ISAs are one of the best savings tools available to UK residents — but the rules have sharp edges. Here are five mistakes that regularly catch people out, and how to avoid them.
Mistake 1: Exceeding the £20,000 Annual Allowance
You can deposit up to £20,000 total across all ISA types in one tax year. Exceed this and HMRC will void the excess deposit — you lose the tax-free status on the overage and may face a penalty. This catches people with multiple ISAs (Cash ISA, Stocks & Shares ISA, LISA) who lose track of their total contributions. Your ISA providers don't automatically communicate with each other — you're responsible for tracking your own total.
Mistake 2: Reopening a Closed ISA in the Wrong Way
If you withdraw from a standard ISA, you cannot simply 'put it back' and retain the tax-free status — you're using a new deposit against your annual allowance. Exception: Flexible ISAs allow you to withdraw and replace in the same tax year without affecting your allowance. Check whether your ISA is flexible before assuming you can dip in and out freely.
Mistakes 3, 4 and 5
Mistake 3: Using a LISA for a property over £450,000. The Lifetime ISA property purchase cap is £450,000. Buy above this and you pay the 25% withdrawal penalty. With London property prices, this is a real risk — always check the property's value before committing to LISA funds. Mistake 4: Taking a LISA payout before 12 months. You must have held the LISA for at least 12 months before using it to buy a home. Open it now even if you're not buying yet — the clock needs to start. Mistake 5: Forgetting the ISA transfer rules. To move money between ISA providers while retaining tax-free status, you must use an official ISA transfer — not withdraw and redeposit. Using the wrong process can mean you lose the tax-free wrapper on years of savings.
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