The story started in October 2024, when Rachel Reeves announced a surprise decision: Capital Gains Tax would increase immediately from 30 October 2024, instead of waiting for the new tax year as usual. I remember how everyone complained about it at the time.
New rates:
Basic rate taxpayers: 10 percent → 18 percent
Higher rate taxpayers: 20 percent → 24 percent
The issue was this: HMRC’s online tax return software for 2024–25 had been programmed long before the rule changed, and it wasn’t something that could be quickly rewritten. As a result, if you sold shares or property after 30 October and filed your return online, the system continued to use the old rates.
This means the tax shown on your return could be incorrect.
And the consequence is that you may be held responsible:
Paying the shortfall
Interest for late payment (8% a year)
A penalty of up to 30 percent if HMRC considers the mistake “careless”
The truth is, taxpayers naturally expect the government’s official system to be accurate. No one imagines that HMRC’s own software could lag behind the law. Most people don’t have the time to review every new rule; they simply rely on the system to get it right. Yet in the end, if something goes wrong, it might still fall on them.
At the moment, HMRC has released an adjustment tool and has started sending reminder letters to anyone whose return appears to contain errors. Taxpayers will have 30 days to check and respond.
If you’re filing CGT, check your figures as soon as you can. Liệu will leave the link to HMRC’s recalculation tool in the comments.
And if you do end up with a penalty, there’s still a chance to appeal, although late-payment interest is unlikely to be waived.