A Cash ISA is a UK savings account where the interest you earn is free from income tax and capital gains tax. For long-term retail investors in Europe, including those in Romania, it is one of the simplest ways to protect a portion of your savings from tax drag. This guide explains the wrapper from first principles, shows how the tax exemption works in practice, and clarifies the limits and rules that govern it.
ISAs were introduced on 6 April 1999, replacing the earlier personal equity plans and tax-exempt special savings accounts. Since 6 April 2017, there have been four adult types: cash ISA, stocks & shares ISA, innovative finance ISA and lifetime ISA. A Junior ISA is available for children and replaced the Child Trust Fund in 2011. Each type offers the same tax protection but holds different kinds of investments. A cash ISA holds only cash deposits, while a stocks & shares ISA can hold equities, bonds, ETFs and funds.
What is an ISA?
An Individual Savings Account (ISA) is a tax-efficient wrapper. The government does not tax the returns generated inside it. Specifically, interest, dividends and capital gains are exempt from income tax and capital gains tax, and no tax is due when you withdraw money. The wrapper itself is not an investment; it is a legal structure that sits around whatever you choose to hold inside it.
The protection applies to UK residents. You must be 18 or over to open an adult ISA, and you must be resident in the UK, or a member of the armed forces or a Crown servant (or their spouse or civil partner) if you do not live in the UK. You cannot hold an ISA with someone else; each account is held in a single individual's name.
Types of ISA
Adults can hold more than one type of ISA in the same tax year, but the combined contributions must stay within the annual limit. The four adult types differ by what they hold.
| ISA Type | What it holds |
|---|---|
| Cash ISA | Bank and building society deposits |
| Stocks & Shares ISA | Equities, ETFs, bonds, funds |
| Innovative Finance ISA | Peer-to-peer loans and other debt |
| Lifetime ISA | Cash or investments for first home or retirement |
Junior ISAs are available for children under 18 and follow the same tax-exemption principle, but with a separate annual allowance.
How tax-free growth works
Inside a Cash ISA, interest is paid gross, meaning the bank or building society does not deduct income tax before crediting it. This is different from a standard savings account, where interest is paid net and the provider reports it to HM Revenue and Customs (HMRC). After 5 April each year, your bank or building society will tell HMRC about any interest they paid to you for the previous year.
Because no tax is deducted at source, the full amount of interest compounds on the original balance. When you withdraw the money, you do not pay any tax on the returns. The same shelter applies to stocks & shares ISAs, where dividends and capital gains are protected.
A simple example makes this concrete. Suppose you hold a sum in a standard savings account paying 4% interest. If you are a higher-rate taxpayer (40%), you would owe tax on the interest earned. In a Cash ISA, the same interest stays in your account. Over many years, that difference compounds, leaving you with a larger balance at the end.
The annual allowance
Each tax year, you have an annual ISA allowance that sets the maximum you can contribute across all ISA types. In the 2026 to 2027 tax year, the maximum you can save in ISAs is £20,000. You can split this allowance among the four adult types as you wish, or put it all into one type. Any allowance you do not use in a given year does not roll over to the next year.
The allowance has changed over time. In the March 2014 Budget, the Chancellor of the Exchequer George Osborne announced that the adult ISA limit would be increased to £15,000 from 1 July 2014, and the Junior ISA limit to £4,000. Since 1 July 2014, savers have been allowed to invest the full amount as cash or stocks and shares, or a mix of both, and are also able to switch stocks and shares ISAs to cash ISAs.
Flexible ISAs
Some providers offer flexible ISAs, which allow you to withdraw funds and replace them within the same tax year without reducing your remaining allowance. For example, if you have a £20,000 flexible Cash ISA and you withdraw £5,000 in June, you can pay that £5,000 back in before the following 5 April and still stay within the £20,000 limit for that year.
Not all ISAs are flexible. If you hold a non-flexible ISA, any withdrawal permanently reduces your allowance for that year. If you are considering a flexible product, check the terms carefully, because the label is not automatic.
Eligible investments
While a Cash ISA holds only deposits, a stocks & shares ISA can hold a broad range of investments. These typically include individual stocks, corporate and government bonds, ETFs, unit trusts and investment trusts. There is no restriction on when or how much money can be withdrawn from the wrapper, and you can switch between ISA types each year as long as you stay within the annual allowance.
For long-term investors, a stocks & shares ISA is often more suitable than a Cash ISA, because equities and equity ETFs have historically delivered higher returns over long periods, and the tax shelter protects those returns from capital gains tax.
ISAs for long-term goals
Because returns inside an ISA are tax-free, the power of compounding works faster than in a taxable account. Over decades, the difference can be substantial. If you are planning for retirement, an ISA can sit alongside a pension as part of a broader strategy. Until the lifetime ISA was introduced in 2017, ISAs were not a specific retirement investment, but the accounts can be useful tools for retirement planning alongside pensions.
A long-term investor who contributes a fixed amount each year to a Cash ISA earning 3% interest will see the tax-free interest compound each year. After 20 years, the balance will be higher than it would be in a taxable account where a portion of the interest is paid away annually. The same principle applies to a stocks & shares ISA, where dividends and capital gains are sheltered.
ISA allowance history
Key takeaways
- An ISA is a tax wrapper where interest, dividends and capital gains are exempt from income tax and capital gains tax.
- The 2026 to 2027 annual allowance is £20,000, which can be split across the four adult ISA types.
- Unused allowance does not roll over to the next tax year.
- Flexible ISAs let you withdraw and replace funds within the same year without losing allowance.
- Cash ISAs hold deposits; stocks & shares ISAs can hold equities, bonds, ETFs and funds.
- ISAs are useful for long-term goals because compounding works faster when returns are not taxed.
Educational only — not personalised investment advice.