What is an ISA and how do the allowances work?
An Individual Savings Account is a UK tax wrapper. Money inside it grows free of income tax and capital gains tax, and withdrawals are tax-free. It is not an investment itself — it is a container that changes the tax treatment of whatever is inside.
The allowance. There is an annual limit on how much you can pay in across all your ISAs combined in a tax year (6 April to 5 April). It has been £20,000 for several years, though limits change and should be checked.
Crucially, the allowance applies to contributions, not balances. Existing ISA holdings do not count against it, so a portfolio built over years can far exceed the annual figure.
Unused allowance does not carry forward. Miss a year and it is gone.
The main types:
Cash ISA — a savings account in an ISA wrapper. Interest is tax-free.
Stocks and shares ISA — investments held in the wrapper. No CGT on gains, no tax on dividends.
Lifetime ISA (LISA) — for buying a first home or for retirement, with a lower annual limit and a 25% government bonus on contributions. Available to those aged 18–39 to open. Withdrawing for any other purpose before 60 incurs a withdrawal charge that can leave you with less than you put in, which is a genuine trap.
Innovative Finance ISA — peer-to-peer lending, notably higher risk with no FSCS protection on the investments.
Junior ISA — for under-18s, with its own separate allowance, locked until 18.
Points worth knowing: rules on paying into multiple ISAs of the same type have been relaxed; always transfer rather than withdraw and re-deposit, since withdrawing loses the wrapper; flexible cash ISAs let you withdraw and replace within the year without using allowance.
Whether an ISA benefits you depends on your tax position — the Personal Savings Allowance already shelters some interest. This is general information, not advice.