Amended tax return: when and how to file one

You filed your tax return and then spotted a mistake — you forgot to report some income, claimed the wrong deduction, or a calculation error slipped through. Now what? If the deadline for filing the original return has already passed, you can no longer submit a corrective return. Instead, you'll need to file an amended tax return. This article explains when you're required to file one, how to fill it in correctly, what the deadlines are, and what it might cost you.

When you must file an amended tax return

The obligation to file an amended return arises in the following situations:

Mandatory filing (tax liability should be higher)

If, after the filing deadline has passed, you discover that your tax liability should be higher than what you originally reported, you are required to file an amended return. Typical situations include:

• You forgot to report income (an invoice you overlooked)
• You claimed an expense that doesn't meet the legal requirements
• You applied the wrong flat-rate expense percentage (higher than you're entitled to)
• You mistakenly claimed a deduction or tax credit you're not entitled to

Voluntary filing (tax liability should be lower)

If you discover that you reported and paid more than you owed, you may voluntarily file an amended return. Typical situations include:

• You forgot to claim a non-taxable allowance (mortgage interest, pension contributions)
• You didn't claim a tax credit you were entitled to
• You forgot to include an expense that legitimately belongs to that tax year
• You applied a lower flat-rate expense percentage than you were entitled to

Filing with no change to the overall tax liability

In exceptional cases, you may file an amended return without any change to the overall tax liability — for example, if you need to correct details that don't affect the amount of tax owed but are important for other reasons (correcting identification details, or correcting income and expense figures that result in the same overall outcome).

Deadlines for filing an amended return

The deadlines differ depending on whether the amended return results in a higher or lower tax liability.

Tax liability should be higher (mandatory filing)

An amended return increasing your tax liability must be filed by the end of the month following the month in which you discovered the error. You must also pay the resulting tax difference within the same deadline.

Tax liability should be lower (voluntary filing)

For an amended return reducing your tax liability, the one-month subjective deadline from the date of discovery does not apply. You may file it at any time within the objective limitation period for tax assessment — i.e., within the three-year preclusive period (§ 148 of the Tax Code).

Objective deadline (for both cases)

An amended return (whether for higher or lower tax) can only be filed within the limitation period for tax assessment under § 148 of the Tax Code. This period is generally 3 years from the deadline for filing the original return.

| Tax year | Original filing deadline (electronic) | End of the three-year period | |----------|----------------------------------------|------------------------------| | 2023 | May 2024 | May 2027 | | 2024 | May 2025 | May 2028 | | 2025 | May 2026 | May 2029 |

How to fill in an amended tax return

An amended return is completed on the same form as the original return (form 25 5405), but with a few specific adjustments.

Practical example

Penalties and the benefits of voluntary filing

If you file voluntarily (higher tax)

Voluntarily filing an amended return for a higher tax liability is favoured by law. The key advantage:

• No 20% surcharge — the surcharge only applies when the tax office assesses additional tax (typically following an audit). If you correct the error yourself, no surcharge is due.
• Late payment interest — unfortunately, this cannot be avoided. Late payment interest is calculated retroactively from the original tax due date to the date the additional amount is paid. The rate is the Czech National Bank repo rate + 8 percentage points (in the first half of 2026, that's 11.50% p.a.).

If you file voluntarily (lower tax)

When the amended return results in a lower tax liability, the situation is more straightforward:

• The tax office will refund the overpayment based on your amended return
• No penalties apply (since you're paying less, the state is returning money to you)
• The refund will be returned within 30 days of the assessment (generally from the date the amended return is filed)

Multiple amended returns

You may file more than one amended return — the law does not limit how many you can submit. Each subsequent amended return must be based on the most recently known tax liability (i.e., the tax figure from the most recent original or amended return that has been assessed).

When this makes sense

• You discovered another error after filing the first amended return
• You received a corrected document from a supplier
• Circumstances changed that affect your tax liability (e.g., retroactive recognition of disability status)

Impact on reports to the Czech Social Security Administration (ČSSZ) and health insurance

If you file an amended tax return and your tax base changes, you must also file amended reports for ČSSZ and your health insurance provider. A change in the tax base affects:

• The assessment base for social insurance contributions
• The assessment base for health insurance contributions
• The amount of advance contributions for the following period
• Any additional amount owed or overpayment on contributions

Special situations

Amended return and the flat-rate tax regime

If you were in the flat-rate tax regime in 2025 and subsequently discover that you didn't meet the conditions (e.g., your income exceeded the limit), you must file a regular tax return for the full year. In this case, it's not an amended return — it's an original return that replaces the flat-rate regime.

Amended return after a tax audit

If a tax audit has been initiated covering a particular period and tax type, you cannot file an amended return for that same period and tax type during the audit (§ 141(6) of the Tax Code). This restriction applies for the entire duration of the audit.

Error in a mortgage interest or pension contribution deduction

If you forgot to claim the deduction for mortgage interest or pension contribution payments in your original return, you can file an amended return for a lower tax liability and claim the deduction retroactively. Make sure to include the relevant supporting documents.

Switching between expense methods

An amended return cannot be used to change your method of claiming expenses (from flat-rate to actual expenses or vice versa). The choice of expense method is a decision that cannot be reversed after the deadline for filing the original return has passed.

Frequently asked questions (FAQ)

How do I know if there's an error in my return?

Errors most commonly come to light when:

• Reviewing bank statements (you find income you didn't report)
• Preparing the return for the following year (you notice a discrepancy)
• Receiving a document that should have been included in a previous period
• Receiving a request from the tax office to explain inconsistencies

Do I need to state a reason for filing?

Yes. In the attachment to the amended return, you must briefly explain why you're filing it — what error you're correcting and how you discovered it.

Can I file an amended return for multiple years back?

Yes, as long as the limitation period for tax assessment is still running (generally 3 years from the deadline for filing the original return). In 2026, you can file amended returns for the 2023, 2024, and 2025 tax years.

What if the tax office rejects my amended return?

The tax office may reject an amended return if:

• It is filed after the preclusive limitation period has expired
• A tax audit is underway for the relevant period and tax type
• The amended return is missing mandatory information

In such cases, the tax office will either ask you to provide additional information or issue a decision rejecting the return, which you can appeal.

How do I file an amended return electronically?

On the MOJE daně portal, select the Personal Income Tax Return form and in the first step choose 'amended' as the return type. The system will automatically display lines 4, 5, and 6 for you to complete. Submit the return in the same way as an original return — via your data mailbox or with an electronic signature.

Avoid errors with DokladBot

Most of the reasons for filing an amended return stem from disorganised record-keeping — a forgotten invoice, an overlooked income item, a missing confirmation. DokladBot is an AI accounting assistant on WhatsApp that helps you keep your records in order throughout the year. Simply photograph each document and send it via WhatsApp — DokladBot processes it, records it, and by the end of the year you have a complete overview with nothing missing.

Get started with DokladBot today at dokladbot.cz — and next year, an amended return won't be necessary.

Official sources and links

• Czech Financial Administration — personal income tax
• Act No. 280/2009 Coll., Tax Code — § 141 (amended tax return)
• Act No. 280/2009 Coll., Tax Code — § 148 (limitation period for tax assessment)
• MOJE daně portal — electronic filing

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This article is intended as a general information guide and does not substitute for individual tax advice. The information is accurate as of the date of publication (February 2026) and is based on legislation in force at that time. For advice on your specific situation, please consult a qualified tax adviser.