Can you amend an old tax return for crypto?

Two different situations

1. The assessment is not yet completed

The assessment is usually completed in the autumn of the year after the tax year, and the date is shown on your tax decision. Until then, missing disposals are added in MyTax as a correction to your tax return, under capital gains and losses. If you only make the correction after the tax return deadline, the consequence is a late-filing fee: for an individual, 50 euros per tax year.

2. The assessment is completed

In that case, the correction is requested with a claim for adjustment, which is also made in MyTax. The claim includes a breakdown of the disposals and the grounds for the change. When you correct missing income on your own initiative, the tax increase (penalty surcharge) is 0.5% of the added income – clearly less than when the Finnish Tax Administration (Verohallinto) discovers the omission itself.

Deadlines by tax year

The deadline is counted three years forward from the start of the year following the end of the tax year. If the last day falls on a public holiday or a weekend, the deadline moves to the next business day.

Why the deadline is worth taking seriously

The three-year deadline applies to claiming what is in your own favour: unconfirmed capital losses, missing acquisition costs and other deductions. Once the deadline passes, they are lost for good.

As a rule, the Tax Administration has the same three years to change an assessment to the taxpayer's detriment. However, if the Tax Administration receives information affecting the assessment from elsewhere, for example as third-party comparison data, the deadline can be extended by a year. This becomes relevant as data reported by crypto service providers starts reaching the Tax Administration. Read more: CARF and DAC8.

Losses matter: a capital loss is first deducted from capital gains of the same year and then from other capital income, and any unused part carries forward to the next five tax years. For example, a confirmed loss from 2023 can still reduce your tax for 2028 – but only if it is confirmed in time.

What a claim for adjustment requires

1. Disposal ledger. Every disposal with its date, the disposal price in euros and the acquisition cost allocated in FIFO order. Crypto-to-crypto swaps are also disposals.
2. Totals per year. Total capital gains and losses, and total disposal prices. For small amounts, below annual thresholds of about 1,000 euros, a gain may be tax-exempt and a loss non-deductible – check the thresholds in the Tax Administration's guidance.
3. Data sources. Where the figures were calculated from: exchange exports and wallet addresses. The Tax Administration may ask for further clarification, so traceability saves time later.
4. Grounds. A short explanation of what is being corrected and why.

If several years need correcting, it is best to calculate them all at once from start to finish, because acquisition costs carry over from one year to the next. Correcting a single year separately easily leads to the same purchase being used twice or not at all.