The 25-Year Mortgage Is a 1990s Relic — At 5.56%, a 35-Year Term and £185 a Month in Your ISA Makes You £232,000 Richer

At 5.56%, a UK mortgage is expensive by post-2008 standards. But it's still the cheapest debt most people will ever hold. Credit cards charge 35%. Personal loans: 8-15%. Car finance: 9%+.

Mortgage debt is secured against an appreciating asset. UK house prices rose 1.3% in the year to January 2026 according to the ONS house price index. Over the last 30 years, they've averaged roughly 5% annually. Your lender is charging you 5.56% to borrow against something that, historically, grows in value.

Every pound you throw at your mortgage above the minimum payment is a pound earning an implicit 5.56% return — guaranteed, yes, but also capped. It will never earn you 7%, 10%, or 15%. It will earn you exactly your mortgage rate, nothing more.

The FTSE 100, including dividends reinvested, has returned roughly 8-10% annualised over the last 30 years. Even stripping out the best years, a diversified global portfolio has comfortably beaten 5.56% over any 25-year period in modern history. The question isn't whether equities beat mortgage overpayment — it's whether you have the patience to let them.

Here's what happens if you take the 35-year term and invest the £185 monthly saving into an ISA instead:

After 10 years: £31,600. After 20: £97,000. After 35 years: £333,000 — all tax-free inside a stocks and shares ISA.

The extra mortgage interest over 35 years? £101,100. So your net gain from choosing the longer term is £232,000.

And this uses 7% — a conservative estimate. The FTSE 100 has averaged closer to 8-9% with dividends reinvested over the long run. At 8%, the portfolio hits £400,000. Your 'extra interest cost' looks like loose change.

Even at a pessimistic 5% return, the ISA grows to roughly £210,000 — still comfortably ahead of the £101,100 extra interest. You'd need returns below about 3.5% annually for the shorter term to win. At that level of persistent underperformance, the entire UK economy would be in serious trouble.

The people who argue for the 25-year term are implicitly saying they'd rather save £101,000 guaranteed than have a strong probability of £333,000. That's a valid risk preference. But calling it 'smart' is a stretch.

The average UK house costs £268,000. The average UK salary is roughly £35,000. After income tax at 20% on the basic rate band and 8% NI, that's about £2,400 a month take-home.

£1,490 for a 25-year mortgage is 62% of that take-home pay. £1,305 for a 35-year term is 54%. That 8-percentage-point difference isn't trivial — it's the difference between financial breathing room and living on a knife edge.

At 62%, you have almost nothing left for a pension, emergency savings, or the kind of investing that actually builds long-term wealth. You can't contribute to a workplace pension beyond the bare minimum. You can't build a six-month emergency fund. You certainly can't take advantage of your £20,000 ISA allowance.

The 25-year mortgage zealots are so focused on minimising one liability that they ignore the opportunity cost of starving every other financial goal. A household with a 25-year mortgage and no ISA, no pension top-up, and no emergency fund is not financially healthy — it's fragile.

With mortgage rates at 5.56% and the Bank of England hinting at potential rate rises, affordability headroom isn't a luxury — it's insurance.

The UK economy grew 0.5% in February — but that was before the Iran-related energy shock. The IMF has already slashed UK growth forecasts to 0.8% for 2026. The BoE governor is publicly discussing the possibility of rate rises for the first time in two years.

In this environment, locking yourself into the highest possible mortgage payment is reckless. A 35-year term gives you a lower mandatory minimum. You can always overpay voluntarily — most lenders allow 10% annual overpayment without penalty. But you can't un-pay if you lose your job or your energy bill doubles.

The 35-year term gives you optionality. Earn a bonus? Overpay the mortgage. Markets crash? Buy cheap equities in your ISA. Redundancy? Your minimum payment is £185 lower, buying you crucial breathing room. Energy bills surge? You can absorb the hit without missing mortgage payments.

The 25-year term commits you to a higher payment regardless of circumstances. In a stable economy, that discipline has value. In an economy facing an energy shock, a potential recession, and the threat of rising rates, it's a straitjacket.

Consider the last two decades: anyone who took a 35-year mortgage in 2006 had the flexibility to survive the 2008 financial crisis, the 2020 pandemic, the 2022 energy crisis, and the 2026 Iran war. Those with the highest possible payments were the ones who defaulted.

The standard case for shorter terms highlights faster equity building. After 10 years on a 25-year term, you've repaid roughly £57,200 in capital versus £30,400 on a 35-year term. A £26,800 difference.

But here's what that comparison misses: the £185/month invested alongside the 35-year mortgage has grown to £31,600 over the same 10 years. Your net wealth position is almost identical — except the ISA money is liquid, tax-free, and diversified across global assets rather than concentrated in a single property.

Property equity is illiquid. You can't spend it without remortgaging or selling. ISA wealth is accessible within days. In a crisis — redundancy, health emergency, opportunity to buy a rental property — that liquidity difference is worth more than any interest saving.

The 35-year borrower at year 10 has £30,400 in property equity plus £31,600 in accessible ISA wealth. The 25-year borrower has £57,200 locked in their walls. One is diversified and flexible. The other has all their eggs in a single, illiquid basket.

For those weighing this against their broader savings strategy, the diversification benefit of holding wealth outside your property shouldn't be underestimated. Our mortgage rate analysis explores the current lending landscape in more detail.

This article is for informational purposes only and does not constitute financial advice. You should seek independent financial advice before making any investment decisions.