How to report capital gains on your Self Assessment

Capital gains go on their own section of the tax return, the SA108, and whether you need to fill it in at all follows two tests. Gains above the £3,000 allowance, obviously. Less obviously, total sale proceeds above £50,000, even when the gains underneath are small or nil, per the reporting rules. Either test alone puts capital gains on your return.

Here is how the reporting actually works, and where people get caught.

The SA108 and what goes on it

The capital gains summary pages want your disposals grouped by type, listed shares in one section, property in another, other assets, which includes crypto, in their own. For each group you report numbers of disposals, total proceeds, total costs, gains and losses. Behind those summary boxes sits the expectation of computations, your working for each disposal, which you attach or hold ready. The summary is easy. The computations are where the year of your financial life gets reconstructed, and they are what HMRC asks for when it checks.

Losses go here too, and this matters more than it looks. A loss only becomes usable once it is claimed, and the return is the normal place to claim it. Skip reporting in a loss year because “there was no tax to pay” and you have failed to bank the one valuable thing that year produced. The four year window and the mechanics get their own treatment in tomorrow’s companion piece on losses.

What does not go through the return

UK residential property gains run on their own track. Tax due on a second home or rental sale must be reported and paid within 60 days of completion through HMRC’s property service, months before any tax return would be due. The disposal then appears on your SA108 as well, with the tax already paid credited against the year. The 60 day deadline catches people precisely because they assumed the return covered it.

There is also a standalone reporting route for people not otherwise in Self Assessment, HMRC’s real time capital gains service, which lets you report a gain without joining the annual return cycle. Useful for the one-off disposal in an otherwise PAYE life.

The paperwork that decides enquiries

Every number on an SA108 is a claim about acquisition cost, and acquisition cost is where the whole edifice rests. Contract notes, completion statements, exchange records, the paper trail from when you bought. HMRC’s guidance on what records to keep is not bureaucratic filler, it is the difference between an enquiry that closes in a letter and one that runs a year. For shares the broker usually holds the history. For crypto, the history is scattered across platforms that may no longer exist, which is why the computation work dwarfs the filing work and why software-generated figures need checking before they become sworn statements on a return.

Filing itself follows the normal Self Assessment calendar, online by 31 January with payment. If you are new to the system, registration comes first, by 5 October, which we covered in how to register for Self Assessment.

Reporting questions

My gains were under £3,000. Do I report at all?

If you are in Self Assessment and proceeds passed £50,000, yes, tax-free or not. Outside Self Assessment with gains under the allowance and no property involved, generally no.

Where does crypto go on the SA108?

The other property, assets and gains section, not listed shares. Each swap, sale and spend is a disposal, aggregated into the summary boxes with computations behind them.

I already paid property tax within 60 days. Why report again?

The 60 day payment is an on-account process. The disposal still belongs on your annual return, where the year’s full picture, other gains, losses, allowance, settles the final figure and credits what you paid.

Can I amend a return I got wrong?

Within twelve months of the filing deadline, yes, directly. Older than that, the disclosure route takes over, which we walked through in the Digital Disclosure Service guide.

If this year’s SA108 involves reconstructing anything, a crypto history, a lost purchase record, an inherited holding, start earlier than feels necessary. Checking a computation is quick. Building one is not. Both are things our free review can scope for you.