Corrective vs Supplementary Tax Returns

The Difference Between an Amended and a Supplementary Tax Return

It might seem like both terms mean the same thing – correcting an error. In reality, that's not the case. This very distinction determines whether the mistake is resolved without penalty or costs you tens of thousands of crowns.

You file an amended tax return if you discover an error before the filing deadline has passed. For the 2025 tax return, this means by 1 April 2026 for paper filings, by 4 May 2026 for electronic filings, or by 1 July 2026 if your return is prepared by a tax advisor or an attorney.

The key advantage is that this correction is not penalised. The Tax Office views it as part of the regular tax process and does not impose any fines or late payment interest.

You file a supplementary tax return if an error is found after the deadline for a regular filing has passed. If the tax liability increases, you must file the supplementary return by the end of the month following the month in which you realised the error. If you are required to file by 1 April and you discover the error on 15 November, you have until 31 December of that year to file the supplementary return.

However, this is where the situation becomes much more complicated. In practice, it pays to consult with experts in tax law about the specific impacts and procedures, especially if there is a risk of late payment interest or penalties. If the tax has increased, you face late payment interest and, in some cases, a penalty. If the tax has decreased, you are not obliged to file a supplementary return, but you have the right to do so – and this is where a multitude of technical rules and exceptions, which the lawyers at our Prague-based law firm handle routinely, come into play.

How Penalties Differ by Return Type

Sanctions, i.e., financial penalties, are what hurt entrepreneurs the most when mistakes are made. And it is precisely here that timeliness has a decisive impact.

Amended Return: No Penalties

If you file an amended return on time, practically nothing bad will happen to you. You will not face fines for late filing, penalties on the additionally assessed tax, or late payment interest. This is a legal window provided specifically so that minor errors can be corrected in time without fear of sanctions.

This is the strongest argument for thoroughly checking your tax return before the deadline expires. It's not just a formality – it's a difference of tens of thousands of crowns. In typical situations, such as loans between a company and its owner, even a minor error in the setup can lead to additional tax assessments and subsequent penalties.

Supplementary Return: Interest and Potential Penalties

When you file a supplementary tax return after discovering that the tax should have been higher, you must pay:

1. The tax difference itself – the amount you originally forgot or miscalculated.

2. Late payment interest – this is calculated for each day from the due date of the original tax until the day the Tax Office receives the money.

Late payment interest rates change throughout the year. Their amount is always calculated based on the Czech National Bank's repo rate increased by 8 percentage points, and it changes twice a year – on 1 January and 1 July. If the interest rate this year were, for example, 15% per annum, for a 100-day delay on an amount of CZK 100,000, you would pay approximately CZK 4,110 in interest alone.

If the tax increased and you voluntarily declare it in a supplementary return, you will not face a penalty. However, if the situation is related to suspicion of intentional misrepresentation or concealment of data, it is appropriate to address the matter from the perspective of criminal law as well. A penalty of 20% of the additionally assessed tax is only at risk if the error is discovered and the tax is assessed by the Tax Office during a tax audit. This means that for the same CZK 100,000, in addition to the tax and late payment interest, you would have to pay another CZK 20,000 as a penalty.

The difference is huge: while with a "voluntary" supplementary return you only pay the tax and late payment interest, with an assessment by the tax administrator, you pay the tax, interest, and a 20% penalty.

If the Tax Decreased

If a supplementary tax return leads to a reduction in tax (e.g., you forgot to claim a deduction or a tax credit), you are not normally subject to penalties or fines. You have the right to file it, but a deadline also applies here – you must do so by the end of the month following the month in which you discovered the error, and also within the period for tax assessment (usually 3 years, in some cases up to 10 years). Subsequently, the lower tax should be refunded to you.

Deadlines for Filing a Supplementary Tax Return

Deadlines are a legal tool to provide certainty for everyone. Once the period for tax assessment expires, it can no longer be assessed. But once you realise an error, you have a precisely defined time to correct it.

The basic deadline: if you discover an error and the tax should have been higher, you must file a supplementary tax return by the end of the month following the month in which you discovered the error.

Example: You file your tax return for 2025 by 1 April 2026. If on 15 November 2026, you notice that you forgot to include an income, you must file a supplementary return by 31 December 2026.

The final deadline for tax assessment: this is generally 3 years from the due date of the regular tax return. In some cases (e.g., tax loss, suspicion of tax fraud), this period can be extended up to 10 years.

This means that if, for example, the tax return was due on 1 April 2026, you can correct it via a supplementary return until 1 April 2029 – and even after a year and a half, you have the right to file a supplementary return without the deadline having closed.

Such deadlines sound simple, but in practice, the question of when the error was "discovered," when everything was "sufficiently" communicated, or whether an exception applies to you can be complex. That is why our legal advisors consistently monitor their clients' individual tax proceedings to ensure deadlines are not missed.

The Difference Between Errors in Accounting and in a Tax Return

You can afford to correct errors if you are conscientious with your accounting. However, many entrepreneurs do not realise that there is a difference between an error in accounting and an error in a tax return – and both problems need to be addressed differently.

Errors in Accounting

An error in accounting means you have recorded something incorrectly – for example, you entered a lower invoice amount, posted it to the wrong month, or did not record the transaction at all. Accounting is kept for this very purpose – so that errors can be identified and corrected retroactively.

If you discover an error before the financial statements are approved, you can correct it simply – open the accounting books and make a correcting entry. Everything remains legible and traceable, as required by regulations. The procedures for correcting errors in accounting are regulated by Act No. 563/1991 Coll., on Accounting.

If you notice the error only after the financial statements have been approved, the procedure is more complicated – you must make corrections in the following accounting period and describe them in the notes to the financial statements.

Errors in a Tax Return

An error in a tax return means that you have transferred data from your accounting incorrectly or calculated it incorrectly. For example, you reported lower revenues than you should have, or you did not claim a deduction you were entitled to. Or you forgot to include rental income, even though it should have been taxable.

This is the key point: an error in a tax return is not resolved by "correcting" it directly in the return. It is resolved by filing a new return – either an amended one (if you are on time) or a supplementary one (if the deadline has passed). Each return is a separate legal declaration, and if you declare incorrect data, you cannot simply "erase" it later.

This is a crucial mindset shift: you can "correct" your accounting records regularly, but you can only correct a tax return within strictly defined deadlines and procedures. That is why it is so important to check the return thoroughly before submitting it.

The Link Between Errors in Accounting and in a Tax Return

If you correct an error in your accounting after the financial statements have been approved (and thus after the tax return has been submitted), it does not necessarily mean that a supplementary tax return can be automatically filed. You must verify whether that error had an impact on the tax.

A practical example: you discovered that you recorded an invoice in your expenses for February, even though it was actually from January. This is an accounting error – but if the tax is calculated for the entire year, it may not have an impact on the annual tax liability. On the other hand: you discovered that you did not claim a VAT deduction that should have been included. This has an impact on the tax and requires a supplementary tax return.

Our legal advisors will help you assess these impacts correctly – because this is precisely where the risk lies that you will address the matter too quickly or too late.

New Rules for Waiving Penalties (Changes from 2026)

A very important resolution came into force on 1 July 2025 – an expansion of the possibilities for having penalties and fines for tax arrears waived. This is crucial for every entrepreneur who has already made a mistake.

Previously, the tax administrator could waive a maximum of 75% of penalties and fines if you demonstrated a "justifiable reason" for the delay – e.g., health problems, particularly difficult personal situations, or a surprising legal change that you could not have anticipated.

From July 2025, up to 100% of penalties and fines can be waived if the reason is truly serious and justifiable. When assessing your request, the tax administrator examines three basic criteria:

1. A justifiable reason for the delay – e.g., illness, death in the family, serious personal reasons.

2. The economic or social circumstances of the tax subject – whether paying the fines and penalties would mean an unmanageable financial burden.

3. The frequency of non-compliance – if you are an honest taxpayer and this is your first mistake, you have a better chance.

A request for a waiver is submitted to the tax administrator on a form – and our legal advisors can prepare it for you and support it with a detailed explanation and evidence that truly carries weight.

In practical terms, this means: even if you have overlooked something and are facing a penalty, you do not have to give up. There is a chance you can reach an agreement with the tax administrator – but only if you act proactively and factually, without unnecessary excuses.

Tax Audit and What Changes When the Tax Office Discovers the Error Itself

The worst-case scenario is when the error is not discovered by you, but by the Tax Office during a routine tax audit. Here, the rules change, and the penalties are significantly stricter.

How the Tax Administrator Initiates an Audit

A tax audit is initiated by the delivery of a notice of commencement of a tax audit, which defines the scope and subject of the audit. The tax administrator has the right to inspect documents that are no older than three years. In some cases (e.g., on suspicion of tax fraud), the period for tax assessment is extended to up to 10 years.

As soon as you receive the notice, you know something has gone wrong – and now procedures will be initiated that are no longer friendly to the taxpayer. To put it simply, if you do not "present evidence," everything is against you.

Proposed Payments in Case of an Audit

If the tax administrator finds during the audit that the tax was calculated incorrectly, you will receive a payment assessment with the amount you have to pay. This amount includes:

1. The additionally assessed tax difference itself.

2. Late payment interest – for each day from the original due date.

3. A penalty of 20% of the additionally assessed tax.

This means that if a tax underpayment of CZK 100,000 is identified, you will pay CZK 100,000 in tax, a few thousand crowns in interest (depending on how long it has been), and another CZK 20,000 in penalties – for a total of approximately CZK 120,000 or more.

This is not a negligible amount. And it is precisely here that it plays an important role whether the error is discovered in the early stages of the audit or only at the end, because interest is calculated for the entire period of delay.

Rights of the Tax Subject During an Audit

You are not defenceless in the proceedings. You have the right to:

• Be notified in a timely manner of the commencement of the audit.

• Know what the audit is focusing on.

• Present evidence that refutes the office's doubts.

• Ask questions and request explanations.

• File a complaint if the process seems unlawful to you.

Our legal advisors can represent you during a tax audit – that is, be present at meetings, analyse the office's requests, prepare your defence, and negotiate with officials in your favour. It is not common practice for a taxpayer to "argue" with the Tax Office on their own – in many cases, representation by a lawyer leads to significantly better results.

Practical Steps to Take When an Error is Discovered

If you act logically and quickly, you can make your life much easier and protect your money.

Step 1: Immediately Verify the Deadline

As soon as you notice the error, determine the exact date you became aware of it. This is crucial – it is what places you in the "timely" or "late" correction category. If it is still within the deadline for filing the regular tax return (this year, by 4 May 2026 for electronic filing), you have room to file an amended return without penalties. If the deadline has passed, you must count on a supplementary return and potential penalties.

Step 2: Consult with an Expert

It is not advisable to handle the matter yourself. Our lawyers and tax advisors will help you:

• Assess whether the error truly needs to be addressed from a tax perspective or if it is purely an accounting matter.

• Calculate precisely what the tax consequences will be.

• Determine what penalties you are facing – and whether they can be minimised.

• Prepare the return flawlessly.

A one-hour consultation can save you weeks of stress and potentially tens of thousands in fines.

Step 3: Prepare the Return

An amended or supplementary tax return is filed on the same form as a regular return. The only difference is checking a box in the header to indicate which type of return it is. In the case of a supplementary return, you must also fill out a special section where you calculate the difference between the original and the newly determined tax liability.

You do not fill in new data as you would with a regular return – you enter the full values and then check them in the "changes" section. If you do this incorrectly, the office may evaluate your correction attempt as faulty, and the matter will become even more complicated for you.

Step 4: Filing and Communication with the Office

Ensure the return is delivered on time. If it is to be filed electronically, submit it before the deadline – never at the last minute. Archive the confirmation of receipt.

If you are filing a supplementary return (and thus admitting to a higher tax), expect that the Tax Office may initiate communication – either to verify the data or to inform you that they have accepted the return. Communicate proactively – never ignore letters from the Tax Office.

Step 5: Payment and Monitoring Deadlines

If the newly assessed tax (the difference) is to be paid, pay it as soon as possible. The sooner you pay, the less late payment interest you will accrue. Every day counts.

If you have questions or are unsure whether everything is correct, contact our law firm (consultation@arws.cz) – do not hesitate, better to be safe than sorry.

Final Summary

Discovering an error in a tax return or accounting does not have to mean financial disaster if you react in time. The crucial difference is between an amended return, which protects you from penalties within the regular deadline, and a supplementary return with late payment interest. However, a quick and voluntary resolution is always the cheapest option.

If the Tax Office discovers the error during an audit, a harsh 20% penalty on the additionally assessed tax will be added to the interest. However, from July 2025, you can defend yourself more effectively. By proving a serious and justifiable reason, the tax administrator now has the authority to waive up to 100% of penalties and fines.

However, tax calculations and communication with authorities are often full of bureaucratic traps. The lawyers at ARROWS will help you with the entire process and, if necessary, prepare a bulletproof request for a waiver of penalties. Write to us at consultation@arws.cz and entrust your case to professionals who are insured for CZK 350 million for your complete peace of mind.