Overview
Most ISA guidance stops at the rule: £20,000 per tax year, shared across every ISA you hold. That tells you the ceiling, not what to do with it. The harder question is allocation — how much goes into cash, how much into investments, and in what order.
That question is about to get less optional. From 6 April 2027 the amount a saver under 65 can direct into cash is capped well below £20,000, so a split many people make by default becomes one the rules make for them. This guide covers the allocation decision; for the mechanics of the limit, see how the £20,000 ISA allowance works.
Quick Answer (Read This First)
Allocate by time horizon, not product preference. Money you may need within roughly five years belongs in a Cash ISA, where the balance does not fall. Money you will not touch for longer can go into a Stocks and Shares ISA, where returns have historically been higher but the value can fall as well as rise.
A workable default order: emergency cash outside the wrapper, then a Lifetime ISA if you qualify, then cash up to your realistic short-term need, then the remainder into investments. From 6 April 2027 that last step stops being optional for under-65s.
Start With the Horizon, Not the Wrapper
The choice between a Cash ISA and a Stocks and Shares ISA is not really a choice between two tax wrappers — both are tax-free in the same way. It is a choice between two risk profiles, and the deciding variable is when you need the money.
Under two years. Cash, without much argument. A short horizon gives no time to recover from a fall in value, and a forced sale after a drop turns a paper loss into a real one.
Two to five years. A grey zone. Cash is the conventional answer, and the more fixed the purpose — a deposit, a wedding, a known bill — the stronger that answer gets.
Five years or more. Long enough that investment risk becomes a considered trade-off rather than a gamble on timing. This is where a Stocks and Shares ISA is conventionally used. It is not a guarantee: investments can fall in value and you may get back less than you put in, over any period.
The wrapper does not change that arithmetic. It only removes Income Tax and Capital Gains Tax from the result.
A Working Order of Priority
Allocation is easier as a sequence than as a single decision. A defensible order for most people:
- Hold an accessible emergency fund first. It need not sit in an ISA, and keeping it outside the wrapper preserves the allowance for money that will stay put. See how much emergency fund you need.
- Clear expensive debt. No tax-free return competes with credit card or overdraft interest.
- Consider a Lifetime ISA if it fits the purpose. The 25% government bonus on up to £4,000 a year is the highest guaranteed uplift inside an ISA, but it is tied to a first home or age 60. See Lifetime ISA rules, bonus and withdrawal penalties.
- Fund the Cash ISA to your realistic short-term need — not more, because every pound of cash is a pound unavailable to the long-term pot this year.
- Direct the remainder to a Stocks and Shares ISA, if the horizon supports it.
Steps 4 and 5 are where the allowance is contested, and where the 2027 change lands.
What Changes on 6 April 2027
The overall allowance stays at £20,000. What changes is how much of it can go into cash. Announced at Autumn Budget 2025:
| 2026/27 tax year | From 6 April 2027 | |
|---|---|---|
| Overall ISA allowance | £20,000 | £20,000 |
| Maximum into a Cash ISA, under 65 | £20,000 | £12,000 |
| Maximum into a Cash ISA, aged 65+ | £20,000 | £20,000 |
| Maximum into a Stocks and Shares ISA | £20,000 | £20,000 |
| Lifetime ISA cap (within the overall limit) | £4,000 | £4,000 |
For a saver under 65 who fills the whole allowance with cash, £8,000 of it can no longer be used that way. It does not vanish — it stays available for a Stocks and Shares, Innovative Finance or Lifetime ISA. But it cannot be redirected to cash, and unused allowance cannot be carried forward, so the choice is to use it in a non-cash wrapper or lose it.
IMPORTANT
The 2026/27 tax year is the last full tax year in which a saver under 65 can commit the entire £20,000 to cash. Money already inside a Cash ISA is unaffected — the new limit applies to new subscriptions, not existing balances. If cash is genuinely where you want a large sum to sit, filling the Cash ISA before 6 April 2027 protects that position permanently.
The government has also published draft anti-circumvention rules intended to stop the lower cash limit being sidestepped by parking cash inside a non-cash ISA. As published, they would restrict transfers from Stocks and Shares or Innovative Finance ISAs into Cash ISAs for under-65s, and apply a flat-rate charge to interest on cash held inside a non-cash ISA. These are draft: treat the detail as subject to change until the final legislation is made.
How the Change Reshapes the Split
The sensible response is not to rush a large sum into investments you would not otherwise have chosen. It is to re-sequence.
If your cash need is above £12,000 a year, the tax years before April 2027 are worth using deliberately, since filling the Cash ISA now locks that money inside the wrapper for good.
If your cash need is below £12,000 anyway, the change is close to irrelevant. Most savers subscribe nowhere near £20,000 in a year, so the cash cap will never bind on them — and savers aged 65 and over keep the full £20,000 cash limit regardless.
If the £8,000 residue has no long-term home, the honest answer may be that some of it sits outside an ISA. Interest on unwrapped savings is tax-free up to your Personal Savings Allowance, which for many savers absorbs the difference. Our comparison of Cash ISA vs easy access savings sets out how to work out which pays more after tax.
Timing Within the Tax Year
Two timing points are worth more than most allocation tweaks.
Pay in early. The allowance is the same in April or the following March, but money paid in early spends longer inside the wrapper, and over a long holding period that compounds.
Do not confuse subscribing with transferring. Moving an existing ISA to a different provider — including between a Cash ISA and a Stocks and Shares ISA — uses no allowance, provided you use the provider transfer process. Withdrawing it yourself and paying it in elsewhere is a fresh subscription. Cash ISA transfers covers the process.
Since 6 April 2024 you may also pay into more than one ISA of the same type in a tax year, making it easier to split cash across providers. The combined £20,000 limit still applies.
Common Allocation Mistakes
Filling the Cash ISA on autopilot. Cash is the default because it is familiar, not because it suits a twenty-year horizon.
Investing money with a two-year deadline attached. A deposit needed next spring does not belong in equities.
Assuming the Lifetime ISA is free money. Withdrawing for anything other than a qualifying first home or age 60 triggers a penalty that can leave you with less than you paid in. See when a LISA helps first-time buyers.
Losing track of subscriptions across providers. With multiple same-type ISAs now permitted, it is easier to breach £20,000 by accident. Our guide on accidentally paying into two Cash ISAs explains the repair process.
Frequently Asked Questions
Looking for more on this topic? Browse all our savings guides.
Sources and Further Reading
This guide is based on UK legislation and official guidance. The following sources cover the rules described above:
- Individual Savings Account Regulations 1998
- ISA reform 2027: anti-circumvention rules factsheet — GOV.UK
- Lifetime ISA — GOV.UK
- HM Revenue & Customs
- FCA Handbook — COBS (Conduct of Business Sourcebook)
Free, impartial money guidance is available from MoneyHelper, the government-backed service run by the Money and Pensions Service.
This guide explains how the rules work. It is not personal advice or a recommendation to buy any particular product, and the value of investments can fall as well as rise.
Related: ISA Allowance: How the £20,000 Limit Works | Lifetime ISA Explained | All savings guides.
