Every year, there is one financial date in the UK that quietly shapes how millions of people save and invest.

5 April.

It marks the end of the tax year. And with it, the moment when one of the most valuable tax allowances available to UK investors disappears and resets.

That allowance is your ISA allowance.

For the current tax year, UK adults can invest up to £20,000 into ISAs, with all growth and income inside the account free from UK income tax and capital gains tax.

But there is one rule that catches people out every year.

If you do not use your ISA allowance before midnight on 5 April, you lose it.

It does not roll over into the next tax year. You cannot reclaim it later. Once the new tax year begins on 6 April, the allowance resets and the previous year’s opportunity is gone.

That is why the weeks leading up to the tax year end are often called ISA season. Investors reviewing their accounts, topping up their contributions and making sure they make the most of their tax free investing.

What is the ISA allowance?

The ISA allowance is the maximum amount you can contribute to ISAs in a single tax year.

For the 2025 to 2026 tax year, the allowance is:

£20,000 per person

You can use that allowance across several types of ISA, including:

• Cash ISA
• Stocks and Shares ISA
• Lifetime ISA
• Innovative Finance ISA

You do not have to put the full amount into one account. Many investors choose to split their allowance between different types depending on their goals.

For example, some people keep emergency savings in a Cash ISA, while investing long term money in a Stocks and Shares ISA.

Why ISAs are so powerful for long term investing

The real value of ISAs comes from their tax efficiency.

Inside an ISA, your investments grow free from:

• Capital gains tax
• Tax on dividends
• Tax on interest

That means every pound of growth stays invested and continues compounding.

Over long periods of time, avoiding tax on investment returns can make a meaningful difference to the size of your portfolio.

For example, imagine investing £20,000 every year for 20 years.

That would mean contributing:

£400,000

If those investments grew at an average annual return of 8 percent, the portfolio could reach roughly:

≈ £988,000

Because the money sits inside an ISA, that growth would be tax free.

Outside an ISA, a portion of those gains could eventually become taxable.

The rule many investors forget

One of the most important things to understand about ISAs is that unused allowance disappears.

Each tax year runs from 6 April to 5 April the following year.

At the start of the new tax year, your ISA allowance refreshes and you can invest another £20,000.

But anything you did not use in the previous year is lost.

For example:

If you invest £5,000 this tax year, the remaining £15,000 allowance disappears once the tax year ends.

Over time, missing ISA allowances can significantly reduce the amount of money you are able to invest tax efficiently.

Cash ISAs vs Stocks and Shares ISAs

When people first open an ISA, they often choose a Cash ISA.

These work similarly to savings accounts. You deposit money and earn interest, but the interest is tax free.

However, many long term investors prefer Stocks and Shares ISAs.

Instead of holding cash, your money is invested in assets such as:

• Global equity funds
• Exchange traded funds (ETFs)
• Shares
• Bonds

Because investments have the potential to grow more than cash over time, Stocks and Shares ISAs are often used for long term wealth building.

The key benefit remains the same.

All growth and income generated inside the ISA remains free from UK tax.

Why investors top up their ISA before the deadline

For many people, the final weeks before the tax year ends become a natural moment to review their finances.

Have they used their ISA allowance?

Have they added enough to their investments this year?

Is there still time to contribute more?

Even if you cannot invest the full £20,000 allowance, using as much of it as possible still helps build your tax efficient investment pot.

Many investors also make ISA contributions in March or early April simply because it is their last chance to use that year’s allowance.

Once the deadline passes, the opportunity disappears.

When is the ISA deadline?

The ISA deadline falls on 5 April each year, the final day of the UK tax year.

You must contribute to your ISA before midnight on 5 April for the investment to count towards that year’s allowance.

From 6 April, a new tax year begins and a new £20,000 allowance becomes available.

The long term habit that builds wealth

Building wealth rarely comes from a single financial decision.

More often, it comes from simple habits repeated over time.

Investing regularly.
Staying invested.
And making the most of the tax efficient accounts available to you.

ISAs were designed to support exactly that.

By using your ISA allowance each year, you gradually build a growing investment pot that can compound over time without unnecessary tax.

And while the ISA deadline returns every year, once it passes, that year’s opportunity is gone.

Which is why many investors take a moment before 5 April to check whether they have made the most of it.

Key takeaways

• The ISA deadline is 5 April, the final day of the UK tax year
• The annual ISA allowance is £20,000 per person
• Any growth inside an ISA is free from UK income tax and capital gains tax
• Unused allowance does not roll over into the next tax year
• Using your ISA allowance each year can help build long term tax efficient wealth

Remember that when you invest, your capital is at risk. Tax treatment is based on personal circumstances and can change in future. This content is educational and is not financial advice.