Lump Sum UK 2026: Student Loan vs Mortgage vs Investing — Full Analysis With Calculator

You've come into some money. Maybe an inheritance, a trust maturing, a redundancy payout, or a chunky bonus. And now you're staring at three options that every UK personal finance forum argues about endlessly:

1. Pay off your student loan
2. Put it towards a house deposit
3. Invest it

All three have merit. None of them are obviously wrong. And the "right" answer depends entirely on your numbers. So let's work through the logic — and the question most people forget to ask.

The Question Nobody Asks First

Before comparing anything, you need to answer this: will you even pay off your student loan?

This sounds odd. You owe the money, so surely you'll pay it back? Not necessarily. Plan 2 student loans get written off 30 years after graduation. If you graduated with a large balance — say £50,000 or more — and you're earning a decent but not spectacular salary, there's a real chance you'll never clear the full amount before it disappears.

Here's the maths that surprises people. Say you have a £65,000 balance, earn £69,000, and graduated 6 years ago. Your annual repayment is 9% of everything above the repayment threshold (around £28,470 for 2026/27) — about £3,648 per year. But interest on a £65,000 balance can easily exceed that — the rate is RPI + up to 3% for the highest earners, and it changes annually (it's been as high as 7.3%). Your balance is actually growing, not shrinking.

Over the remaining 24 years, you might repay over £100,000 in total repayments — and still have a balance left that gets written off. Or you might clear it with a few years to spare if your salary grows fast enough.

The point is: if the loan would be partially written off anyway, paying it off early means spending more than you'd ever have repaid. That's money you could have used for a deposit or invested.

This is why you need to run the numbers for your specific situation. The UKPF wiki on student loans is a good starting point, and our lump sum calculator models this year by year.

Option 1: Pay Off the Student Loan

The case for it: You save 9% of your income above the threshold every month. That's immediate, guaranteed cash flow. No more deductions from your payslip. Psychologically satisfying.

The case against it: Student loan repayments aren't like normal debt repayments. They're income-contingent — if your income drops, your repayments drop automatically. Lose your job? You pay nothing. Go part-time to raise kids? You pay less. The loan is essentially self-insuring.

And here's the kicker: overpayments are irreversible. Once you pay money towards your student loan, you can't borrow it back on the same terms. That £65,000 you used to clear the loan? Gone. If you need it six months later for a deposit or an emergency, tough luck.

When it makes sense: When you're on track to repay the full balance well before write-off — meaning every pound you pay off saves you interest. If you'd repay it all in 15 years anyway, clearing it now saves you years of interest. The higher your salary and the lower your balance, the more likely this is.

When it doesn't: When your loan would be partially written off. In that scenario, you're effectively paying money you'd never have had to pay.

Option 2: Boost Your Mortgage Deposit

The case for it: A bigger deposit means a lower loan-to-value (LTV) ratio. Lenders price mortgages by LTV bands — drop below a key threshold and your rate improves. The standard thresholds are 95%, 90%, 80%, 75%, and 60%.

Say you're buying a £400,000 property. A £120,000 deposit gives you 70% LTV. Add £65,000 from your lump sum and you're at 54% LTV — below the 60% threshold, which typically gets you the best rates available.

The difference between a 4.8% rate at 80% LTV and a 3.8% rate at 54% LTV on a £280,000 mortgage over 25 years? Around £45,000 in total interest. That's real money.

The case against it: The money is illiquid. Once it's in your house, you can't easily get it back without remortgaging. And the benefit is only significant if your lump sum actually pushes you across a threshold. Going from 72% to 68% LTV probably doesn't change your rate at all.

When it makes sense: When the lump sum crosses a meaningful LTV threshold — especially 90% to 80%, or 80% to 75%. The interest saving is guaranteed, tax-free, and immediate. The UKPF mortgage overpayments page has a thorough comparison.

When it doesn't: When you're already well within a band and the lump sum doesn't get you to the next threshold.

Option 3: Invest It

The case for it: Historically, global equities have returned around 7% per year nominally (roughly 4-5% after inflation). Over a 20+ year horizon, a £65,000 lump sum invested could grow substantially — potentially more than the interest saved on either of the other options.

The first £20,000 each year goes into an ISA, sheltered from tax entirely. Even in a General Investment Account, you have a £3,000 annual capital gains tax allowance (2025/26).

The case against it: Returns are not guaranteed. Markets crash. You might panic-sell at the worst time. And unlike the guaranteed savings from a better mortgage rate or clearing a student loan, investment returns involve real risk.

When it makes sense: When you have a long time horizon (10+ years), you're comfortable with volatility, and the other two options don't offer compelling guaranteed returns. Also worth considering if you haven't used your ISA allowance — it's use-it-or-lose-it each year.

When it doesn't: When you need the money within 5 years, or when a mortgage deposit boost would save you more in guaranteed interest than investments might reasonably return.

You Don't Have to Pick Just One

Here's something the internet arguments usually miss: you can split the money. Put £30,000 towards the deposit to cross an LTV threshold, invest £35,000 in an ISA. Or clear the student loan and invest the remainder.

The optimal split depends on your marginal returns. Sometimes £20,000 in the deposit crosses a threshold and the next £20,000 doesn't help at all — in which case, invest the rest.

The Trade-Offs That Don't Show Up in a Spreadsheet

Numbers matter, but so do these:

Flexibility. Investments are liquid — you can access them anytime. Student loan overpayments are gone forever. House equity is locked until you sell or remortgage.

Peace of mind. Some people sleep better with no student loan. Others sleep better with a fat ISA. Neither is irrational.

Life changes. If there's any chance you might need this money in the next few years — career change, starting a business, time off for family — keeping it liquid has genuine value. Student loans adapt to your circumstances automatically. Your ISA doesn't care if you lose your job. But your mortgage payment stays the same.

Run Your Own Numbers

The general principles are useful, but the actual answer depends on your salary, your loan balance, your property price, your risk tolerance, and your time horizon. Two people with seemingly similar situations can get completely different answers.

We built a free lump sum calculator that models all three options side by side with real UK numbers — student loan write-off, LTV mortgage bands, ISA and GIA tax treatment. It takes about two minutes and you'll see exactly where you stand.

Because "it depends" is true but useless. Your numbers aren't.