Your £20,000 ISA Allowance Expires on 5 April — Every Penny Should Be Sheltered

Every unused pound of ISA allowance is a gift to HMRC.

Consider a basic-rate taxpayer with £20,000 in a standard savings account earning 4.5%. That's £900 in annual interest. The Personal Savings Allowance covers the first £1,000, so they're fine — for now. But add that to existing savings, a workplace cash bonus, or a maturing fixed-rate bond, and they breach the threshold fast.

A higher-rate taxpayer? Their PSA is just £500. At 4.5%, only £11,111 of savings stays fully tax-free outside an ISA. Everything above that is taxed at 40%.

Inside an ISA, there is no limit on tax-free interest. No threshold to breach. No reporting to HMRC. The shelter is permanent — money contributed to an ISA in 2025/26 remains tax-free for life, regardless of what happens to allowances in future years. The ISA rules set by HMRC make this explicit: there is no lifetime cap on how much your ISA pot can grow, and withdrawals are never taxed.

The Personal Savings Allowance was introduced in April 2016, and at the time it seemed generous enough to make cash ISAs redundant. That argument made sense when the Bank Rate was 0.5% and savings accounts paid next to nothing. Today, with the Bank Rate at 3.75% and competitive accounts paying above 4%, the PSA is easily overwhelmed. A saver with £30,000 across easy-access accounts earns £1,350 at 4.5% — £350 above the basic-rate PSA, triggering a £70 tax bill. A higher-rate taxpayer with the same balance faces £340 in tax. The ISA eliminates all of it.

If you're wondering whether you actually need an ISA given the PSA, our analysis of cash ISAs versus savings accounts walks through the exact thresholds where the ISA becomes essential.

The argument that cash ISAs aren't worth it died in 2024. The best easy-access cash ISAs now pay 4.68% — Trading 212's promotional rate — with Plum at 4.66% and several building societies above 4.4%. Fixed-rate ISAs stretch to 4.23% for one year and 4.20% for two.

With CPI inflation running at roughly 3%, these are real positive returns. Tax-free positive returns. That combination hasn't been this attractive since before the 2008 financial crisis.

For the risk-averse, the case is closed. A cash ISA paying 4.5% on £20,000 generates £900 a year — entirely tax-free. Outside an ISA, a higher-rate taxpayer keeps only £540 of that. The ISA wrapper saves £360 annually, compounding every year the money stays sheltered.

It's worth understanding exactly how much the tax saving compounds over time. After five years at 4.5%, a higher-rate taxpayer sheltering £20,000 in a cash ISA accumulates £4,685 in tax-free interest. The same money outside an ISA, after 40% tax on interest, grows by just £2,811. That's £1,874 lost to tax — on a single year's contribution. Over a decade, the gap widens to over £4,000.

For those willing to take investment risk, stocks and shares ISAs delivered an average 11.2% return in the 12 months to February 2026. Tax-free capital gains and dividends on that growth make the ISA wrapper even more valuable for investors. If you're weighing up which ISA type to prioritise, our comparison of stocks and shares ISAs versus cash ISAs breaks down the decision by risk profile and time horizon. See also our guide to investing your ISA allowance for how to get started.

The Autumn Budget 2025 confirmed that from 6 April 2027, anyone under 65 can only contribute £12,000 to cash ISAs per year. The remaining £8,000 of the £20,000 total must go into stocks and shares, innovative finance, or Lifetime ISAs.

This makes the 2025/26 tax year the last chance to put £20,000 into pure cash. For cautious savers who prefer the certainty of cash, that's a closing window.

But there's a subtler point. Money already inside a cash ISA is grandfathered — the new rules only apply to fresh contributions from April 2027. Every pound you shelter in a cash ISA before 5 April 2026 stays tax-free forever, regardless of future rule changes. You're locking in rights that new savers won't have. The FCA's ISA regulations confirm that existing ISA holdings are fully protected from future contribution limit changes.

Think of it as ISA allowance arbitrage. The government is explicitly telling you it plans to restrict this benefit. The rational response is to maximise it while you can.

This is also why the interaction between your ISA and pension allowances matters. The annual pension allowance of £60,000 also resets on 5 April. If you have spare cash, our analysis of ISA versus pension priorities explains which shelter to fill first — and our deeper look at the £60,000 pension allowance deadline makes the case that pensions should usually come before ISAs for higher-rate taxpayers.

The Treasury's rationale for the cash ISA cap is to encourage more productive investment — channelling savings into equities and business lending rather than bank deposits. Whether you agree with that policy or not, the practical implication is clear: if you value the flexibility to hold your full allowance in cash, 2025/26 is your final opportunity. After April 2027, savers under 65 who want to shelter more than £12,000 will be forced into investment risk they may not want.

You don't have to choose one ISA type. The £20,000 allowance can be split across cash, stocks and shares, innovative finance, and Lifetime ISAs in any combination, as MoneyHelper's ISA guidance confirms.

A pragmatic split for 2025/26:

• £6,000 in an easy-access cash ISA at 4.5%+ — your emergency fund, tax-free and instantly accessible
• £14,000 in a stocks and shares ISA — broad index trackers like a global equity fund or FTSE All-Share tracker

This gives you liquidity for emergencies while capturing the long-term return premium of equities. Over 20 years, the tax savings on investment growth dwarf anything a cash ISA produces.

Not sure how to divide your allowance? Our detailed guide on how to split your £20,000 ISA allowance before 5 April walks through five different allocation strategies based on your age, risk tolerance, and existing savings.

Already have an emergency fund sorted? Put the full £20,000 into equities. Already hold investments outside an ISA? Use the bed and ISA technique — sell holdings in your general account and rebuy them inside your ISA wrapper. Same investments, permanent tax shelter. The capital gains tax annual exempt amount for 2025/26 is just £3,000, so sheltering gains inside an ISA is more important than ever.

For couples, remember that the ISA allowance is individual. Two adults in a household have £40,000 of combined ISA capacity. If one partner earns more and pays higher-rate tax, it may be worth prioritising their ISA first — the tax saving per pound sheltered is double that of a basic-rate taxpayer.

Every year, platforms report a last-minute surge in ISA applications during the final week of March. Every year, some people miss the deadline because transfers don't settle in time.

ISA transfers between providers can take up to 15 business days for cash ISAs and up to 30 days for stocks and shares ISAs. If you're planning to transfer an existing ISA to a better rate, start now — not in the first week of April.

For new contributions, most online platforms process cash ISA deposits within one business day. Stocks and shares ISA purchases settle in two. But if you're opening a new account, identity verification and anti-money-laundering checks can add days.

There are a few common mistakes to avoid in the final days of the tax year. First, don't confuse ISA transfers with new contributions — transferring an existing ISA to a new provider does not use your annual allowance, but withdrawing from one ISA and depositing into another does. Second, check your provider's cut-off time; while the tax year officially ends at midnight on 5 April, some platforms stop accepting contributions at 11pm or earlier. Third, remember that you can only pay into one cash ISA and one stocks and shares ISA per tax year with the same provider — though since April 2024, the updated ISA rules allow multiple subscriptions to the same type across different providers.

The deadline is 5 April at midnight. The smart deadline is today.

If you can't afford to shelter the full £20,000, don't let that stop you from contributing what you can. Even £5,000 sheltered in a cash ISA earning 4.5% saves a higher-rate taxpayer £90 a year in tax — every year, compounding indefinitely. Over ten years, that single contribution shields over £1,000 from tax. Small amounts matter because the ISA wrapper is permanent: once inside, your money never faces tax again regardless of how large the pot grows.

For those who have already used some of their allowance earlier in the tax year, check your ISA provider's dashboard to confirm exactly how much headroom remains. The £20,000 limit applies across all ISA types combined, so a £4,000 Lifetime ISA contribution earlier in the year leaves just £16,000 for cash and stocks and shares ISAs.

This article is for informational purposes only and does not constitute financial advice. You should seek independent financial advice before making any investment decisions. Tax treatment depends on your individual circumstances and may change in the future.