# Missing crypto cost basis: rebuild your UK tax records

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.

## Key facts

## 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.
## 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 hierarchy for a missing transaction

| 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.

Source: [GOV.UK: filling in your Self Assessment return](https://www.gov.uk/hmrc-internal-manuals/self-assessment-manual/sam121190)
## 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.

[Build a practical tax preparation checklist](https://digital-assets.co.uk/tools/crypto-tax-checklist/)

[What HMRC records to keep](https://digital-assets.co.uk/tax/crypto-record-keeping/)

## Visual explanation
Turn transaction history into evidence
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.
A reproducible record connects the original transaction to the calculation. Keep a dated explanation of adjustments rather than silently changing the raw export.

## FAQs
### 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.

## Sources

- [HMRC: selling or disposing of cryptoassets](https://www.gov.uk/guidance/check-if-you-need-to-pay-tax-when-you-sell-cryptoassets)
- [HMRC: Cryptoassets Manual](https://www.gov.uk/hmrc-internal-manuals/cryptoassets-manual)
- [GOV.UK: estimated and provisional return figures](https://www.gov.uk/hmrc-internal-manuals/self-assessment-manual/sam121190)

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— Digital Assets UK (https://digital-assets.co.uk/tax/crypto-missing-cost-basis/), reviewed 2026-09-30. Source: https://www.gov.uk/guidance/check-if-you-need-to-pay-tax-when-you-sell-cryptoassets