Why Pension Savers Keep Being Overtaxed
The root cause is something called an emergency tax code. When you take a taxable lump sum from your pension for the first time in a tax year, your provider usually doesn't have an up to date tax code for you on file.
Rather than wait, HMRC applies a temporary emergency code instead, and that code makes an assumption that isn't very kind to you: it treats your one-off withdrawal as if you're going to take the exact same amount every single month for the rest of the tax year.
So a single withdrawal of £10,000 can end up taxed as though your annual income is £120,000, pushing you into a much higher tax band than you actually belong in and leaving you noticeably short on the money you were relying on. It's not a mistake on HMRC's part exactly, more a built-in quirk of how the system handles a first withdrawal, but it means thousands of people are routinely charged far more than they owe and then have to actively ask for it back.
You can read more about how emergency tax codes work and why they catch out so many people, not just pension savers, throughout the year