If you have a cash ISA, you won’t lose any tax benefits by making withdrawals, but fees and penalties may apply. With an instant access cash ISA, you can withdraw money without any restriction. With a fixed term cash ISA you will have to pay a penalty to access your money, typically a certain number of days’ interest. If your ISA is ‘flexible’ you can withdraw money and replace it during the same tax year without it affecting your current year’s allowance.