How do I report Capital Gains Tax in my Self Assessment?

This guide explains how to report Capital Gains Tax in your Self Assessment including calculating gains, entering figures correctly, using losses and understanding HMRC requirements.

If you have sold or given away an asset that has increased in value you may need to report Capital Gains Tax through Self Assessment. Many people worry about this process because the forms look complicated at first glance, yet in my opinion reporting Capital Gains Tax is actually quite straightforward once you understand what HMRC wants. The key is to gather the right information before you start, understand the difference between gains and sale prices, and follow the correct steps in the return.

This guide explains how to report Capital Gains Tax in your Self Assessment from start to finish. It covers which gains must be reported, how to calculate the gain, what HMRC requires, how to enter the figures correctly in the online tax return, how the tax is calculated, when payment is due, and the common mistakes you should avoid. Whether you sold shares, crypto, property or inherited assets, this guide will help you complete your return confidently.

First: who must report Capital Gains Tax?

You must report Capital Gains Tax to HMRC if any of the following apply:

  • Your total gains before applying your allowance exceed the Annual Exempt Amount

  • You disposed of an asset with proceeds over £50,000 even if the gain is small

  • You sold UK residential property that triggered a Capital Gains Tax bill

  • You sold cryptoassets that generated a gain

  • You gave away an asset other than to a spouse

  • You are claiming a loss to carry forward

  • HMRC specifically asked you to file a Self Assessment return

Annual Exempt Amount

The Capital Gains Tax allowance is currently £3,000 per person.

If your total gains for the year exceed this amount you must report them.

In my opinion

Even if your gains fall within the allowance you should still report them if you want to carry losses forward because HMRC only recognises losses that are reported.

Before you start: gather your information

Reporting CGT is much easier if you collect all your documents first. You will need:

1. Sale proceeds

The amount you sold the asset for.

2. Purchase price

The amount you originally paid for the asset.

3. Associated costs

These reduce your CGT bill. They include:

  • Broker fees

  • Stamp duty

  • Legal fees

  • Improvement costs for property

  • Transfer fees

  • Valuation fees

4. Dates of purchase and sale

You must enter these exactly.

5. Evidence of losses

You can offset losses from the same tax year or from previous years.

6. Ownership history

If the asset was transferred from a spouse you take on their purchase price.

7. Special allowances

Such as Business Asset Disposal Relief or private residence relief.

In my experience the biggest delays happen because people begin the return without having all the information they need.

Step by step: how to report Capital Gains Tax in the Self Assessment return

Below is the full process for reporting your gains.

Step 1: Start your Self Assessment return online

Log into your HMRC account and begin a new return for the correct tax year.

Go to:

Income Tax → Self Assessment → Complete your return

You must select the section that states:

“Tell us about your capital gains”

This activates the Capital Gains pages.

Step 2: Confirm whether you disposed of any assets

HMRC will ask:

“Did you dispose of any chargeable assets?”

You must answer Yes if:

  • You sold an asset

  • You gave away an asset

  • You received compensation for an asset

  • You exchanged one asset for another

Answering No when you did in fact dispose of something is one of the most common errors people make.

Step 3: Enter property gains (if applicable)

If you sold UK residential property you must complete the property disposal section.

You will enter:

  • Address

  • Date purchased

  • Date sold

  • Sale proceeds

  • Purchase costs

  • Allowable costs

  • Private residence relief if appropriate

  • Capital gains already reported through the 60 day CGT return

Important

If you filed a 60 day CGT return you must still report the disposal in Self Assessment. The tax you paid earlier will be deducted from your final CGT bill.

Step 4: Enter gains on shares or investments

This section is used for shares, funds, ETFs and similar assets.

You will enter:

  • Number of disposals

  • Sale proceeds

  • Purchase costs

  • Allowable costs

  • Gains or losses

  • Whether you used pooled share calculations

If you have multiple share disposals you must calculate the pooled cost basis yourself or via your broker.

In my opinion most mistakes happen here because people do not use the pooled cost method correctly.

Step 5: Enter gains on cryptoassets

Cryptoassets are treated like shares for CGT purposes. You must include:

  • Date sold or exchanged

  • Proceeds received

  • Original cost

  • Costs such as exchange fees

  • Total gain for the year

  • Losses to carry forward

If you made dozens of small trades you can enter the total gains as a single figure.

Step 6: Enter other asset disposals

This includes:

  • Land

  • Second homes

  • Business assets

  • Collectibles worth more than £6,000

  • Art

  • Jewellery

  • Vehicles used for business

  • Inherited assets

Enter all details exactly as HMRC requests.

Step 7: Enter losses

You can enter losses from the same tax year or from previous years.

Reporting losses is essential because:

  • They reduce your CGT bill

  • They can be carried forward indefinitely

  • HMRC only recognises losses if you report them

Example

You have £5,000 of losses carried forward.
You sell shares and make a gain of £8,000.
Your taxable gain becomes £3,000 which is below the allowance so no CGT is due.

In my opinion many people forget to claim losses which increases their tax unnecessarily.

Step 8: Apply reliefs

Reliefs that may apply include:

  • Private residence relief

  • Business Asset Disposal Relief

  • Investor relief

  • EIS and SEIS sale relief

  • Rollover relief

These reliefs can significantly reduce or eliminate your tax.

Step 9: Review the HMRC calculation

HMRC will calculate:

  • Your total gains

  • Your total losses

  • Your taxable gain

  • Your Capital Gains Tax bill

Check this carefully. If your income crosses into higher rate band HMRC will divide the gain between basic rate CGT and higher rate CGT.

CGT rates

  • 10 percent for most assets in basic rate band

  • 20 percent for most assets in higher rate band

  • 18 percent for residential property basic rate

  • 28 percent for residential property higher rate

Step 10: Submit your return

Once you review everything you can submit the return.

HMRC will confirm:

  • Tax due

  • Due date

  • Any adjustments

When do you pay the Capital Gains Tax?

For residential property

You must pay within 60 days of completion using a separate CGT on property return.

For all other gains

You pay your CGT bill by:

  • 31 January following the end of the tax year

  • At the same time as the rest of your Self Assessment tax

Example

CGT for gains in the 2024 to 2025 tax year is due by 31 January 2026.

Common mistakes when reporting CGT in Self Assessment

Mistake 1: Reporting sale proceeds instead of gains

HMRC wants the gain, not the sale price.

Mistake 2: Forgetting to deduct allowable costs

Legal fees and broker fees reduce your CGT bill.

Mistake 3: Not reporting the disposal because the gain is small

Disposals over £50,000 must be reported even if the gain is small.

Mistake 4: Forgetting previous losses

This often leads to unnecessary tax.

Mistake 5: Not reporting a disposal already declared via the 60 day CGT return

HMRC must see both.

Mistake 6: Incorrect pooled cost for shares

Using the wrong cost basis inflates the gain.

In my opinion

Most CGT errors come from missing information, not deliberate mistakes.

Real world examples

Example 1: Reporting share gains

Tom sold £12,000 worth of shares with a gain of £4,500. He had no other disposals. He entered this in the shares section and paid CGT on £1,500 after his allowance.

Example 2: Reporting a rental property

Sarah sold a rental property and filed the 60 day CGT return. She included the gain again in Self Assessment, entered the tax already paid, and HMRC deducted the earlier payment.

Example 3: Using losses

Amir had £8,000 of losses from crypto in previous years. He sold shares and made a gain of £5,000. His Self Assessment showed no tax due once losses were applied.

Example 4: Basic rate and higher rate split

Claire had a large disposal and part of her gain fell into the basic rate band at 10 percent while the rest was taxed at 20 percent. HMRC calculated this automatically.

In my opinion: the essential steps to follow

If I had to simplify the process it would be:

  1. Calculate your gains properly.

  2. Include all relevant allowable costs.

  3. Report losses so you do not lose them.

  4. Enter the figures correctly in the CGT section.

  5. Check the final HMRC calculation.

  6. Pay by the correct deadline.

Once you understand these steps the process becomes much easier.

Final thoughts

Reporting Capital Gains Tax in your Self Assessment is not as daunting as it seems. The key is understanding what HMRC is asking for and keeping clear records of your acquisitions, costs and disposals. Whether you are reporting property, shares, crypto, business assets or personal possessions, the process follows the same structure. In my opinion the most important thing you can do is calculate your gains accurately and use any losses or allowances available to you so you never pay more tax than necessary.