Extend Mortgage Term Calculator
Extending your mortgage term — spreading the same balance over more years — is one of the most direct ways to reduce a monthly payment when money is tight, and it genuinely works for that specific purpose. What it does not do is make the mortgage cheaper overall; stretching the same debt over a longer period means paying interest for longer, and the total interest bill goes up even though each individual payment goes down. This calculator shows exactly how much your payment would fall, and how much more interest that lower payment actually costs over the full term.
Extend Mortgage Term / Interest Only
See how extending your term or switching to interest-only affects monthly payments and total interest.
Lower Payments Now, More Interest Later — Always
This trade-off is mathematically unavoidable: for the same balance and the same interest rate, a longer term always produces a lower monthly payment and a higher total interest cost, with no exceptions. The relationship is not linear either — stretching a term by a relatively modest number of years can meaningfully reduce the monthly payment while adding a surprisingly large amount to the lifetime interest bill, since interest continues accruing on a slower-shrinking balance for longer. This calculator shows both numbers side by side specifically so the decision is made with the full picture, not just the monthly figure that first catches the eye.
The extra total interest tends to surprise people more than the monthly saving does, precisely because monthly payment changes are immediate and visible on a bank statement, while the extra total interest only becomes apparent when it is calculated over the whole remaining term — which is exactly what this calculator does explicitly, rather than leaving it as an abstract, easy-to-underestimate figure.
Interest-Only Isn’t a Free Lunch Either
Switching to interest-only payments reduces the monthly cost to its absolute minimum — covering only the interest, with none of the payment reducing the actual balance owed. This calculator shows that figure alongside the extended-term option specifically because it is easy to see “lowest monthly payment” and assume it is the better choice without registering what it actually means: the full balance remains outstanding at the end of the term, needing to be repaid in full from savings, investments, or the sale of the property. Interest-only is a genuine tool in the right circumstances, but it defers the capital repayment entirely rather than simply spreading it further, which is a meaningfully different trade-off from a term extension.
Lenders are generally cautious about interest-only mortgages specifically because of this outstanding-balance risk, and typically require a credible, evidenced plan for repaying the capital before agreeing to it — it is not simply a lower-payment option available on request in the way a term extension often is.
Seeing the Three Options Together
A £180,000 balance at 5% with 20 years remaining carries a monthly payment in the region of £1,190. Extending the same balance to 30 years lowers the payment to somewhere around £966 a month — a meaningful monthly saving, but with total interest over the full term rising by tens of thousands of pounds compared with the shorter period. Switching to interest-only on the same balance and rate lowers the monthly payment further still, to around £750 — but leaves the entire £180,000 still owed at the end of the term, a fundamentally different outcome from either repayment option.
This Doesn’t Have to Be Permanent
Many lenders allow a term extension to be reversed later, shortening the term back down once your financial situation improves, sometimes at no cost at the point of a routine remortgage. This makes a term extension a genuinely useful short-to-medium-term tool for getting through a period of tighter finances — a career break, reduced hours, a temporary drop in household income — without it needing to be a permanent, decades-long commitment to paying more interest overall. It is worth checking your specific lender’s flexibility on this before assuming the extended term is locked in for good.
When Extending Genuinely Makes Sense
A term extension makes the most sense as a deliberate, temporary response to a real change in circumstances — not as a default first move whenever finances feel tight, since the interest cost is real and adds up. If the goal is simply to reduce long-term cost rather than free up monthly cash flow, moving in the opposite direction — making overpayments to shorten the term and cut interest — achieves the reverse effect. Our Mortgage Overpayment Calculator shows that side of the equation. If you are reconsidering your mortgage more broadly, including how much you can realistically afford going forward, our Mortgage Affordability Calculator is a useful starting point.
Frequently Asked Questions
Will extending my term require a new affordability assessment?
Often yes — lenders typically reassess affordability for a term change, similar to a standard remortgage application, since it is a material change to the loan agreement.
Is there an age limit on how long I can extend my term?
Yes — most lenders have a maximum age by which the mortgage must be repaid, commonly somewhere around 70 to 80, which can limit how far a term can realistically be extended depending on your current age.
Does extending my term affect my credit score?
Not directly in most cases, since it is a change to an existing agreement rather than a new credit application, though any associated credit check as part of the affordability reassessment could have a small, standard effect.
Can I extend my term and still make overpayments?
Yes — these are not mutually exclusive. Extending the term lowers the required minimum payment, but many mortgages still allow voluntary overpayments on top, which effectively lets you choose flexibility now while still paying down faster when you are able to.
Does switching to interest-only require lender approval?
Yes — lenders generally require evidence of a credible repayment plan for the capital, such as savings, investments, or planned property sale, before agreeing to switch some or all of a mortgage to interest-only.
Is there a fee for extending my mortgage term?
This varies by lender — some process a term change with no fee as a straightforward account adjustment, while others may charge an administration fee, so it is worth checking with your specific lender before assuming either way.
Important Information
This calculator provides an estimate based on the figures you enter and does not constitute financial advice. It does not confirm whether your specific lender would approve a term extension or interest-only switch. For advice specific to your circumstances, consult a mortgage broker or FCA-regulated adviser. See our Disclaimer for further information.