Reporting CGT on Self Assessment

Key facts

Capital gains are reported on the SA108 (Capital Gains Summary) pages of your Self Assessment return.
You must report if your total gains exceed the £3,000 Annual Exempt Amount, or total disposal proceeds exceed 4 × the AEA (£12,000).
The filing deadline for online Self Assessment returns is 31 January after the end of the tax year.
If you filed a 60-day property return, you still need to include the gain on your SA return — with credit for tax already paid.
You can report capital losses on your return even if you have no gains — protecting them for future use.

When You Must Report Capital Gains

You must complete the SA108 capital gains pages of your Self Assessment return if any of the following apply during the tax year:^[1]

Your total chargeable gains (before losses and the AEA) exceed the £3,000 Annual Exempt Amount
Your total disposal proceeds exceed £12,000 (4 × the AEA of £3,000)
You want to claim an allowable capital loss
You want to claim a CGT relief or exemption (such as BADR, holdover relief, or rollover relief)
You have already filed a 60-day CGT property return

The £12,000 proceeds rule: Even if your gain is below the AEA, you must still report if total disposal proceeds exceed £12,000 in the tax year. This catches situations where you sell high-value assets but make a small or zero gain — for example, selling shares for £15,000 that cost £14,500.^[3]

The SA108 Capital Gains Pages

SA108 is the supplementary form for reporting capital gains and losses. If you file online (which most people do), the SA108 sections are built into the online return and you simply complete the relevant screens.^[1]

The SA108 is divided into sections covering different asset types:

Section	What It Covers
Listed shares and securities	Disposals of shares listed on a recognised stock exchange
Unlisted shares and securities	Disposals of shares in private companies, AIM-listed shares
Property and other assets	UK and overseas property, chattels, crypto assets, and any other chargeable disposals
Additional information	BADR claims, Investors’ Relief, holdover relief, and other reliefs

Information You Need

For each disposal, you will need to provide:

Description of the asset (e.g. “200 ordinary shares in XYZ plc” or “34 High Street, Anytown”)
Date of disposal
Disposal proceeds (or market value if gifted or sold to a connected person)
Acquisition date
Acquisition cost (or market value at acquisition)
Allowable costs (improvement expenditure, incidental costs of acquisition and disposal)
Any reliefs claimed (PRR, BADR, holdover, rollover, etc.)

Tip: Keep records of all share purchases and property costs as you go. Trying to reconstruct acquisition costs years later is difficult, and without evidence HMRC may challenge your figures. You must keep CGT records for at least 5 years and 10 months after the end of the tax year of disposal.

Reporting Losses

Capital losses should be reported even if you have no gains in the year. This is essential because:^[3]

Losses must be claimed within 4 years of the end of the tax year they arise
Unreported losses cannot be carried forward and may be permanently lost
Current-year losses must be offset against current-year gains (even if this wastes the AEA)
Excess losses carry forward indefinitely to offset future gains

On the SA108 pages, you enter the total of your losses in the current year and also any brought-forward losses you are using. HMRC tracks your cumulative brought-forward loss balance from return to return.

Interaction with the 60-Day Property Return

If you sold UK residential property and filed a 60-day CGT property return, you still need to include the gain on your annual SA return. The process is:^[2]

Report the property disposal on SA108 in the normal way
Enter the amount of CGT already paid via the 60-day return in the designated box
HMRC calculates your overall CGT for the year, taking into account all gains, losses, the AEA, and your income (which determines the rate)
Any overpayment is refunded; any underpayment is added to your Self Assessment bill

Why might the amount change? When you filed the 60-day return, you may have estimated your income for the year. By the time you file your SA return, your actual income is known. If your income is different from the estimate, the CGT rate may change (from 18% to 24% or vice versa), resulting in an adjustment.^[2]

The Real Time Capital Gains Service

HMRC offers a real time capital gains service as part of your Government Gateway account. This allows you to:

Report gains and losses during the tax year, rather than waiting until you file your return
See a running total of gains, losses, and remaining AEA
Calculate provisional CGT liabilities

This is separate from the 60-day property return. Using the real time service does not replace your obligation to complete SA108 on your annual return.

Filing Deadlines and Penalties

Filing Method	Deadline
Online Self Assessment return	31 January after the end of the tax year (e.g. 31 January 2027 for 2025/26)
Paper Self Assessment return	31 October after the end of the tax year

Late filing penalties for Self Assessment are:^[5]

Delay	Penalty
1 day late	£100 fixed penalty
3 months late	£10 per day for up to 90 days (maximum £900)
6 months late	£300 or 5% of the tax due (whichever is greater)
12 months late	Further £300 or 5% of the tax due (whichever is greater)

CGT payment is also due by 31 January (for non-property gains). Interest runs on late payments at the Bank of England base rate plus 2.5%.

Paper vs Online Filing

Most taxpayers file their Self Assessment return online, but paper returns are still accepted (with an earlier deadline of 31 October). Key differences:

Online: HMRC calculates the tax automatically; you can see the calculation immediately; deadline is 31 January
Paper: You must submit by 31 October if you want HMRC to calculate the tax for you; you complete the SA108 pages by hand
Either method: You must include all disposals, claim all losses and reliefs, and declare any 60-day return amounts

Frequently Asked Questions

When do I need to report capital gains on Self Assessment?

You must report if your total chargeable gains for the tax year exceed the £3,000 Annual Exempt Amount, or if total disposal proceeds exceed £12,000 (4 × the AEA). You must also report if you want to claim an allowable capital loss or if you have previously been sent a tax return by HMRC.

What is the SA108 form?

SA108 is the Capital Gains Summary supplementary pages of the Self Assessment tax return. It covers all types of capital gains — shares, property, crypto, and other assets. If you file online, the SA108 pages are integrated into the return.

Do I still report on SA108 if I filed a 60-day property return?

Yes. The 60-day return is an interim report. You must also include the gain on your annual SA108 pages and enter the amount of CGT already paid via the 60-day return. HMRC will give credit for the tax paid, and any overpayment or underpayment will be adjusted.

What happens if I miss the Self Assessment deadline?

If you miss the 31 January online filing deadline, you will face an initial £100 late filing penalty. Further penalties apply after 3 months, 6 months, and 12 months. Interest also runs on any unpaid CGT from the due date.

Sources

Self Assessment: Capital gains summary (SA108) — HMRC
Report and pay Capital Gains Tax — GOV.UK
Capital Gains Tax: what you pay it on, rates and allowances — GOV.UK
HS275 Entrepreneurs’ Relief — HMRC
Self Assessment tax returns — GOV.UK