rule:

• Overpay if: Your mortgage rate is higher than the after-tax return you could earn elsewhere. Overpaying at 5% mortgage rate is a guaranteed 5% return (risk-free).
• Invest if: You expect investment returns to exceed your mortgage rate after tax. Stock market average returns are ~7%, but they are not guaranteed and come with volatility.
• Always first: Build a 3-6 month emergency fund before aggressively overpaying. Being mortgage-free is no help if you need to borrow at higher rates for an emergency.

A common balanced approach: overpay modestly (£100-200/month) while also investing in a stocks and shares ISA. This hedges both directions.

Offset Mortgages

An alternative to overpayment: offset mortgages link your savings account to your mortgage. Your savings balance is "offset" against the mortgage, so you only pay interest on the difference. £20,000 in savings against a £200,000 mortgage means you pay interest on £180,000. The savings remain accessible, unlike overpayments which are usually non-refundable. First Direct, Barclays Offset, and Scottish Widows Bank run the better-known offset products in the UK market as of April 2026. Offset rates typically sit 0.2-0.5 percentage points above a comparable straight repayment deal, so the break-even depends on how much you keep in savings.

UK Mortgage Market Context in June 2026

The Bank of England base rate sits at 3.75% heading into the 18 June 2026 MPC meeting, having been held at that level since the 30 April vote (8-1 in favour of holding), down from the 5.25% peak in 2023-2024. Average fixed-rate mortgage pricing has not fallen in lockstep because of sticky swap rates - per Moneyfacts on 8 June 2026 the average 2-year fix was 5.64% and the average 5-year fix 5.60%. The average Standard Variable Rate remains around 7.15%, which is why overpayments or a remortgage make the biggest difference for borrowers rolling off a fix onto SVR.

The scale of the UK mortgage book is enormous: Bank of England Mortgage Lenders and Administrators data shows outstanding residential mortgage balances reached £1,746.1 billion in Q1 2026, up 2.6% year on year. Despite higher rates, arrears are easing - UK Finance reported homeowner mortgages in arrears fell 2% quarter on quarter to 0.91% of the homeowner stock, with the value of balances in arrears down to £20.1 billion (the lowest since Q3 2023).

The practical takeaway: the gap between a typical fixed rate (~5.6%) and easy-access savings (around 4.3-4.5% AER from top UK banks as of June 2026) has widened slightly versus the spring, so the maths for overpaying has improved at the margin. Overpaying is still a guaranteed return at the mortgage rate, and the spread over cash savings is now around a full percentage point again for most borrowers.

How Lenders Actually Apply Overpayments

Not every lender applies overpayments the same way. The two common approaches are "reduce the term" (your monthly payment stays the same and the mortgage ends sooner) and "reduce the payment" (the term stays the same and the next monthly direct debit falls). The default on most UK mortgages is to reduce the term, which gives the biggest interest saving - but you typically have to call or write to the lender to confirm, and some lenders default to reducing the payment instead.

Here is what to check with your specific lender:

• How is interest recalculated? Daily (most UK lenders now), monthly, or annually. Daily recalculation means your overpayment starts saving interest the day it lands, which matters most for lump sums.
• Is there a minimum overpayment? Some lenders require £50 or £500 minimum per transaction.
• Do unused 10% allowances carry forward? Usually not - the allowance resets each year.
• Does the 10% cap reset on the calendar year or your mortgage anniversary? Most lenders use the mortgage anniversary (the date your deal started).

What Happens to the Interest Saving Over Time?

The earlier you overpay, the bigger the saving, because interest compounds across the remaining term. A £5,000 lump sum in year 1 of a 25-year mortgage at 4.5% saves roughly £8,700 in interest. The same £5,000 in year 20 only saves about £1,100 in interest. This is why financial writers like Martin Lewis emphasise overpaying early in the term, when the outstanding balance (and therefore monthly interest cost) is highest.

Year of £5,000 Lump Sum	Interest Saved	Months Knocked Off
Year 1	~£8,700	~11
Year 5	~£6,500	~10
Year 10	~£4,300	~9
Year 15	~£2,600	~8
Year 20	~£1,100	~5

Figures assume a £200,000 mortgage at 4.5% over 25 years with the lump sum applied at the start of that year.

Common Mistakes to Avoid

• Draining your emergency fund. Mortgage overpayments are usually non-refundable (unless you have a flexible or offset mortgage). If you lose your income three months later, you cannot get that money back without remortgaging.
• Overpaying while carrying expensive debt. Credit card APRs of 20-30% dwarf a 4-5% mortgage rate. Clear cards, overdrafts, and personal loans first.
• Ignoring the pension tax relief trade-off. A higher-rate taxpayer adding £1,000 to their pension costs only £600 net. Overpaying the mortgage costs £1,000 of net income. Over long horizons pension contributions often beat overpayments, especially with employer matching. HMRC figures show pension relief cost the Treasury £48.7 billion in 2022-23 - that is the scale of the tax break available.
• Missing the 10% cap. Breaching the annual allowance on a fixed deal triggers an ERC of typically 1-5% on the excess. Always check your annual statement for the current year's allowance before sending a big payment.
• Forgetting to ask for "reduce the term". Lenders that default to reducing the payment cut the headline monthly cost but reduce the interest saving dramatically.

How This Compares to Other Mortgage Decisions

Overpaying is only one lever borrowers can pull. The others are remortgaging to a better rate, switching to a shorter term at remortgage, and extending the term to free up cash flow. To compare the starting position, use the mortgage calculator for a standard repayment schedule, the loan calculator to model an offset-equivalent balance, or the compound interest calculator to model what you would earn investing the same money in a stocks and shares ISA instead. For a fuller picture of home-buying costs, the stamp duty calculator covers SDLT in England and Northern Ireland.

Frequently Misunderstood Details

Does overpaying lower my monthly payment or my term? By default on almost every UK mortgage, the term. The monthly direct debit stays the same until you specifically ask the lender to recalculate it downward.

Can I overpay on an interest-only mortgage? Yes, and it is the only way to reduce the capital balance before the end of the term. Most interest-only products still cap annual overpayments at 10% unless you switch to repayment.

Do I get tax relief on mortgage overpayments? No. UK mortgage interest relief was phased out for residential properties years ago. Landlords get a basic-rate tax credit on interest via the Section 24 regime, not relief on overpayments.

Can I get my overpayments back? Only on a flexible mortgage (sometimes called a "drawdown" or "reserve" facility). Standard repayment mortgages treat overpayments as gone for good.

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Sources

• Bank of England - Bank Rate and monetary policy
• Bank of England - Mortgage Lenders and Administrators Statistics, Q1 2026
• UK Finance - Mortgage arrears and possessions, Q1 2026
• HMRC - Personal pension tax relief statistics
• Moneyfacts - Early Repayment Charges explained
• HomeOwners Alliance - Best UK mortgage rates
• Halifax - Early repayment charges
• FCA - Mortgages consumer guidance