When you must file a self-assessment with crypto activity
HMRC's filing triggers for UK crypto holders are:
- Capital gains: file SA108 if any of these apply in the tax year:
- Net chargeable gains exceed the AEA (£3,000 in 2025/26)
- Total disposal proceeds exceed 4× the AEA (£12,000 in 2025/26) — even if net gain after costs is below £3,000
- You want to claim capital losses to carry forward
- Income from crypto activity: file SA100 (and supplementary pages as relevant) if you received:
- Staking rewards above £1,000 (or below £1,000 if claiming actual expenses rather than the trading allowance)
- Mining income (miscellaneous or trading)
- Airdrops that required action (Uniswap UNI, ENS, ARB etc.) above £1,000
- NFT royalties
- Yield farming or DeFi lending interest treated as income
- Self-employed trading: file SA103 if your activity crossed into commercial trading (frequent NFT flipping, professional mining operation, etc.)
If any of these apply, you must file a UK self-assessment for the tax year. Even if all of these are below thresholds, voluntary filing is allowed and often advisable for record-keeping.
Which forms — SA108 vs SA100 box 17 vs SA103
| Crypto activity | Form / box |
|---|---|
| Buy and sell crypto (investor) | SA108 (Capital Gains Summary) |
| Staking rewards (small scale, misc income) | SA100 box 17 (Other UK Income) |
| Mining (hobbyist / misc) | SA100 box 17 |
| Mining (commercial trading) | SA103 (Self-Employment) + SA108 on later disposals |
| Active NFT flipper crossing into trader | SA103 + possibly SA108 |
| NFT royalties (creator) | SA100 box 17 |
| Action-required airdrops above £1k | SA100 box 17 |
| Foreign exchange holdings with FTC considerations | SA106 (Foreign) + SA108 |
Most UK retail crypto users only need SA108 (CGT). Mixed-activity users may need SA108 + SA100 box 17 together.
Registering for self-assessment — first-time filers
If you've never filed a UK self-assessment before but now need to (because of crypto activity that crosses the thresholds), you must register with HMRC.
- Register by 5 October following the end of the tax year in which you incurred the liability. For 2025/26 (ending 5 April 2026), the deadline is 5 October 2026.
- Register online at gov.uk/register-for-self-assessment. Choose "Not self-employed" if your only income is crypto-related and below trading threshold.
- HMRC will issue you a Unique Taxpayer Reference (UTR) by post within ~10 working days.
- You then enrol for online Self-Assessment using the UTR.
- File the return by 31 January following the tax year (31 Jan 2027 for 2025/26).
Late registration is technically a separate penalty regime from late filing, but in practice if you register and file by 31 January, late-registration penalties are usually waived. Don't rely on that — register on time if you know you need to.
The SA108 boxes explained — what goes where
SA108 has dedicated rows for cryptoassets ("Other property and assets and gains" was historically used; HMRC added explicit crypto sections in recent versions). The key boxes:
| Box | What goes there |
|---|---|
| 14 | Other property and assets and gains — number of disposals |
| 15 | Total disposal proceeds (GBP, summed across all crypto disposals) |
| 16 | Total allowable costs (s.104 pool basis, same-day matches, 30-day matches) |
| 17 | Total gains (positive figures only) |
| 18 | Total losses (positive figures, will be deducted) |
| 19 | Tax adjustments |
Important: HMRC accepts aggregated reporting — you submit the totals, not line-by-line per trade. However, you must retain the detailed workings (per-disposal date, asset, quantity, proceeds, cost basis) and produce them if requested. Crypto tax software (Koinly, Recap, CryptoTaxCalculator) produces an SA108-ready summary plus the underlying disposal-by-disposal workings as a PDF or CSV.
Worked example — from wallet history to filed return
Scenario: UK higher-rate taxpayer in 2025/26. Three sources of crypto activity:
Activity
- BTC disposals. Bought 1 BTC in Jan 2022 for £25,000. Sold 0.5 BTC in Jul 2025 for £30,000 (i.e. 0.5 BTC at £60k/BTC market price).
- ETH staking rewards. Received 0.4 ETH staking rewards across the year. FMV in GBP at receipt dates: £1,200.
- Uniswap UNI airdrop (already received in 2020). No income tax event in 2025/26; just sold the UNI tokens in Nov 2025 for £8,000 against original FMV-on-receipt-base-cost of £2,400.
Calculations
CGT — BTC disposal:
- Disposal: 0.5 BTC × £60,000 = £30,000 proceeds
- Cost basis from s.104 pool: 0.5 × £25,000 = £12,500
- Gain: £30,000 − £12,500 = £17,500
CGT — UNI disposal:
- Disposal proceeds: £8,000
- Cost basis: £2,400 (the FMV at receipt in 2020, which was income-taxed at the time)
- Gain: £8,000 − £2,400 = £5,600
Total CGT calculation:
- Total proceeds: £30,000 + £8,000 = £38,000
- Total cost basis: £12,500 + £2,400 = £14,900
- Total gain: £23,100
- Less AEA: £23,100 − £3,000 = £20,100 chargeable
- At 24% higher-rate CGT (post Oct 2024): £20,100 × 24% = £4,824 CGT
Income tax — staking:
- Total misc income: £1,200
- £1,000 trading allowance not optimal here (real "expense" is just zero); claim allowance = £1,200 − £1,000 = £200 taxable
- At 40% higher rate: £200 × 40% = £80 income tax
What gets filed
- SA108 box 14: 2 (disposals)
- SA108 box 15: £38,000
- SA108 box 16: £14,900
- SA108 box 17: £23,100
- SA100 box 17 (Other UK Income): £200 (post £1k allowance)
- Total additional tax owed: £4,904
Plus the detailed disposal-by-disposal workings retained (Koinly / Recap PDF export, etc.).
What workings to attach (or have ready)
HMRC doesn't require detailed workings to be filed alongside the return, but they must be retained and producible if requested. The standard package:
- Per-disposal record: date, asset, quantity disposed, proceeds in GBP (with exchange rate source), acquisition date(s) matched, cost basis applied (same-day / 30-day / s.104 pool)
- S.104 pool reconciliation: opening balance, additions, disposals, closing balance per token
- Wallet inventory: list of all wallets and exchange accounts used in the tax year, even if no activity (helps explain coverage)
- Transaction CSV exports: from each exchange, plus blockchain-derived records for self-custodied wallets
- Source for GBP valuations: consistent reference (CoinGecko, CoinMarketCap, exchange rate at hour of disposal etc.)
- Tax software output: Koinly / Recap / CryptoTaxCalculator export — typically a "Tax Summary" PDF plus underlying disposal CSV
If HMRC opens an enquiry into your return, they'll typically ask for the underlying workings within 30 days. Having it pre-built and exportable is much less stressful than reconstructing months later.
Deadlines, penalties, and the late-filing creep
| Date | Deadline |
|---|---|
| 5 October following tax year end | Register for self-assessment if first time |
| 31 October following tax year end | Paper return deadline |
| 31 January following tax year end | Online return deadline AND tax payment deadline |
| 31 July (mid-year) | Second payment on account (if applicable) |
Late filing penalties:
- 1 day late: £100 fixed penalty
- 3 months late: additional £10/day for up to 90 days (max £900)
- 6 months late: additional 5% of tax due or £300, whichever higher
- 12 months late: additional 5% or £300, plus potential serious-non-compliance penalties
Late payment penalties (separate from filing):
- 30 days late: 5% of unpaid tax
- 6 months late: additional 5%
- 12 months late: additional 5%
- Interest accrues from 1 February at the prevailing rate (currently 7.75%)
For crypto-specific cases where you're disclosing past underpayment: voluntary disclosure attracts lower penalties (often 10-30% of tax owed) than waiting for an HMRC enquiry / discovery (30-100%). The Worldwide Disclosure Facility (WDF) is the standard route.