Adding attachments to invoices in KSeF – what does the FA(3) structure change?

Paweł Gorzelec
Nov 17
3 min read

With the upcoming mandatory implementation of the National e-Invoice System (KSeF), which will come into effect on February 1, 2026, the Ministry of Finance is working intensively to adapt the system to the real needs of businesses. One of the most frequently reported requests was the ability to add attachments to structured invoices.

Until now, KSeF lacked this functionality, and many taxpayers rely on attachments in their daily transactions (e.g., in the energy, technical, and manufacturing sectors). In response to these needs , the FA(3) structure has introduced the ability to attach attachments. However, these will not be traditional PDF or JPG documents, but special XML data blocks that constitute an integral part of the invoice.

What can an attachment contain?

Attachments will be in the form of an extended XML structure. This means they can include, among other things:

tables with headers and rows,
detailed technical parameters,
energy consumption data,
tariffs, rates and units of measurement,
quantitative or qualitative descriptions of goods and services,
data required by regulated industries.

This is a great convenience for companies settling sales based on extensive statements.

Note: Attachments cannot include typical business documents such as:

price lists,
offers,
contracts,
acceptance protocols,
order confirmation.

Such materials must be transferred outside KSeF.

How to technically add an attachment?

The Ministry of Finance introduces a clear restriction: ➡ Attachments can only be added via the KSeF API .

Free MF tools will only allow you to read and download attachments—not create them. Therefore, companies using attachments will need to use software integrated with the API.

Additional technical limitations:

error in the attachment = need to issue a corrective invoice ,
the maximum size of an invoice with an attachment is 3 MB (without an attachment – 1 MB),
documents are stored in the system for 10 years from the end of the year of issue .

The 3 MB limit is particularly important – in the case of large summaries, this may be a barrier.

Who can add attachments?

The basis is authorization to issue invoices in KSeF. Attachments can also be received by individuals authorized to receive invoices.

However, something else is crucial:➡ the entity must first notify the intention to send invoices with attachments and obtain system acceptance .

The notification is made exclusively via the KSeF API – i.e. through accounting or ERP software compatible with the system.

What must the application contain?

taxpayer identification data,
justification for the need to add attachments (e.g. complicated pricing structure, technical parameters),
expected start date,
estimated number of invoices per month,
indication of the type of activities to which the annex applies.

Once approved, the taxpayer receives two years of use . After this period, the application must be resubmitted.

Deadline for response

The Ministry of Finance has 30 days to review the application. Anyone wishing to issue invoices with attachments from February 1, 2026 , should submit their application in advance.

The "Additional Description" field – a small element, a big impact

While this field is not mandatory, it can be crucial in practice. It allows you to provide information that cannot be included in an attachment, such as:

order number,
e-mail address for sending additional PDF files,
link to the documents folder,
references to contracts, protocols, specifications.

This will significantly simplify work in many companies – especially where orders and protocols are an integral part of the commercial process.

Summary

Adding attachments in the FA(3) structure is a significant improvement, but it comes with significant limitations. XML attachments are primarily intended for companies that handle technical or quantitative data. Other documents still need to be transferred outside of the KSeF, preferably using the "Additional Description" field to link them to the invoice.