The tax rules for an ISA are simple at their core: any interest, dividends, or capital gains earned inside the wrapper are free from UK income tax and capital gains tax, you can put in up to £20,000 each tax year across all the ISAs you hold, and the shelter lasts as long as the money stays inside.1GOV.UK. Individual Savings Accounts (ISAs) – How ISAs Work There is no lifetime cap on how large the pot can grow. The complications come from the boundaries: who counts as eligible, what happens when you move money in or out, and where the shelter stops.
What an ISA Actually Shelters
Three separate taxes fall away inside an ISA.
Interest earned in a cash ISA is free from income tax. Outside an ISA, savings interest above your Personal Savings Allowance is taxed at your marginal rate. The allowance is £1,000 for basic-rate taxpayers, £500 for higher-rate, and nothing for additional-rate taxpayers.2GOV.UK. Tax on Savings Interest – How Much Tax You Pay ISA interest does not use up any of that allowance, which leaves it available for taxable savings elsewhere.
Dividends from shares held in a stocks and shares ISA are also tax-free.3GOV.UK. Tax on Dividends Outside the wrapper, dividend income above the £500 dividend allowance is taxed at 10.75% for basic-rate taxpayers, 35.75% for higher-rate, and 39.35% for additional-rate taxpayers in 2026/27.
Capital gains on investments sold inside the ISA are exempt too. Without the wrapper, profits from selling shares or funds face capital gains tax at 18% for basic-rate taxpayers or 24% for higher-rate and additional-rate taxpayers.4GOV.UK. Capital Gains Tax – What You Pay It On, Rates and Allowances Inside the ISA that liability disappears no matter how large the gain gets.
The £20,000 Limit and How It Splits
The overall ISA allowance for 2026/27 is £20,000, running from 6 April to 5 April.1GOV.UK. Individual Savings Accounts (ISAs) – How ISAs Work You can spread it across any mix of ISA types. Put £4,000 into a Lifetime ISA and you have £16,000 left for the rest. Unused allowance does not carry forward; if you use £12,000 this year, the other £8,000 is gone.
Since April 2024, you can pay into more than one ISA of the same type in a single tax year — several cash ISAs with different providers, for instance, or two stocks and shares ISAs.5GOV.UK. Tax-Free Savings Newsletter 11 Lifetime ISAs and Junior ISAs still cap you at one subscription of that type per year. Whichever way you split things, the £20,000 combined ceiling stands, and no single provider can see your contributions elsewhere. Keeping a running total is your job.
Some providers offer flexible ISAs. These let you take money out and put it back in the same tax year without the replacement counting as a new contribution. Deposit £15,000, withdraw £3,000 in August, replace the £3,000 before 5 April, and you still have £5,000 of fresh allowance left. Not every provider offers flexibility, and it never applies to Junior ISAs or Lifetime ISAs. Without it, any redeposit eats into your allowance like a new subscription.
Who Can Open One
To open an ISA you must be 18 or over and resident in the UK for tax purposes. Crown servants posted overseas and their spouses or civil partners also qualify.6GOV.UK. Individual Savings Accounts (ISAs) – Overview A Lifetime ISA additionally requires you to be under 40 at the time of opening.7GOV.UK. Lifetime ISA
Applications need your National Insurance number. If you have never been issued one but are otherwise eligible, you can make a declaration to that effect when opening a cash, stocks and shares, or innovative finance ISA. A Lifetime ISA always requires the actual number.8GOV.UK. How to Open an ISA as an ISA Manager
Rules That Differ by ISA Type
The tax shelter is identical across ISA types, but the surrounding rules are not.
Cash ISA
Works like a savings account. Interest is tax-free, access is usually unrestricted, and this is the lowest-risk option.
Stocks and Shares ISA
Holds equities, bonds, and funds. Dividends and capital gains are sheltered. Market risk applies; the value can fall.
Innovative Finance ISA
Holds peer-to-peer loans. Interest is tax-free, but if a borrower defaults you can lose capital, and the money is not covered by the Financial Services Compensation Scheme.
Lifetime ISA
Contributions cap out at £4,000 per year, which sits inside the £20,000 overall limit. The government adds a 25% bonus, up to £1,000 a year.7GOV.UK. Lifetime ISA Withdrawals for anything other than a qualifying first home or retirement after age 60 trigger a 25% government charge on the amount withdrawn. That charge eats into your own money, not just the bonus. HMRC’s own example: save £800, receive a £200 bonus, and you have £1,000; the 25% charge on £1,000 is £250, leaving you £750, which is £50 less than you put in.9GOV.UK. Withdrawing Money From Your Lifetime ISA You can only hold one Lifetime ISA at a time.
Junior ISA
Opened by a parent or guardian for a child under 18.10GOV.UK. Junior Individual Savings Accounts (ISA) – Overview Comes in cash or stocks and shares form, with its own annual limit of £9,000 for 2026/27 that sits outside the adult £20,000 allowance.11GOV.UK. Junior Individual Savings Accounts (ISA) – Add Money to an Account The child takes control at 16 and can withdraw at 18.
Moving an ISA Without Losing the Shelter
Switching providers has to go through the formal transfer process. If you withdraw the money to your bank account and redeposit it with a new provider, current-year contributions eat into your annual allowance, and previous years’ savings lose their tax-free status altogether.
You start the transfer with the new provider, who then contacts the old one. Cash ISA transfers should complete within 15 working days. Stocks and shares and innovative finance ISA transfers should complete within 30 calendar days.12GOV.UK. Individual Savings Accounts (ISAs) – Transferring Your ISA Your money may be out of the market during the transfer window, so time investment transfers with that in mind.
Breaking the Rules
Going over £20,000 does not bring a formal fine. HMRC will repair the ISA by removing the excess contribution and stripping tax relief from any income or gains earned on that excess from the date of the invalid subscription onward.13GOV.UK. How to Close, Void or Repair an ISA The rest of the ISA is left alone.
Bigger breaches — opening an ISA when you are not eligible, or holding non-qualifying investments — can void the entire account. Voiding removes all tax exemptions and forces both the invalid subscription and any related income to come out.13GOV.UK. How to Close, Void or Repair an ISA HMRC audits ISA managers regularly, checking for excess subscriptions, non-UK addresses, and missing National Insurance numbers.14GOV.UK. ISA Audits by HMRC
Inheritance Tax and the Spouse Allowance
The ISA shelter does not extend to inheritance tax. When you die, the value of your ISA holdings is counted as part of your taxable estate, and if the estate exceeds the inheritance tax threshold your ISA savings are caught with everything else.15GOV.UK. Individual Savings Accounts (ISAs) – If You Die
A surviving spouse or civil partner gets something useful in return. They receive an Additional Permitted Subscription allowance equal to the value of the deceased’s ISA holdings, on top of their own £20,000 annual allowance. The APS amount is the higher of the ISA value at the date of death or the value when the ISA stops being a continuing account of the deceased. Cash APS subscriptions have to be made within three years of the date of death, or within 180 days of completing the estate administration, whichever is later.16GOV.UK. How to Manage Additional Permitted Subscriptions
If You Leave the UK
Once you stop being a UK tax resident, you cannot make new contributions. Your existing ISA stays open and the UK tax-free status of what is already inside is preserved, though your new country of residence may tax the income or gains under its own rules.17GOV.UK. Individual Savings Accounts (ISAs) – If You Move Abroad Contributions made accidentally during a non-resident year have to come out along with any related growth.
Narrow exceptions let non-residents make Additional Permitted Subscriptions after a spouse’s death and replace flexible-ISA withdrawals taken before moving. A Junior ISA can keep receiving subscriptions even if the child becomes non-resident, though no new Junior ISA can be opened at that point.
US Citizens and Green Card Holders
The ISA is a UK concept. The IRS does not recognise the shelter, and the US-UK tax treaty contains no provision that extends ISA protection to a US tax return. Every pound of interest, dividends, and capital gains earned inside your ISA is fully taxable on the US side because the US taxes its citizens on worldwide income wherever they live.
Stocks and shares ISAs create a second problem. Most UK-domiciled funds are classified by the IRS as Passive Foreign Investment Companies, which triggers punitive tax treatment and a Form 8621 filing requirement for each fund held.18Internal Revenue Service. About Form 8621 – Information Return by a Shareholder of a Passive Foreign Investment Company or Qualified Electing Fund Preparation fees alone can run to hundreds of pounds per fund.
Separately, if the combined value of your foreign financial accounts, ISAs included, tops $10,000 at any point in the calendar year, you have to file a Report of Foreign Bank and Financial Accounts with FinCEN.19FinCEN.gov. Report Foreign Bank and Financial Accounts For US-UK dual taxpayers, a cash ISA holding only cash deposits is generally the least awkward option because it avoids the PFIC issue, but the interest is still US-reportable income. Talking to an adviser who handles US expatriate returns before opening any ISA is worth the fee.