UK Self Assessment Crypto Checklist: 10 Things HMRC Actually Wants

If HMRC opens an enquiry into your crypto Self Assessment, they will ask for specific evidence. Knowing what those records are before you file makes the filing itself faster and the enquiry, if it happens, painless. Here are the ten records HMRC actually wants, in the order you should gather them.

1. A list of every wallet you own or have signed transactions from

Self-custody wallets, exchange custodial wallets, hardware wallets, multi-sig wallets, mobile wallets. For each one: the address (or the exchange account email), the date you first used it, and whether you still have access. HMRC asks "what are all the wallets you used in 2025/26" — being able to answer that question completely is the first sign you're filing seriously.

2. CSV exports from every centralised exchange

Binance, Coinbase, Kraken, Bitstamp, Crypto.com, Gemini, Bitfinex, KuCoin, Bybit, OKX — whatever you used. The CSVs should cover the full tax year 6 April 2025 to 5 April 2026, and ideally a few weeks either side to catch boundary-effect disposals. Download these BEFORE you might need them — exchanges occasionally delete or paywall historical exports without warning.

3. On-chain transaction histories for every self-custody wallet

CryptoLens handles this automatically via wallet address scanning. The alternative is downloading transaction history from each chain's block explorer (Etherscan for EVM, Solscan for Solana, etc.) and parsing them manually. Either way, you need the full list of disposals, including DEX swaps, NFT mints, gas paid, and any token migrations.

4. Cost basis evidence for every starting position

If you held BTC, ETH, or any other asset on 6 April 2025 (i.e. at the start of 2025/26), you need to know its cost basis. Three valid sources:

- A prior year's SA108 / tax tool output showing your Section 104 pool average cost. - The original purchase receipts (exchange transaction records, bank transfer screenshots). - Last year's filed tax return.

If you started crypto in 2025/26 from scratch and have all-fresh acquisitions, this is easy. If you've been holding since 2017 and never tracked cost basis, this is the single biggest piece of work — but HMRC will want it.

5. Records of disposals, broken down

For each disposal, HMRC wants:

- Date and time (HMRC accepts dates for matching; intra-day matching uses dates not minutes). - Asset disposed and quantity. - Proceeds in GBP at the time of disposal. - Cost basis applied and the matching method (same-day, B&B, or Section 104). - Allowable costs (fees, gas, bridge fees). - Net gain or loss.

The SA108 PDF from CryptoLens produces exactly this layout per disposal. If you file via your accountant's spreadsheet, every row needs these fields.

6. Income-side records, separate from CGT

Some crypto activity is income, not capital gains:

- Staking rewards: FMV in GBP at the date of receipt. - Mining proceeds: same. - Airdrops you didn't actively earn (free token drops): FMV at receipt. - Crypto received as payment for work: FMV at receipt, plus the work context. - Lending interest from CeFi platforms: actual GBP interest received.

These go on SA100 / SA107 (foreign / miscellaneous income), NOT SA108. Don't double-count: the FMV at receipt becomes your cost basis for the subsequent CGT calculation if you later sell the same asset.

7. Fee records — they reduce your gains

Gas fees, swap fees, exchange trading fees, bridge fees: all deductible under CG15250 as incidental costs of acquisition (added to cost basis) or disposal (deducted from proceeds). HMRC accepts the on-chain record as evidence — you don't need separate receipts. CryptoLens captures the per-transaction fee where it's available from the chain.

You CANNOT deduct: cost of a hardware wallet, your internet bill, time spent doing research, gym membership, or the cost of this kind of advice content. Costs must be directly attributable to specific transactions.

8. Negligible value claims, if applicable

Under TCGA s.24, you can claim a loss on a token that has become "of negligible value" — typically a rugged memecoin or a project whose token has dropped to near-zero with no realistic recovery prospect. You make the claim in writing (in your Self Assessment, in a separate letter, or both), specifying:

- The asset and how it became negligible. - The date you consider it became negligible. - The cost basis you're claiming.

The asset must be IN YOUR POSSESSION at the claim date — you can't claim s.24 on tokens you've already sold or sent to a burn address. CryptoLens surfaces negligible-value candidates for review but does not auto-claim, because HMRC requires the claim to be a deliberate filing.

9. Carry-forward losses from prior years

If you had a net loss in 2024/25 (or earlier years), and you correctly claimed it in that year's Self Assessment, you can carry it forward to offset 2025/26 gains. CryptoLens loads these automatically if you have a prior Self Assessment output.

CRITICAL: a loss had to have been REPORTED in the year it arose. You cannot retroactively "remember" a 2022/23 loss in your 2025/26 filing if you never reported it before. The window to amend a prior return is 12 months past its filing deadline.

10. The matching method evidence

The single thing most likely to come up if HMRC enquires: how did you decide which acquisition matched which disposal? They want to see that you applied the rules in order:

- Same-day first (s.105 TCGA). - Then 30-day bed-and-breakfast (s.106A). - Then Section 104 pool.

The SA108 PDF from CryptoLens shows the matching method as a column on each disposal row. That's the audit trail. If you filed manually via a spreadsheet, your matching column needs to be in the spreadsheet — not in your head.

What HMRC has copies of, automatically

You don't need to re-supply data HMRC already has — though you do need to make sure your filing agrees with what they have:

- CARF reports from every UK-supervised crypto exchange and several major overseas ones (since January 2026). Your gross fiat-in, fiat-out, and total token volumes are visible to them. - Bank account information showing fiat-on-ramp deposits via Open Banking. - Information shared between tax authorities under the OECD Common Reporting Standard for non-crypto accounts that may indirectly evidence crypto activity.

Discrepancies between what you file and what they have are the trigger for enquiry. CryptoLens lets you cross-check by uploading the same exchange CSVs the exchanges sent to HMRC — if the engine produces a different gross-volume number than your CSV says, there's a reconciliation issue to resolve before you file.

What to do this week if 31 January 2027 still feels far away

Run a dry-run reconciliation against last year's filing (2024/25) using your current 2024-25 data. Does the prior year's output match what you filed? If not, fix it now — amending a prior return is easier than defending one that doesn't match your current records. Then you have a working baseline going into 2025/26.

CryptoLens runs this against your existing wallets in about 60 seconds. The output is your honest starting position for the 2025/26 SA108.