£20,000 per year, sheltered from tax permanently. Every pound of unused allowance that reaches 5 April is gone forever — it cannot be recovered or carried forward to the next year.
An Individual Savings Account (ISA) is a tax wrapper — a container that shelters whatever you put inside from Income Tax, Capital Gains Tax, and Dividend Tax. Once money is inside an ISA, all growth, interest, and income are permanently and completely tax-free — regardless of how large the pot grows or how long it is held. The protection is for life.
The annual subscription limit is £20,000 per person per tax year (6 April to 5 April). This limit applies to new subscriptions only — existing ISA balances are unaffected by the annual limit and continue to grow tax-free indefinitely. Unused allowance cannot be carried forward. It resets — and the previous year's unused capacity is permanently lost.
The £20,000 annual limit applies across all your ISAs combined (with the exception of the Junior ISA, which has its own separate limit). You can split your allowance across multiple ISA types however you choose.
Outside an ISA, three separate taxes erode investment returns. Dividend Tax applies to distributions above the £500 annual allowance at 8.75% (basic rate) or 33.75% (higher rate). Capital Gains Tax applies to profits above the £3,000 annual exemption at 18% or 24%. Interest on cash savings above the Personal Savings Allowance (£500 for higher rate, £1,000 for basic rate) is taxed as income.
Inside an ISA, none of these taxes apply — ever, regardless of how large the portfolio grows. A portfolio that grows from £50,000 to £500,000 inside an ISA generates no tax liability on any of that growth. The same portfolio outside an ISA would generate substantial annual tax bills on dividends, interest, and eventually on any gains realised.
The compounding effect of tax-free growth versus taxed growth is enormous over a long investment horizon. Every pound kept inside the ISA wrapper compounds faster — because none of it is being handed to HMRC along the way.
Since 2016, some ISA providers have offered "flexible ISA" status. A flexible ISA allows you to withdraw money and replace it within the same tax year without that replacement counting against your annual allowance. For example, if you have contributed £15,000 and then withdraw £5,000 to cover an emergency, a flexible ISA allows you to re-contribute that £5,000 later in the same tax year — leaving you with your full £20,000 available, not just £5,000 remaining.
Not all ISAs are flexible — it depends on the specific provider and product. Check with your provider before making withdrawals in the expectation of replacing them.
Exceeding the £20,000 limit: HMRC will identify over-subscriptions and apply a tax charge to the excess. The process of unwinding over-subscriptions is time-consuming and creates administrative complexity. Track your contributions carefully, especially if you hold ISAs with multiple providers.
Assuming all ISAs are flexible: Most standard ISAs are not flexible. If you withdraw from a non-flexible ISA, that allowance is gone for the year — you cannot replace it within the same tax year even if you have unused allowance remaining.
Missing the deadline: ISA platforms can be very busy in late March. Ensure any deposits clear before 5 April — a failed or delayed payment can cause you to miss the deadline.
A married couple can each subscribe up to £20,000 per year — £40,000 combined. Each of their children can hold a Junior ISA with a £9,000 annual allowance. A family of four could shelter £58,000 per year across ISAs. Over a decade, this amounts to £580,000 in tax-free wrappers — generating dividends, interest, and capital gains entirely free from UK tax.
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