Each tax year the Government limits how much you can save tax-free in an ISA. This is called your allowance, and the limit for 2017/18 is £20,000. Since 2016, cash ISAs can be ‘flexible’. This means you can take out and replace funds throughout a tax year without this affecting your ISA allowance.
Because cash ISAs can be flexible, you could end up in a situation where money has gone out and in of your ISA – but you haven’t actually used any of your allowance by the end of the tax year.
When this happens, the next time you try to add funds, you’ll need to fill out an ISA renewal form.