How the ISA Allowance Actually Works

The confusing part is not the number. It is that £20,000 is one allowance covering several different accounts, and nearly everyone assumes at some point that each ISA gets its own.

One allowance, several accounts

Open a cash ISA and a stocks and shares ISA in the same tax year and you still have £20,000 in total, not £40,000. Pay £12,000 into the cash ISA and you have £8,000 left for everything else. That single rule is the source of most of the mistakes.

The Lifetime ISA sits inside that limit with a ceiling of its own. You can pay in up to £4,000 a year and the government adds 25%, so £4,000 becomes £5,000. That £4,000 still counts against your £20,000, leaving £16,000 for other ISAs. The bonus is genuinely free money, with the catch that taking it out before 60 for anything other than a first home carries a withdrawal charge.

It is worth doing the arithmetic on what the shelter is actually worth to you. £20,000 in a cash ISA paying 4.2% earns £840 in a year. Held outside an ISA, a basic-rate taxpayer with a £1,000 personal savings allowance pays nothing on that. A higher-rate taxpayer has a £500 allowance, so £340 gets taxed at 40%, which is £136. Useful rather than transformative at that balance. The case strengthens every year you add to the pot, because the shelter compounds along with the money.

What does not count against your allowance

Growth and interest inside an ISA do not use up allowance. If your stocks and shares ISA rises from £30,000 to £34,000, that £4,000 is not a contribution. Neither are transfers of money you subscribed in earlier tax years: moving an old cash ISA to a better rate is a transfer, not a fresh subscription, as long as you use the provider's transfer process rather than withdrawing and paying it back in yourself. Junior ISAs are separate as well, with their own £9,000 limit, and paying into a child's JISA does not touch your £20,000.

The deadline is 5 April and it does not move

Unused allowance does not roll forward. Whatever is left of your £20,000 when the tax year closes on 5 April simply stops existing, and a fresh £20,000 begins on 6 April. Pension annual allowance can be carried forward for up to three years, but ISA allowance cannot, which is why so much money moves into ISAs in the last fortnight of March.

Withdrawals and the flexible ISA trap

If your ISA is flexible, money you take out and replace within the same tax year does not use fresh allowance. If it is not flexible, and plenty of stocks and shares ISAs are not, replacing a withdrawal counts as a new subscription. Take £5,000 out in June from a non-flexible ISA and put it back in October and you have used £5,000 more of your £20,000 for nothing. Check the wording on the account before you move money out.

How to use the ISA allowance calculator

Enter what you have already paid into each type this tax year, cash, stocks and shares, and Lifetime, and the calculator returns how much of the £20,000 remains and how much Lifetime ISA headroom is left inside it.

Use the date you paid in, not the date the money cleared or the date it got invested. A transfer made in December that sits as pending until January still counts as a December subscription. When you are working out what you can realistically set aside each month, start from net pay rather than salary, since the take-home figures are the ones that actually land in your account. If your question is what the pot grows to rather than what you can pay in, the ISA growth walk-through handles that side of it.

Allowance figures are set by the Treasury and can change between tax years, so confirm the current limit on gov.uk before you plan around it.

Common questions

Can I pay into two cash ISAs in one year? Yes, as long as your total across every ISA stays inside the £20,000. The rules were loosened for this, so older guidance saying one ISA per type per year is out of date. Lifetime ISAs are the exception and still allow only one subscription per year.

What happens if I go over the £20,000? HMRC picks it up when providers report subscriptions after the tax year closes, and the excess is normally unwound with any interest or growth on it made taxable. Do not try to fix it by quietly withdrawing money; contact the provider and let them correct it.

Does unused allowance carry over? No. It disappears at midnight on 5 April. If you have £6,000 of allowance left in March and no spare cash, that allowance is simply gone on 6 April.

Do I have to declare ISA interest on a tax return? No. Interest, dividends and capital gains inside an ISA are not reportable and do not count towards your personal savings allowance or dividend allowance, which is most of the point of the wrapper.