HMRC has confirmed that its Self Assessment system does not automatically apply the split Capital Gains Tax rates that took effect on 30 October 2024. Anyone who sold assets in the 2024 to 2025 tax year may need a Capital Gains Tax Self Assessment adjustment to avoid overpaying.
Key figures at a glance:
- The main Capital Gains Tax rates rose on 30 October 2024, from 10% to 18% for basic rate and 20% to 24% for higher rate.
- The 2024/25 Self Assessment online system uses a single CGT rate for the whole tax year and cannot split disposals either side of 30 October.
- Business Asset Disposal Relief was 10% until 5 April 2025, rose to 14% on 6 April 2025 and rises to 18% on 6 April 2026.
- Taxpayers have until 31 January 2027 to amend a 2024/25 online return, and up to four years to claim overpayment relief after that.
Why the 2024/25 rates changed mid-year
At the Autumn Budget on 30 October 2024, the government raised the main Capital Gains Tax rates with immediate effect. The lower rate jumped from 10% to 18%, and the higher rate rose from 20% to 24%.
Residential property rates were not changed, staying at 18% and 24%. Business Asset Disposal Relief was 10% until 5 April 2025, rose to 14% that day and rises again to 18% from 6 April 2026.
Making a Capital Gains Tax Self Assessment adjustment
HMRC has published an adjustment calculator on GOV.UK for anyone completing a 2024/25 return as an individual, trustee or personal representative. It applies to those whose total gains exceed the annual exempt amount.
The calculator produces an adjustment figure that is entered into the return alongside a written statement. Without this step, a gain after 30 October 2024 could be taxed at the wrong rate — potentially leaving the taxpayer owed a refund.
Taxpayers using commercial software should confirm the correct split rates were applied to each disposal. HMRC’s own adjustment guidance is available on GOV.UK.
What to do if you may have overpaid
Anyone who sold shares, crypto, a second home, a buy-to-let property or business assets during the 2024 to 2025 tax year should review their return. The risk is highest where disposals fall on either side of 30 October 2024.
If your 2024/25 return used a single CGT rate:
- Use HMRC’s Capital Gains Tax adjustment calculator on GOV.UK to work out the correct figure.
- Log in to your Self Assessment account via the personal tax account and amend the relevant boxes on the 2024 to 2025 return.
- Attach the adjustment statement as required and submit the amendment by 31 January 2027.
Missing the 31 January 2027 deadline does not entirely close the door on a refund. Overpayment relief claims can still be made in writing for up to four years after the end of the 2024 to 2025 tax year.
Could you be owed a refund?
Overpayments can happen where too much tax was paid on gains made before 30 October 2024. They can also occur where the return used default rates rather than the split calculation.
HMRC reported more than 135,500 scam referrals in the 10 months to late 2025, including around 29,000 referring to fake tax refund claims. Genuine refunds are handled through a Self Assessment amendment or overpayment relief, not via links in unsolicited messages.
Myrtle Lloyd, HMRC’s Chief Customer Officer, has urged Self Assessment filers to use official channels only. A professional review of your Capital Gains Tax position for the 2024 to 2025 tax year can confirm whether an adjustment is needed.
Our Capital Gains Tax guide sets out how the split rates apply and when a refund claim might be appropriate.
Key Takeaways
- The main Capital Gains Tax rates rose at the Autumn Budget on 30 October 2024, creating a split tax year.
- HMRC’s Self Assessment system does not automatically apply the two rates, so a Capital Gains Tax Self Assessment adjustment is often needed on 2024/25 returns.
- A dedicated adjustment calculator on GOV.UK works out the figure to enter on the return.
- Online returns for 2024/25 can be amended until 31 January 2027, with overpayment relief available for up to four years after that.
- Scam reports referring to fake tax refund claims reached around 29,000 in the 10 months to late 2025, so only use official channels to claim.
