Missing crypto cost basis: rebuild your UK tax records
Quick answer: Missing cost basis is an evidence problem, not proof that your acquisition cost was zero. Recover exchange files, bank records and on-chain history, reconcile transfers, and document any reasonable estimate. Keep unresolved items visible and explain provisional figures where the return requires it.
Reconstruct missing crypto transaction history, GBP prices and acquisition costs, reconcile wallet transfers and document estimates for HMRC.
Start with a complete account inventory
List every exchange, wallet and chain used, including closed accounts. Record when each account was opened and which tax years it covers. Export full transaction history rather than only the year being filed: an old acquisition can determine the pool cost of a current disposal. Back up the untouched exports before editing anything.
Turn transaction history into evidence
Keep the original files beside your working copy
A reproducible record connects the original transaction to the calculation. Keep a dated explanation of adjustments rather than silently changing the raw export.
Read the diagram as text
- 01. Collect. Export every exchange and wallet, including older purchases.
- 02. Reconcile. Pair transfers and investigate duplicates or missing entries.
- 03. Explain. Record prices, classifications and manual adjustments.
- 04. Preserve. Keep the calculation, source files and submission evidence.
Source: HMRC cryptoasset record keeping
Recover the evidence in layers
Try the exchange account export and support route first. Then reconcile bank transfers, emails, trade confirmations, wallet addresses and transaction hashes. A bank payment can establish money sent to an exchange, but does not alone prove which tokens were bought or their quantity.
| Evidence | What it helps establish | What to check |
|---|---|---|
| Exchange trade confirmation | Token, quantity, price, fee | Correct account and trading pair |
| Wallet transaction hash | Quantity and chain timestamp | Ownership and economic purpose |
| Bank statement | Fiat funding or withdrawal | Not necessarily a token acquisition |
| Historical price source | GBP valuation estimate | Timestamp, liquidity and methodology |
Fix transfers before accepting a zero-cost warning
An incoming transfer may look like a purchase with no cost when the sending wallet was not imported. Match the outgoing and incoming legs before changing acquisition values. Keep one ownership history across accounts: a transfer usually does not reset the Section 104 pool or create a fresh tax-free allowance.
When an exact figure cannot be recovered
Document what is missing, your attempts to recover it, the evidence used and the reason for an estimate. HMRC’s Self Assessment guidance explains how to identify provisional or estimated figures and replace provisional figures when available. A filing deadline is a reason to seek help promptly, not to invent a purchase price.
Keep a change log your future self can follow
Record original value, revised value, evidence reference, reason and date. Save both the raw and adjusted datasets and reconcile opening holdings plus acquisitions minus disposals to closing holdings. Investigate unexplained negative balances before relying on a tax-software report.
- Recover full history, including acquisitions before the current tax year.
- Remove duplicate API and CSV imports.
- Reconcile asset quantities before tax totals.
- Review GBP prices for illiquid or delisted tokens.
- Give the accountant a list of unresolved questions.
Frequently asked questions
Should I set missing crypto cost basis to zero? +
Not automatically. Try to reconstruct the actual allowable cost and retain evidence. A zero value can overstate gains; an invented value can understate them.
Do I need old transactions for this year’s return? +
Yes, earlier purchases may determine current pooled costs. Importing only sales from the filing year can produce incorrect gains.