Capital Gains Tax Self Assessment: Rates, Deadlines, and Reliefs

If you made a taxable profit selling shares, a second property, or other valuable assets during the tax year, you report and pay Capital Gains Tax through Self Assessment. For the 2024/25 tax year, the online return and any tax owed are both due by 31 January 2026, and the annual tax-free allowance is £3,000.1GOV.UK. Capital Gains Tax: What You Pay It On, Rates and Allowances You calculate the gain, enter it on the capital gains pages of your return, and pay what you owe. Miss the deadline and penalties apply automatically, regardless of how small the bill.

When You Need to Report a Gain

You must complete the capital gains pages of your Self Assessment return if your total gains for the year, before deducting losses, exceed the £3,000 annual exempt amount.1GOV.UK. Capital Gains Tax: What You Pay It On, Rates and Allowances You also need to report if your total proceeds from all disposals exceed £50,000, even where the gains themselves fall below the allowance. The one exception is a home fully covered by Private Residence Relief, which does not need to appear on the capital gains pages.

If you have never filed a return before and now have a CGT liability, you are legally required to notify HMRC within six months of the end of the tax year in which the gain arose. That means by 5 October.2Legislation.gov.uk. Taxes Management Act 1970 – Section 7 You register using form SA1, either through the Government Gateway or on paper.3GOV.UK. Register for Self Assessment if You Are Not Self-Employed Register early. Waiting for your Unique Taxpayer Reference to arrive can take several weeks, and you cannot file without it.

Key Deadlines

The Self Assessment year runs to 5 April, and the same calendar applies every year. For the 2024/25 tax year:

One important boundary: selling a UK residential property triggers a separate, faster reporting duty. You must report the disposal and pay any CGT within 60 days of completion through HMRC’s Capital Gains Tax on UK property service, which is not the Self Assessment portal.5GOV.UK. Report and Pay Your Capital Gains Tax – If You Sold a Property in the UK on or After 6 April 2020 If you also file Self Assessment, the same gain still goes on your annual return, where HMRC reconciles the two and adjusts any over- or underpayment.

Penalties for Missing the Deadline

A return filed one day late attracts an automatic £100 penalty. After three months, HMRC adds £10 per day for up to 90 days, reaching a maximum of £900. At six months late, a further penalty of 5% of the tax due or £300 applies, whichever is greater. At twelve months, the same again.6GOV.UK. Self Assessment Tax Returns – Penalties Interest also runs on unpaid tax, at 7.75% as of January 2026.7GOV.UK. HMRC Interest Rates for Late and Early Payments

What Rate You’ll Pay

Following the Autumn Budget 2024, CGT rates on most assets now match the rates that previously applied only to residential property. For disposals from 30 October 2024 onward, basic-rate taxpayers pay 18% and higher-rate taxpayers pay 24% on gains from shares, personal possessions, and other non-property assets.8GOV.UK. Capital Gains Tax – Rates of Tax Residential property gains continue at the same 18% and 24%.

Which band applies depends on your total taxable income plus the gain. A large gain can push you from the basic-rate band into the higher-rate band, so part is taxed at 18% and the rest at 24%. Trustees and personal representatives pay a flat 24% on all gains.

Two special reliefs carry their own rates. Business Asset Disposal Relief taxes qualifying gains at 14% for disposals between 6 April 2025 and 5 April 2026, rising to 18% from 6 April 2026.9GOV.UK. Business Asset Disposal Relief Investors’ Relief follows the same pattern, at 14% for 2025/26 and 18% from April 2026, with a lifetime limit reduced to £1 million for disposals on or after 30 October 2024.10HM Revenue & Customs. Capital Gains Manual – CG63515 Investors Relief Rates From April 2025 and From April 2026

Working Out the Gain

The formula is simple: sale proceeds, minus what you paid for the asset, minus allowable expenses, equals your gain. What counts as an allowable expense is broader than most people expect. Section 38 of the Taxation of Chargeable Gains Act 1992 sets out three categories:11Legislation.gov.uk. Taxation of Chargeable Gains Act 1992 – Section 38

  • Acquisition costs: the purchase price plus incidental costs like stamp duty, solicitor fees, surveyor fees, and any commission
  • Enhancement expenditure: money spent improving the asset (not maintaining it), where the improvement is still reflected in the asset when you sell. An extension counts; repainting does not.12GOV.UK. Tax When You Sell Your Home – Work Out Your Gain
  • Disposal costs: estate agent fees, solicitor fees, advertising, and valuation fees paid when selling

Interest on a loan used to buy the asset is not deductible. Keep receipts for every cost you claim.

Using Losses

Losses on other disposals can reduce your gains. Current-year losses must be applied first, against current-year gains. If your gains still exceed the £3,000 exempt amount after that, you can then deduct losses carried forward from earlier years, but only enough to bring the gain down to the exempt amount. The rest of the carried-forward losses stay banked for future years.13GOV.UK. Capital Gains Tax: What You Pay It On, Rates and Allowances – Losses

You have four years after the end of the tax year in which the loss arose to claim it. If you are not registered for Self Assessment, you can write to HMRC to report a loss instead. Even in a year with no gains, reporting a loss is worth doing: it locks it in for a future year when you may need it.

Reliefs That Can Reduce or Eliminate the Bill

Private Residence Relief takes your main home out of CGT entirely, provided you lived in it as your only home the whole time you owned it, you never let any of it out (a lodger does not count), you never used any part exclusively for business, the grounds are under 5,000 square metres, and you did not buy it purely to make a profit.14GOV.UK. Tax When You Sell Your Home – Private Residence Relief Married couples and civil partners can only nominate one property as their main home at a time. Where the conditions are only partly met, partial relief may still apply, most commonly for a property that was your home for some years and then let out.

Business Asset Disposal Relief applies when you sell all or part of a qualifying business, or shares in a trading company where you hold at least 5% and work for the company. Gains are taxed at 14% in 2025/26, with a £1 million lifetime limit on qualifying gains.

Investors’ Relief applies to gains on shares in unlisted trading companies that you subscribed for directly and held for at least three years, at the same 14% rate for 2025/26.

Filing the Return

You file through HMRC’s online Self Assessment portal, signing in via Government Gateway or GOV.UK One Login.15GOV.UK. HMRC Online Services – Sign in or Set Up an Account When you tailor the return, tick the box to include capital gains. Your figures then go on the SA108 supplementary pages.16HM Revenue & Customs. Self Assessment – Capital Gains Summary (SA108) The form asks you to categorise each disposal (listed shares, residential property, or other assets) and to enter acquisition costs, disposal proceeds, and any reliefs claimed.

HMRC gives you a submission reference once you file. Save it. Payment is due by the same 31 January deadline, by Direct Debit, online banking, bank transfer, or debit card. Credit cards are not accepted for personal tax. Some payment methods take several working days to clear, and a first-time Direct Debit needs at least five working days, so do not leave payment to the final day.

Records to Keep

HMRC expects you to keep records for at least one year after the filing deadline for the relevant tax year, and longer if you filed late or an enquiry is opened. For CGT that means holding:

  • Purchase contracts and completion statements
  • Sale contracts and estate agent correspondence
  • Receipts for improvement work
  • Solicitor invoices and stamp duty certificates
  • Dividend statements and share transaction confirmations from your broker

If you cannot produce supporting documents during an enquiry, the inspector can reject claimed deductions and recalculate the gain using HMRC’s own figures.

The Enquiry Window

Filing does not close the matter permanently. HMRC can open a formal enquiry within twelve months of the date you filed, provided the return was in by the 31 January deadline.17GOV.UK. Self Assessment Legal Framework – SALF403 Time Limits for Notice of Enquiry If you filed late, the twelve months run from the date HMRC actually received the return, which in practice gives them longer. An enquiry is not an accusation. HMRC runs risk-based checks and random compliance reviews. It does mean they can ask for every document behind every figure, so organised records before filing save the trouble of scrambling later.