he SA108 form is an extra page that sometimes needs to be sent to HMRC with your Self Assessment tax return. You’ll need to deal with the SA108 (or its online equivalent) if you’ve made a profit from selling assets such as shares, a second property, or cryptocurrency.
But here’s the key point:
You only complete a physical SA108 form if you are filing a paper tax return.
If you file your Self Assessment online, you won’t submit a separate SA108. Instead, you’ll fill in the Capital Gains section digitally.
In this guide, we’ll explain:
- When the SA108 form is required
- What counts as a capital gain
- How to complete it properly
- How it links to the 60‑day property reporting rules
What Is the SA108 Form?
The SA108 form is the Capital Gains section of a Self Assessment tax return.
It’s not a separate tax return on its own. It simply reports any capital gains or losses you made during the tax year.
If you’re filing by post, you download the SA108 pages for the relevant tax year, complete them, and send them to HMRC along with your main tax return (SA100).
If you’re filing online, the same questions appear within the digital return, so there’s no need to print or attach anything separately.
The form allows you to report:
- Profits from selling shares (outside an ISA)
- Sales of second homes or buy‑to‑let properties
- Cryptocurrency disposals
- Sales of valuable personal possessions
- Capital losses
- Claims for certain tax reliefs
In short, if you’ve made a taxable capital gain, this is where it gets reported.
When Do You Need to Use the SA108 Form?
You’ll need to complete the SA108 pages if:
- You are filing a paper Self Assessment return, and
- You made capital gains or losses during the tax year
A capital gain is the profit you make when you dispose of an asset.
“Dispose” doesn’t just mean sell. It can also mean:
- Gifting an asset to someone (other than your spouse or civil partner)
- Swapping it for something else
- Receiving compensation for it
Common situations where SA108 is required
You may need to report gains if you have:
- Sold shares that weren’t held in an ISA
- Sold a second property or buy‑to‑let
- Sold cryptocurrency
- Sold valuable items like art, antiques, or jewellery
- Made gains above the £3,000 annual Capital Gains Tax allowance
- Made losses and want to carry them forward
Even if no tax is ultimately due, you may still need to report the disposal depending on the amounts involved.
Capital Gains Tax Rates and Allowances (2025/26)
Before you complete the SA108 form, it helps to understand how Capital Gains Tax (CGT) is worked out.
The Capital Gains Tax allowance
For the 2025/26 tax year, the Capital Gains Tax allowance is:
| Individuals | Most Trustees |
| £3,000 | £1,500 |
This means you only pay tax on gains above this amount.
So if your total gains for the year are £8,000 and you have no losses, you would normally only pay tax on £5,000 (£8,000 minus the £3,000 allowance).
What tax rate will you pay?
The rate depends on two things:
- What you sold
- How much other taxable income you have
Your capital gains are added on top of your income to see which tax band they fall into.
Shares and most other assets
- 10% if the gain falls within the basic rate band
- 20% if it falls within the higher or additional rate band
Residential property (that isn’t fully exempt)
- 18% within the basic rate band
- 24% within the higher or additional rate band
It’s not always a simple flat rate. If your income sits near a tax band threshold, part of your gain may be taxed at one rate and part at another.
That’s why it’s important to calculate the figures properly before submitting your SA108 form.
How to Complete the SA108 Form Step by Step
If you’re filing a paper tax return, you’ll need to download the SA108 pages for the correct tax year from GOV.UK and send them in with your SA100 form.
Here’s how to approach it clearly and calmly.
Step 1: Gather your information
For each asset you disposed of, you’ll need:
- The date you bought it
- The date you sold or disposed of it
- The amount you sold it for (or its market value if gifted)
- What you originally paid
- Any related costs (legal fees, broker fees, Stamp Duty, etc.)
- The cost of any improvements
- Details of any capital losses
Having this ready before you start makes the form much easier to complete.
Step 2: Work out your gain
The basic calculation is:
Sale proceeds – purchase cost – allowable costs and improvements = your gain
You can then:
- Deduct any capital losses from the same year
- Deduct unused losses from previous years (if properly claimed)
- Apply the £3,000 annual allowance
What’s left is your taxable gain.
If you already reported a property sale under the 60‑day rule, you must include the details again here so everything is reconciled correctly.
Step 3: Fill in the relevant sections
The SA108 form is split into sections depending on what you sold, such as:
- Shares and securities
- Residential property
- Other assets
- Losses and claims for relief
You only complete the parts that apply to you.
The form usually asks for totals rather than detailed breakdowns, but you should keep your full calculations in case HMRC asks for them later.
Step 4: Submit on time
If filing by post:
- The deadline is 31 October following the end of the tax year.
If filing online:
- The deadline is 31 January.
Any Capital Gains Tax due is normally payable by 31 January (unless it was already paid earlier under the 60‑day property rule).
Missing deadlines can lead to penalties and interest, so it’s worth double‑checking everything before sending it off.
Do You Need the SA108 Form If You File Online?
No, not as a separate document.
If you complete your Self Assessment online, the Capital Gains section is built into the digital return. You’ll answer a series of questions and enter your figures there.
So:
- Paper return → You must complete and attach the SA108 pages.
- Online return → You complete the Capital Gains section digitally instead.
The information required is broadly the same, it’s just submitted differently.
If you’ve already reported a property sale using the 60‑day service, you still need to include it in your Self Assessment so everything is properly finalised for the year.
The 60‑Day Property Reporting Rule Made Simple
If you sell a UK residential property and Capital Gains Tax is due, you usually cannot wait until your Self Assessment deadline.
You must:
- Report the gain within 60 days of completion, and
- Pay an estimate of the Capital Gains Tax due
This applies to:
- Buy‑to‑let properties
- Second homes
- Inherited properties that aren’t fully exempt
If you’re not sure whether tax is due, it’s important to check. Missing the 60‑day deadline can trigger penalties and interest.
How this links to the SA108 form
If you also complete a Self Assessment tax return, you must still include the property disposal in your SA108 (or online equivalent).
Think of the 60‑day report as an early payment.
Your Self Assessment return then confirms the final position for the year.
Common Mistakes to Avoid
The form itself isn’t usually the difficult part. The mistakes tend to happen in the calculations.
Here are some of the most common issues:
Forgetting allowable costs
Legal fees, Stamp Duty, estate agent fees and improvement costs can usually reduce your gain. If you don’t include them, you may overpay tax.
Not reporting capital losses
Losses can reduce your bill, but only if they’re properly declared.
Using the wrong tax rate
Your income affects how much of your gain is taxed at each rate. It isn’t always a flat 10% or 20%.
Missing the 60‑day deadline
This is one of the most common causes of penalties.
Getting cryptocurrency reporting wrong
Crypto transactions can be complex. Swapping one token for another can trigger a taxable disposal.
If you’re unsure about any of these areas, it’s better to check before submitting your return.
Can You Complete the SA108 Form Yourself?
In straightforward cases, yes.
If you’ve made a single disposal, have clear records and no complicated reliefs to claim, completing the form yourself may be manageable.
However, things become more technical when:
- You’ve made multiple disposals
- You’re using brought‑forward losses
- Your income pushes part of the gain into a higher tax band
- You’re dealing with property that was partly your home and partly rented
- You’re claiming Business Asset Disposal Relief
- You have complex crypto activity
The biggest risk isn’t filling in the form incorrectly, it’s calculating the gain incorrectly.
If you’re uncertain, getting expert advice before submitting can prevent unnecessary tax or avoidable penalties.
SA108 FAQs
Is the SA108 form always required?
No. It’s only required as a physical document if you’re filing a paper Self Assessment return and need to report capital gains or losses.
What is the deadline for submitting the SA108?
If filing by post, the deadline is 31 October.
If filing online, it’s 31 January.
Tax due is usually payable by 31 January.
Do I need to report gains below £3,000?
Possibly. Even if no tax is due after the annual allowance, you may still need to report the disposal depending on the amounts involved.
Can I amend my Capital Gains figures after filing?
Yes. You can usually amend your Self Assessment return within 12 months of the filing deadline.
What happens if I miss the 60‑day property deadline?
Penalties and interest may apply, even if you later report the gain on your Self Assessment return.