Validation rules to tighten from September 2025

Since its introduction in 2018, the NAV Online Invoice System has undergone continuous development, aimed at more accurate data reporting and improved data quality. The new business validations, effective from 15 September 2025, will once again bring stricter requirements for businesses. The changes will affect not only invoicing software but will also have a direct impact on accounting and tax processes.

The new rules will already be available in the test system from 1 September 2025, giving companies and developers the opportunity to prepare in advance.

Why are the changes necessary?

The aim of the Hungarian Tax and Customs Authority (NAV) is to ensure that the invoice data submitted by businesses is as accurate and reliable as possible. This not only facilitates the work of the authority but also benefits businesses, as digital services built on accurate data – such as the eVAT system – can operate more smoothly.

NAV submitted the proposed changes for a two-month professional consultation, during which several market participants provided feedback. Based on these responses, the draft was modified at several points, so the finalised system takes professional practice better into account.

The main changes in the NAV Online Invoice System

From September, the following modifications will take effect:

• Three new WARN messages will be introduced.

• Three existing WARN messages will be discontinued and will no longer be displayed.

• One INFO message will become a WARN, highlighting content-related issues.

• Fifteen WARN messages will become ERROR messages, which will prevent data submission in the future.

What is the difference between ERROR and WARN?

• ERROR messages are critical: data submission cannot be completed until the error is corrected.

• WARN messages do not block the submission of invoice data but indicate a content issue that may, in the long term, also lead to incorrect or inaccurate reporting.

Ignoring WARN messages is therefore risky, as NAV’s aim is to ensure that all details of XML-based data reporting are correct and consistent.

List of new ERROR rules

NAV has specified exactly which WARN validations will become blocking errors. Among them:

• 330 – Closing date of performance period cannot be earlier than the opening date.

• 434 – In case of unique unit of measure (OWN), the corresponding XML element is missing.

• 560 – The modification document number cannot be identical with the original invoice number.

• 581–584 – Incorrect VAT marking linked to various tax codes.

• 591, 593 – VAT data present despite exemption or “outside scope of VAT Act” marking.

• 596 – In case of domestic reverse charge, the buyer must be a domestic VAT taxpayer.

• 620 – Performance date required for collective invoice item.

• 701 – VAT data present in summary despite “outside scope of VAT Act” marking.

• 1150 – Unrealistic invoice modification sequence number.

• 1300, 1310 – Incorrect or extreme exchange rate provided.

These errors will automatically prevent data submission in the future, making it essential to review invoicing processes.

New WARN and INFO rules

In the new system, three new WARN messages will be introduced:

• 435 – Incorrect invoice line item data.

• 734 – Incorrect summary data.

• 1311 – Incorrect invoice header data.

In addition, the previous 11400 INFO message (incorrect performance date) will operate as a WARN, thus receiving greater emphasis during data reporting.

How can businesses prepare?

The September transition may pose serious technical and organisational challenges for companies. Preparation is essential, especially for those who regularly encounter WARN messages in the Online Invoice System.

Steps for preparation:

1. Review processes – Analyse WARN messages generated in the last 12 months, as many will become ERRORs.

2. Contact invoicing software developers – Ensure that necessary updates and XML modifications are implemented in time.

3. Use the test environment – From 1 September, companies can verify whether their system complies with the new rules, reducing risks at go-live.

4. Establish internal monitoring – Regular tracking and analysis of WARN messages helps prevent future disruptions.

Detailed information and the regulatory changes are available on NAV’s GitHub page:
https://github.com/nav-gov-hu/Online-Invoice/discussions/1144?sort=new

Why is preparation important?

The new validation rules are not merely administrative. Failure to correct ERROR messages in time may lead to reporting disruptions, late filing penalties, or other tax authority consequences.

Moreover, the eVAT system and other machine-processing-based services rely on accurate data, meaning that erroneous or incomplete XML files could cause significant issues for businesses in the future.

The September 2025 changes to the NAV Online Invoice System will raise the bar for data reporting requirements. With WARN messages turning into ERRORs, companies must pay much closer attention to invoicing and accounting processes.

Our advisors are ready to use automated tools to analyse the data communication history between your invoicing software and the tax authority’s system. Based on this, we identify the points where your invoicing practices or system operation may need to be adjusted in order to comply with the stricter validation requirements taking effect in September

Those who prepare in time will avoid disruptions caused by errors and ensure that their reporting complies with NAV requirements.