10 Most Common Mistakes in the OSVČ Tax Return

Every year, tax authorities process hundreds of thousands of tax returns from OSVČ. And every year, the same mistakes appear — from minor typos and incorrect calculations to critical oversights that can result in fines or additional tax assessments. Yet most of these mistakes are easy to avoid once you know what to watch out for. In this article, we'll walk through the 10 most common mistakes self-employed individuals make in their 2025 tax returns, with specific advice on how not to repeat each one.

Mistake #1: Using the Wrong Form

One of the most basic — and surprisingly common — mistakes is filling in the wrong form. OSVČ with business income (Section 7 of the Income Tax Act) must use the Personal Income Tax Return form (form 25 5405) together with Appendix No. 1 (form 25 5405/P1), where the partial tax base from self-employment is reported.

Why This Happens

• An OSVČ mistakenly uses the form intended for employees, or a form from the previous year.
• When downloading the form from the internet, an outdated version is downloaded that does not correspond to the current tax period.
• A self-employed individual has income from both business activity and rental or capital gains, and forgets to attach the relevant appendices.

How to Avoid It

Always use the current forms from the MOJE daně portal or the Czech Financial Administration website. The electronic form on the MOJE daně portal will automatically suggest the correct appendices based on your income types.

Mistake #2: Missing Appendices and Required Certificates

Even if you fill in the main return form correctly, the tax authority will not consider it complete without the required appendices. The most commonly forgotten attachments include:

• Appendix No. 1 (business income) — mandatory for every OSVČ
• Confirmation of mortgage interest paid — if you are claiming a home loan deduction
• Confirmation of pension savings / supplementary pension savings — for deducting contributions above CZK 1,700 per month
• Life insurance confirmation — if you are claiming this deduction
• Charitable donation confirmation — if you are claiming a deduction for donations (blood, bone marrow, financial donations)
• Employer confirmation — if you were also employed during the year

Practical Impact

Without the supporting certificates, the tax authority will not recognise your non-taxable portions of the tax base or tax credits. In the best case, they will ask you to provide the missing documents; in the worst case, they will disallow the deductions and assess a higher tax liability.

How to Avoid It

Create a simple checklist of all required appendices before you start filling in the form. Request confirmations from your bank, insurance company, and pension provider well in advance — they are usually sent out automatically in January and February.

Mistake #3: Incorrect Calculation of the Tax Base

Calculating the tax base is the key step of the entire return. Errors here have a direct impact on the amount of tax you owe.

Most Common Calculation Errors

• Including income that is not subject to tax — for example, tax-exempt income or income already taxed at source by withholding tax
• Omitting certain income — typically income from short-term agreements (DPP) up to CZK 10,000 per month on which withholding tax was deducted, where including it in the return would actually be more beneficial for the taxpayer
• Incorrect rounding — the tax base is rounded down to the nearest hundred crowns, not up, and not to the nearest whole crown
• Arithmetic errors — a simple addition or subtraction in the form rows does not add up

How to Avoid It

Use the electronic form on the MOJE daně portal, which performs all calculations automatically. If you are filling in the return manually, double-check every total and make sure you are rounding the tax base down to the nearest hundred crowns.

Mistake #4: Choosing the Wrong Flat-Rate Expense Percentage

Flat-rate expenses are a popular way to claim deductions because they don't require you to keep records of individual expense documents. The problem arises when an OSVČ applies the wrong percentage.

Overview of Correct Rates for 2025

Where Mistakes Most Commonly Occur

• An IT specialist with an unregulated trade licence claims 80% instead of 60% — the 80% rate applies only to craft trades, not unregulated licences.
• An author earning royalties claims 60% instead of 40% — creative authorship falls under other self-employment.
• A self-employed person with multiple income types incorrectly applies a single rate to all income — if you have income from different activities, you must apply the corresponding percentage to each type.
• Exceeding the maximum cap — if you earn CZK 3,000,000 from an unregulated trade, your flat-rate expenses are not CZK 1,800,000 (60%) but only CZK 1,200,000 (the maximum).

How to Avoid It

Check what type of trade licence you have registered in the trade register. The type of trade licence directly determines which flat-rate expense percentage you can apply. If you are unsure, look up your trade register extract at rzp.cz.

Mistake #5: Missing the Filing Deadline

Missing the deadline is a mistake that will cost you money. The tax authority does allow a 5 working-day grace period, but after that, a penalty starts accruing automatically.

Penalties for Late Filing

• Penalty of 0.05% of the assessed tax for each day of delay (from the 6th working day after the deadline)
• Penalty of 0.01% of the reported tax loss for each day of delay
• Maximum 5% of the tax or loss, but no more than CZK 300,000
• The minimum penalty is CZK 500, but penalties up to CZK 1,000 are waived

How to Avoid It

Add the key deadlines to your calendar at the start of the year. Electronic filing has a later deadline (4 May 2026), giving you an extra month compared to paper filing.

Mistake #6: Overlooked Tax Credits and Child Tax Benefits

Many OSVČ fail to claim all the credits and benefits they are entitled to. The most frequently forgotten items include:

Basic Taxpayer Credit

The standard credit of CZK 30,840 for 2025 is available to every taxpayer automatically. Yet some people fail to fill it in — particularly if they are filing a return for the first time.

Spouse Credit

A credit of CZK 24,840 can be claimed if your partner's income did not exceed CZK 68,000 in 2025 and they are also caring for a child under the age of 3. For a spouse who holds a ZTP/P disability card, the credit is doubled: CZK 49,680.

Child Tax Benefit

| Child | Annual benefit | |-------|----------------| | 1st child | CZK 15,204 | | 2nd child | CZK 22,320 | | 3rd child and beyond | CZK 27,840 |

For a child with a ZTP/P disability card, the amount is doubled.

Non-Taxable Portions of the Tax Base

• Mortgage interest (up to CZK 150,000, or CZK 300,000 for older contracts)
• Pension savings (up to CZK 48,000)
• Life insurance (up to CZK 48,000)
• Donations to public benefit causes
• Blood donation (CZK 3,000 per donation)

How to Avoid It

Go through the complete list of credits and non-taxable portions and ask yourself for each item whether you are entitled to it. The electronic form on the MOJE daně portal will prompt you about most of them automatically.

Mistake #7: Incorrect Bank Account Number

If your return shows a tax overpayment or a child tax bonus, the tax authority will send the money to the bank account stated in your return. An error in the account number means the money won't arrive.

Where Errors Occur

• A typo in the account number — a single wrong digit is enough
• Entering a business account instead of a personal one (or vice versa) — the tax authority sends money to exactly the account you specify
• Not entering an account number at all — if you are requesting a refund of an overpayment, you must fill in the account number in the refund request section and sign it (even when filing electronically)

How to Avoid It

When filling in the form, copy your account number directly from your online banking. Once entered, check it again digit by digit.

Mistake #8: Forgetting to Sign the Return

Even in the digital age, returns sometimes go unsigned. With paper returns, it's the classic forgotten signature on the last page. With electronic filing, it's the situation where an OSVČ completes the form but does not submit it with a valid electronic signature or through a data mailbox.

Specific Situations

• The overpayment refund request is technically a separate submission that requires its own signature. Even if you sign the main return, forgetting to sign the refund request means the tax authority will not refund your overpayment.
• Returns filed on behalf of a spouse — if you are filing on your partner's behalf, you must have and attach a power of attorney.

How to Avoid It

When filing via a data mailbox, the submission is automatically considered signed upon sending. When filing via the MOJE daně portal, make sure you have completed the submission and received a confirmation of receipt (an acknowledgement).

Mistake #9: Wrong Tax Period

It may seem trivial, but errors in the tax period appear more often than you'd expect. A return filed in 2026 covers income earned in 2025. Problems arise when:

• An OSVČ accidentally enters the tax period 2026 instead of 2025
• Invoices issued in January 2026 are included in the 2025 return
• Expenses from 2024 that should have been in the previous return are counted

Income and Expense Documents — What Belongs in 2025

For OSVČ keeping tax records (most self-employed individuals), the principle of income and expenses actually received/paid in 2025 applies:

• Invoice issued in December 2025, paid in January 2026 — the income belongs to 2026
• Invoice received in November 2025, paid in December 2025 — the expense belongs to 2025
• Invoice received in December 2025, paid in January 2026 — the expense belongs to 2026

How to Avoid It

When preparing your return, always check first that you are working with the correct period. Filter your bank statements to cover exactly 1 January 2025 to 31 December 2025.

Mistake #10: Not Keeping a Copy of the Return and Supporting Documents

The final mistake many OSVČ make is failing to keep a copy of the return and the supporting documents after submitting it.

Why This Is a Problem

• The tax authority can carry out an audit up to 3 years retrospectively (and in some cases even longer). If you don't have a copy of the return or the documents at that point, you have nothing to defend yourself with.
• Inability to file an amended return — if you later discover an error, you won't know what to correct without a copy of the original return.
• Summaries for the ČSSZ and health insurance company are based on the figures in your return — without a copy, you cannot fill them in correctly.

How Long to Keep Documents

| Type of document | Minimum retention period | |------------------|--------------------------| | Tax return (copy) | At least 3 years after the filing deadline | | Accounting records and tax records | At least 3 years (5–10 years recommended) | | Issued and received invoices | At least 3 years | | Documents relating to depreciated assets | Throughout the entire depreciation period + 3 years | | Payroll records (if you have employees) | 30 years |

How to Avoid It

Immediately after submitting your return, save:

1. A PDF copy of the submitted return (the MOJE daně portal allows you to download it)
2. The confirmation of receipt (the acknowledgement from your data mailbox)
3. All supporting documents in digital or paper form

Bonus Mistakes: Other Things Self-Employed Individuals Often Overlook

In addition to the 10 mistakes above, there are other common oversights we regularly encounter in practice:

Not Filing Summaries for the ČSSZ and Health Insurance Company

After filing your tax return, you are required to submit an Income and Expenditure Summary for the ČSSZ and your health insurance company. The deadline is one month after the tax return filing deadline. If you file your tax return electronically by 4 May 2026, the summaries must be submitted by 4 June 2026.

Not Accounting for Tax Advance Payments

If your tax liability for 2024 exceeded CZK 30,000, you were required to make advance income tax payments during 2025. These advance payments must be stated in your return, as they are deducted from the final tax amount. If you don't include them, your calculated balance due will appear higher than it actually is.

Incorrect Application of the Co-operating Person Rules

If you allocate a portion of income and expenses to a co-operating person (your spouse), both of you must file separate returns with the correct allocation. A common mistake is applying the wrong split ratio, or the co-operating person failing to file their own return.

Checklist Before Submitting Your Return

Frequently Asked Questions (FAQ)

What should I do if I find a mistake after submitting my return?

It depends on when you spot the error:

• Before the filing deadline (e.g. before 4 May 2026) — file a corrective return. The tax authority will only take into account the last return submitted.
• After the deadline — you must file a supplementary tax return. If you find that you owe more tax, you are required to file it by the end of the month following the discovery of the error. If you overpaid, you may file a supplementary return voluntarily.

Can I avoid a penalty for late filing?

The tax authority allows a 5 working-day grace period. If you file within 5 working days after the deadline, no penalty will be charged. Additionally, penalties up to CZK 1,000 are waived. You can apply to the tax authority for waiver of a higher penalty if you can demonstrate serious grounds (hospitalisation, natural disaster, etc.).

Do I have to file electronically?

In practice, yes. Since 2023, all OSVČ have been automatically assigned a data mailbox. If your data mailbox is active (enabled), you are required to file your tax return exclusively electronically. Filing on paper when you have an active data mailbox carries a fine of CZK 1,000.

How do I know whether the tax authority has received my return?

When filing via a data mailbox, you will receive a delivery confirmation. When filing via the MOJE daně portal, you will receive a confirmation of receipt. You can monitor the status of your submission on the MOJE daně portal after logging in.

Can artificial intelligence help me with my return?

Yes. Modern AI tools can help with organising documents, checking calculations, and sending deadline reminders. However, it is always important to verify the results against official forms and current legislation.

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Official Sources and Links

• Czech Financial Administration — personal income tax
• MOJE daně portal — electronic filing
• Czech Financial Administration — tax forms

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This article is intended as a general informational guide and does not replace individual tax advice. The information is accurate as of the date of publication (February 2026) and is based on currently applicable legislation. For advice on your specific tax situation, please consult a qualified tax advisor.