Mortgage interest is the largest single cost most households ever pay, and it is compounding — which means small changes in the rate have outsized consequences over decades. A 1% difference sounds trivial on a monthly basis. Over 25 years, it is anything but.
This guide shows, with worked numbers, what a 1% rate change actually costs, and why the term you choose matters just as much as the rate.
The headline numbers on a £200,000 mortgage
Take a £200,000 repayment mortgage over 25 years. Here is what different rates produce:
| Rate | Monthly payment | Total interest over 25 years |
|---|---|---|
| 4.0% | £1,056 | £116,702 |
| 4.5% | £1,112 | £133,536 |
| 5.0% | £1,170 | £150,932 |
| 5.5% | £1,228 | £168,443 |
The jump from 4% to 5% costs about £34,000 in extra interest — on the same amount borrowed, over the same period. That is more than a full year of the mortgage payments themselves, lost purely to the rate.
Why 1% is not really 1%
The reason a 1% change has such a large effect is that the interest is charged on the outstanding balance every month, and the balance stays high for years. In the early years of a repayment mortgage, almost all of your payment is interest, so a higher rate is applied to a large balance repeatedly.
It also works in reverse in a way that surprises people: a small rate cut saves relatively little in the first month but a great deal over the full term. The compounding works in your favour when rates fall, just as it works against you when they rise.
The term matters as much as the rate
Lenders often present a longer term as a way to lower the monthly payment. It does — but at a substantial cost.
Compare a £200,000 loan at 4.5%:
| Term | Monthly payment | Total interest |
|---|---|---|
| 20 years | £1,265 | £103,600 |
| 25 years | £1,112 | £133,536 |
| 30 years | £1,013 | £164,812 |
| 35 years | £947 | £197,700 |
Stretching from 25 years to 35 years lowers the monthly payment by £165 but adds roughly £64,000 in total interest. If you can afford the higher payment, a shorter term is almost always the better financial decision.
How much can you borrow?
Affordability is assessed against your income, and most lenders cap borrowing at around 4 to 4.5 times your annual salary, then stress-test your budget against a higher hypothetical rate. A £40,000 salary might support borrowing of £160,000 to £180,000, depending on outgoings and existing debt.
Self-employed applicants, those with variable income, and anyone carrying significant unsecured debt will typically be offered less. Because lenders also model a rate rise of several percentage points when testing affordability, a higher income does not automatically unlock proportionally higher borrowing.
Fixed vs variable: the real trade-off
- Fixed rate — your payment is guaranteed for the fixed period, usually two to five years. You pay a premium for that certainty, and early repayment charges usually apply.
- Tracker — follows the Bank of England base rate, moving up and down with it. Often cheaper to arrange, but your payment can rise sharply and unexpectedly.
- Standard variable rate — the lender’s default rate, usually the most expensive. It is what you fall onto when a deal ends, and it is the main reason remortgaging matters.
The “right” answer depends on your tolerance for uncertainty and how tight your budget is. A fixed rate is insurance: you pay for certainty, and it is worth it if a rate rise would genuinely strain your finances.
What this means for your budget
Before committing, it is worth understanding your take-home pay precisely, because that is what the mortgage payment comes out of. A £40,000 salary takes home around £2,693 a month; a mortgage payment of £1,170 at 5% is 43% of that, which is at the upper end of what most lenders and advisers consider comfortable.
Our mortgage calculator lets you model repayments at any rate and term, and see the total cost. Combining it with your take-home pay gives the full picture before you borrow. If you are also buying property, the stamp duty calculator shows the one-off tax you will owe.
Key takeaways
- On a £200,000 loan over 25 years, 4% vs 5% costs about £34,000 more in total interest.
- A longer term lowers the monthly payment but adds tens of thousands in interest.
- Most lenders cap borrowing at around 4 to 4.5 times income.
- Fixing is insurance: you pay for certainty, and it is worth it if your budget is tight.
- Always compare the total cost, not just the monthly payment.