In the UK, you do not have to declare ISA interest, income, or capital gains on a tax return, as they are tax-free. However, if you are a U.S. citizen or green card holder, you must report ISA income and gains to the IRS, as the U.S. does not recognize the tax-exempt status of ISAs.
All interest, income and capital gains within an ISA are tax-free, and you don't need to include them on a tax return. Learn more about the different types of ISAs in our guide to ISAs and other tax-efficient ways to save or invest.
US taxpayers are required to report all ISA income and capital gains on their annual US tax return. The nature of the income determines its tax treatment: Interest income, ordinary dividends, and short-term capital gains are taxed at ordinary tax rates.
This is called the ISA allowance. The annual ISA allowance for the 2025/2026 tax year is £20,000. This means you can save up to £20,000 across different types of ISAs, including: Cash ISAs: Save money with a fixed or variable interest rate.
Stocks and Shares ISAs allow investments in mutual funds, ETFs, and individual stocks. While these accounts offer the potential for tax-free capital gains and dividends in the UK, US tax rules require all capital gains and dividends to be reported as taxable income.
You do not pay income tax and capital gains tax on investments held within individual savings accounts (ISAs). This means that: the income your ISA generates (normally interest or dividends) is exempt from income tax.
Disadvantages: Interest rates may decrease, funds might be locked in fixed-rate ISAs, and not all accounts permit transfers, sometimes incurring exit fees.
Individual Savings Accounts (ISAs)
The government sets a maximum amount that you can invest in ISAs. Until 2031 the annual limit is £20,000. You pay no Income Tax on the interest or dividends you earn within an ISA and any profits from investments are free of Capital Gains Tax.
Isa providers are obliged to provide contribution histories to HMRC. If you go over your limit without realising it, HMRC will contact you and you can arrange to correct the underpaid tax.
The UK government has announced significant changes to the tax treatment of cash held within stocks and shares Isas, targeting a loophole that could allow savers to bypass newly imposed caps on tax-free cash savings.
As ISAs are tax-free, you do not need to declare your interest, capital gains or income on your tax return.
Can I keep my ISA if I move abroad? Yes, you can keep your cash ISA when you move abroad. However, you won't be able to make any more contributions to it.
Can I put more than £20,000 in an ISA? Technically, yes, but not all at once. There's no limit to how much money can be in an ISA. The ISA allowance limit applies to how much you can pay in during each tax year (6 April to 5 April the following year).
Investments that pay interest (like government and corporate bonds), or rental income (like some property funds) provide 100% tax-free income if held within an ISA. Everyone gets a £500 tax-free Dividend Allowance. This is on top of your personal allowance – the amount you can earn each tax year before paying tax.
At a glance
The minimum income amount to file taxes depends on your filing status and age. For 2025, the minimum income for Single filing status for filers under age 65 is $15,750 . If your income is below that threshold, you generally do not need to file a federal tax return.
To avoid the UK's 60% tax trap (an effective 60% rate on income between £100k-£125k), the key is to reduce your adjusted net income back below £100,000 by making tax-efficient contributions, primarily via pension contributions, which reclaim your full £12,570 Personal Allowance, and also through salary sacrifice for benefits like childcare or cycle-to-work, and Gift Aid donations to charity.
If you complete a tax return, you do not need to declare any ISA interest, income or capital gains on it.
Document any legitimate reasons for income fluctuations, such as a new business venture or a change in your personal circumstances. Large or frequent cash transactions can be a red flag, particularly if they are not typical for your industry or personal financial habits.
HMRC's process for addressing excess payments
According to HMRC, 'We'll only take action after the end of the tax year, once we have the audit data in from the ISA companies. ' Providers are also given the task of calculating the gain or interest arising on any excess amount.
Currently, you are able to contribute up to £20,000 each tax year into a cash ISA (or you can split this allowance between other types of ISA). However, the Chancellor has confirmed the cash ISA limit will be reduced to £12,000 a year from April 2027.
In the U.K., ISAs are used to shelter savings and investments from local taxes. But here's the catch—the IRS does not recognize the tax-free status of ISAs. This means any growth, interest, or dividends in your ISA must still be reported—and potentially taxed on your U.S. return.
Consider Fixed-Term Options: If you don't need immediate access to your funds, fixed-term cash ISAs often provide higher interest rates compared with easy-access accounts—but ensure that locking away money aligns with your needs.
You can get around 7% interest on savings in the UK primarily through Regular Saver accounts, with top offers from Zopa (7.1% variable), First Direct (7% fixed), and the Co-operative Bank (7% variable), though these often require you to have their current account and limit monthly deposits, while Principality Building Society has offered rates near this (7.5%) on fixed-term savers, so check MoneySavingExpert and MoneyWeek for current deals.
Cash ISAs are tax-free. You won't pay tax on any interest you earn. At NatWest, we offer an instant access Cash ISA, and a Fixed Rate ISA with a set term. On the other hand, the interest you make on normal savings accounts may be taxed, if it's more than your Personal Savings Allowance.