As part of the upcoming implementation of the mandatory National e-Invoicing System (KSeF) in Poland from 2026, the Ministry of Finance has announced a long-awaited feature: the ability to send invoices with attachments via KSeF. While this may seem like good news for many businesses, the details reveal a more nuanced picture.
Who can use invoice attachments?
Although the new feature will technically be available to all taxpayers, it has been specifically designed with certain industries in mind:
- energy,
- fuel,
- gas,
- telecommunications.
These sectors often issue detailed attachments alongside invoices, such as consumption data by point of delivery or line-item breakdowns for telecom services. For such use cases, KSeF will allow a structured attachment to be submitted as an integral part of the e-invoice.
How will it work?
- to use this functionality, the taxpayer must first submit a declaration of intent via the e-Tax Office (e-Urząd Skarbowy),
- the functionality will be available starting January 1, 2026,
- attachments must be in a structured data format, consisting of blocks and tables with descriptions,
- the FA(3) logical structure will be extended with a new optional node called “Zalacznik” (Attachment),
- while flexible and adaptable to industry-specific needs, the attachment must conform to the logical structure of the system.
What cannot be attached?
Unfortunately, this feature will not solve all invoicing challenges. Businesses in sectors such as construction, B2B services, or consulting will not be able to include:
- work schedules,
- cost estimates,
- acceptance protocols,
- specifications of completed work.
Such documents, which are often key to understanding and verifying an invoice, will have to be sent separately, for example by email, along with a visualization of the e-invoice from KSeF.
What does this mean for businesses?
In practice, companies will need to maintain a dual document workflow:
Legally, invoices must go through KSeF, but commercially, supporting documents will still need to be sent directly to business partners. This may result in:
- a need to adjust internal invoicing and document processes,
- investment in commercial invoicing software (the free KSeF app does not support attachments),
- additional communication with clients to ensure they receive all necessary information.
Summary
The option to include attachments in KSeF is a step forward — but with important limitations. KSeF will not yet serve as a universal platform for all business communication. Companies should begin reviewing their invoicing processes now to determine which documents can be integrated into KSeF and which will still need to be shared externally.
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