The headlines about the new cash ISA rules have, understandably, focused on the lower limit. Far less has been said about the rules sitting quietly behind it, and those are the ones most likely to catch out the careful saver who believed everything was in order. HMRC published its Tax Information and Impact Note on 17 September 2026, setting out how the changes announced at Budget 2025 will work in practice. Here is what it means, and what is worth doing before the rules arrive.

What is changing.

From 6 April 2027, anyone aged under 65 will be able to pay up to £12,000 a year into cash ISAs, down from £20,000. The overall ISA allowance does not change. It remains at £20,000, so the balance can still go into a stocks and shares ISA or an Innovative Finance ISA. Savers aged 65 and over keep the full £20,000 limit for cash.

The government has been fairly open about its reasoning. The stated aim is to encourage more people to invest rather than hold cash, on the basis that investment has historically delivered better long-term returns. Whether that suits any particular person is another matter entirely. Cash has its place, especially for anyone saving towards a near-term goal, or who simply sleeps better knowing the balance will not fall.

Who is actually affected.

For most cash ISA savers, the new limit on its own will change very little. HMRC’s figures show that in 2022/23, 78% of cash ISA savers under 65 paid in less than £12,000. The remaining 22% paid in more, and they are the people the headline change is aimed at. HMRC also identifies a second group, which has received much less attention: anyone holding cash or money market funds inside a stocks and shares ISA or an Innovative Finance ISA. Many people in that group will have no idea they are in it.

The cash sitting in a stocks and shares ISA.

A great many people treat a stocks and shares ISA as a waiting room. Money goes in before the end of the tax year to use the allowance, then sits as cash for months, sometimes years, while a decision is made about where to invest it. Under the current rules, that is a perfectly sensible habit.

From April 2027, it becomes a costly one. A flat-rate charge of 22% will apply to interest paid on cash held within non-cash ISAs, and to the equivalent returns under alternative finance arrangements. The ISA provider deducts the charge and pays it to HMRC, so there is no bill to settle and nothing to report. The interest that reaches you is simply smaller, which makes it remarkably easy never to notice.

Why the cash cannot simply be moved across.

The obvious response would be to transfer that idle cash into a cash ISA. For savers under 65, that route will generally be closed. The new rules restrict transfers from stocks and shares ISAs and Innovative Finance ISAs into cash ISAs, specifically to stop people paying £20,000 into a stocks and shares ISA and then moving it into cash. Once the rules take effect, cash held in a non-cash ISA will broadly face three choices: be invested, stay put and bear the charge, or come out of the ISA altogether.

Money market funds.

Money market funds have become a popular near-cash option on investment platforms. Under the new rules they will be treated as “cash-like” investments. They can still form part of a wider portfolio, but a non-cash ISA invested entirely in them will not qualify. Providers will also need to report the market value of money market fund holdings to HMRC, so this is an area that will be watched closely.

What does not change.

Money already held in a cash ISA is not affected by the lower limit. Nothing needs to be withdrawn, and existing balances keep their tax-free status. The change applies to new money paid in from 6 April 2027.

What is worth doing now.

The single most useful step is to review your ISAs before April 2027 rather than after.

  • If you are under 65 and prefer to save in cash, the current tax year, ending on 5 April 2027, is the last in which the full £20,000 can go into a cash ISA.
  • If cash is sitting idle in a stocks and shares ISA, it is worth deciding deliberately what should happen to it while transfers into a cash ISA are still permitted.
  • If you hold money market funds, it is worth checking whether they make up the whole of your portfolio.
  • If you are approaching 65, it is worth confirming exactly when the higher limit will apply to you.

None of this is cause for alarm. For most people the changes are modest, and there are still nearly six months to arrange things sensibly. The savers who will feel it are those who carry on exactly as before, without realising the ground has shifted beneath them.

If a careful look at how your savings are arranged would help, or you would simply value a straight answer from someone who knows this area well, do send a message.

A short conversation now can save a good deal of irritation later.