Accounting and taxes

The most common mistakes when filling out a tax return

Author

Good teacher

Published

February 24, 2026

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Misunderstood exemption of rental income up to €500

When it comes to rental income, many landlords misinterpret the threshold 500 €.

According to Section 9(1)(g) of the Income Tax Act:

If rental income does not exceed €500, are exempted in full. Nare not included in the tax base and they do not need to be listed in the tax return.
If income exceeds €500, is taxed only amount over €500.
Example: income €800 → the tax base includes 300 € (800 – 500).

Incorrectly used form or outdated version

Choosing the right form is essential. The wrong form can mean corrections, challenges, and delays in processing your return.

Applies to:

Tax return type A: only income from dependent activities (employment, agreements).
Tax return type B: business, trade, rental, author's income, other § 6 income or a combination of employment + other income.
Tax return of a legal entity: sro, as, cooperatives, selected non-profit entities.

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The taxpayer files a return but does not pay the tax

This is more common than you might think. The taxpayer files the return but postpones the payment until later. The problem is that the filing deadline and the tax payment deadline are the same.

Consequences: the occurrence of arrears and interest on late payment, or further steps by the tax administrator.

Incorrect tax payment identification (wrong account number or prefix)

Tax is not paid into one universal account. Each taxpayer has their own account, which consists of:

prefixes by tax type,
taxpayer's personal account,
State Treasury code.

Examples of prefixes:

self-employment income tax → 500259
business and rental income tax → 500208

Procedure:

choose correctly prefix,
verify personal taxpayer number (according to the tax number/tax ID via the financial administration),
generate IBAN (e.g. via the NBS IBAN generator).

Incomplete or incorrectly completed return

Formal errors will cause the declaration to be incomplete or unenforceable.

Most common:

missing signature (when submitting on paper),
missing date,
it is not marked whether it is proper / corrective / additional confession,
empty lines to be filled in,
errors in typesetting, rounding, and conversions.

Submitting multiple versions of the same return

When filing electronically, some taxpayers get scared that "it didn't go through" and resubmit the form. Or they find an error and submit another return.

Correct procedure: the declaration is filed once. If you made a mistake:

submit corrective declaration, if you are still within the deadline,
submit additional declaration, if you discovered the error after the deadline.

Missing attachments (especially income confirmation)

For type A, it is classic: missing certificate of taxable income from the employer.

Consequences: the tax office cannot verify the data → request for additions → delay in processing and possible overpayment.

Missing social security number or VAT number on each page

When filling out the form manually, it is mandatory to provide the registration number/tax ID number on each page of the form.

How to avoid the mistake: Electronic filing will eliminate this problem automatically. With paper, go through it page by page.

Missing SK NACE designation (entrepreneurs)

Entrepreneurs with income according to Section 6, Paragraphs 1 and 2 often forget to add the main activity according to SK NACE.

How to avoid the mistake: Check the SK NACE by ID number in the register of the Statistical Office and fill it in the introductory part of the declaration.

Failure to indicate the method of application of expenses (actual vs. flat rate)

For type B, it is necessary to clearly state whether you are applying:

actual (provable) expenses – require registration and documentation,
lump sum expenses – percentage of income, without documentation of real costs.

Consequences: a request for correction or incorrect tax base.

Incorrectly applied child tax bonus

A common mistake is dividing children between parents. The law does not allow this.

Applies to:

can claim the tax bonus in one month only one parent,
and that for all children at once,
However, parents can alternate after months (e.g. 6 months father, 6 months mother).

The tax return deferral is only announced

Many are waiting for the tax office to confirm the deferral. The deferral is does not confirm, it is enough to file the notification correctly and on time.

If you have mandatory electronic communication, the deferral must also be made electronically through the Financial Administration portal.

How to avoid the mistake: After sending, check your email for notification.

Errors when remitting 2 % from tax

Even a small mistake can cause 2 % to not be credited.