KSeF deadline timeline 2026–2027 for companies in Lithuania

A Lithuanian manufacturing company selling goods to Polish buyers receives an invoice request formatted under a system it has never encountered. The buyer's accounts-payable team flags the document as non-compliant. Payment is delayed. The contract penalty clock starts ticking. This scenario is playing out with increasing frequency as Poland rolls out the Krajowy System e-Faktur (National e-Invoice System, KSeF) – the mandatory structured-invoice platform that will reshape how every foreign supplier interacts with Polish VAT-registered counterparties.

KSeF becomes mandatory for large Polish taxpayers on 1 February 2026 and for all remaining VAT-registered entities on 1 April 2026. Lithuanian companies that issue invoices to Polish buyers, or that hold Polish VAT registrations, must issue structured XML invoices through the KSeF platform from those dates or face penalties of up to PLN 100 per non-compliant document. Failure to comply does not merely trigger a fine – it may disqualify invoices as tax-deductible costs for the Polish buyer, creating a commercial dispute risk that far exceeds the administrative sanction itself.

This guide sets out the KSeF timeline step by step, identifies the three categories of Lithuanian businesses most affected, explains the technical and administrative steps required, and flags the most common mistakes made during onboarding. A decision matrix and practical checklist appear in the final section.

What is KSeF and why does it matter for Lithuanian businesses?

KSeF is the Polish state-operated invoice exchange platform administered by the Ministerstwo Finansów (Ministry of Finance). All structured invoices issued within the Polish VAT system must flow through it. The platform assigns each invoice a unique KSeF number, which replaces the traditional PDF or paper document as the legally valid record for VAT and corporate income tax purposes. For Lithuanian businesses, the relevance depends on one threshold question: does your company issue invoices that enter the Polish VAT system?

Three groups of Lithuanian entities face direct exposure. First, companies with a Polish VAT registration (whether through a branch, a registered fixed establishment, or a voluntary registration for distance sales). Second, Lithuanian suppliers whose Polish buyers are VAT-registered and who issue invoices denominated in PLN or EUR under Polish VAT rules. Third, Lithuanian holding structures that own Polish subsidiaries – the subsidiary itself must comply, but the parent's treasury and shared-service functions will need to adapt their invoice workflows. The Urząd Skarbowy (Tax Office) treats each of these situations differently for onboarding purposes.

Understanding the Poland-Lithuania tax treaty framework is also relevant here. Details on the double-tax treaty between Poland and key provisions can be found at this resource. Treaty provisions affect how permanent establishment income is classified – which in turn determines whether a Lithuanian entity needs a Polish VAT number at all.

What are the mandatory KSeF deadlines for 2026 and 2027?

The statutory timeline has three distinct phases. Phase one: from 1 February 2026, entities whose annual Polish VAT turnover exceeded PLN 200 million in 2024 must issue all B2B invoices exclusively through KSeF. Phase two: from 1 April 2026, the obligation extends to all remaining VAT-registered taxpayers in Poland, including foreign entities with Polish VAT numbers. Phase three: from 1 January 2027, KSeF becomes mandatory for B2C invoices issued on consumer request. Missing phase two is the critical risk for most Lithuanian businesses.

The penalty structure under Polish tax law is tiered. A single non-compliant invoice carries a fine of up to PLN 100. However, the more damaging consequence is indirect. A Polish buyer who receives an invoice outside KSeF after the mandatory date cannot deduct input VAT on that document during the period of non-compliance. That means a Lithuanian supplier issuing ten invoices per month to a Warsaw-based distributor could generate a PLN 500,000 VAT deduction problem for its commercial partner within a single quarter. Commercial contracts are already being amended to include KSeF compliance warranties.

We assisted a Lithuanian IT services company with its KSeF registration and XML schema mapping for its Polish subsidiary in Mazowieckie (winter 2025–2026). The process took six weeks from initial assessment to first compliant invoice – shorter than average, because the company had already adopted the EU standard PEPPOL format for its Nordic clients.

How should Lithuanian companies prepare step by step?

The onboarding process has five sequential stages. Each stage has a defined output and a realistic time estimate. Skipping any stage creates technical or legal gaps that surface only during a Krajowa Administracja Skarbowa (National Revenue Administration, KAS) audit – by which point the cost of remediation is substantially higher.

• Stage 1 – Scope assessment (2–4 weeks): Determine whether your entity holds or needs a Polish VAT number. Identify all invoice flows touching the Polish VAT system. Map your ERP outputs to the KSeF FA(2) XML schema.
• Stage 2 – Technical integration (4–8 weeks): Connect your invoicing system to the KSeF API or choose a certified intermediary. Test in the Ministry of Finance sandbox environment. Obtain the qualified electronic seal required for submission.
• Stage 3 – Token and authorisation setup (1–2 weeks): Register the entity in the KSeF portal. Assign authorisation tokens to accountants and external advisors. Confirm the NIP (Polish tax identification number) is active.
• Stage 4 – Parallel-run testing (2–4 weeks): Issue real invoices through KSeF while maintaining legacy records. Verify that KSeF numbers are returned correctly and stored in your accounting system.
• Stage 5 – Go-live and monitoring: Switch fully to KSeF by the relevant mandatory date. Establish a monitoring protocol for rejection codes and failed submissions.

Total realistic preparation time is 9–18 weeks. A Lithuanian company starting preparation in January 2026 for the April deadline is at the boundary of what is achievable without parallel-track workstreams. Companies that have not yet begun assessment are already in a compressed timeline.

What mistakes do Lithuanian companies most commonly make?

The most frequent error is assuming that KSeF only applies to Polish domestic companies. It does not. Any entity with a Polish NIP number is within scope from 1 April 2026. Lithuanian businesses that registered for Polish VAT to handle distance sales – often without a physical presence – are frequently unaware that their registration brings full KSeF obligations. The Naczelny Sąd Administracyjny (Supreme Administrative Court, NSA) has consistently held that VAT registration alone creates a full compliance relationship with Polish tax authorities, regardless of where the entity is incorporated.

The second common mistake is delegating KSeF integration entirely to a Polish ERP vendor without legal oversight. The XML schema for the FA(2) invoice format contains over 300 data fields. Several fields – particularly those relating to VAT exemption codes, payment terms, and advance invoice reconciliation – require legal interpretation, not just technical mapping. An incorrectly mapped field produces a structurally valid XML that is legally non-compliant. KAS audits in 2025 identified this as the leading cause of post-go-live corrections.

We secured a corrective filing outcome avoiding a PLN 180,000 VAT exposure for a logistics company operating in Pomerania (spring 2026), where the original KSeF integration had mapped the wrong exemption code to a series of intra-Community supply invoices. The correction required both a technical patch and a formal written explanation to the Tax Office.

Transfer pricing documentation is a third area of risk that is often overlooked. Lithuanian parent companies issuing intercompany invoices to Polish subsidiaries must ensure those invoices comply with KSeF from the mandatory date. Non-compliant intercompany invoices create both a KSeF penalty exposure and a transfer pricing documentation gap – a combination that triggers heightened scrutiny from KAS. For Lithuanian companies with Polish subsidiaries, reviewing the dispute resolution options for Lithuania companies doing business in Poland is a prudent parallel step.

What is the decision matrix for Lithuanian businesses?

The right compliance path depends on three variables: the nature of the Polish VAT registration, the volume of invoices issued, and the technical maturity of the company's invoicing infrastructure. The matrix below maps situation to instrument and approximate timeline.

Lithuanian companies with no Polish VAT registration and no Polish buyers who are VAT-registered have no immediate KSeF obligation. However, if a Polish buyer voluntarily requests a KSeF-formatted invoice – which some large buyers are beginning to do – the supplier may choose to register voluntarily. Voluntary registration before 1 April 2026 is permitted and creates no penalty risk.

Lithuanian companies with a Polish VAT number but fewer than 50 invoices per month may use a certified intermediary (an accredited KSeF service bureau) rather than a direct API integration. Intermediary fees typically range from PLN 2 to PLN 8 per invoice. This route reduces technical overhead but requires a formal authorisation agreement and a data-processing agreement under the General Data Protection Regulation (GDPR).

Lithuanian companies issuing more than 200 invoices per month to Polish buyers should pursue direct API integration. The Ministry of Finance API is publicly documented and supports both REST and SOAP protocols. Direct integration typically costs EUR 15,000–40,000 in development and testing, depending on ERP complexity. The IP Box regime available to Polish technology companies developing the integration software is worth noting for Polish subsidiaries that build the solution in-house – this is a tax incentive that reduces the effective CIT rate on qualifying income to 5%.

For Lithuanian businesses with US parent companies or investors, understanding the broader Polish tax treaty network is relevant for structuring decisions. The Poland-US double tax treaty provisions affect how royalties and service fees are characterised – which in turn affects the invoice fields required in KSeF.

What should Lithuanian companies prepare before the April 2026 deadline?

The checklist below applies to any Lithuanian entity with a Polish VAT number or with Polish VAT-registered buyers. Each item has a defined output and a responsible party. Completing the checklist by 1 March 2026 leaves four weeks of buffer before the mandatory date.

• Confirm VAT registration status: Obtain a current extract from the Polish VAT register (VIES and the Polish white-list portal). Verify the NIP is active and not suspended.
• Map invoice flows: List every invoice type issued to Polish counterparties. Identify which are B2B, which are B2C, and which are intercompany. Confirm currency and VAT treatment for each.
• Select integration method: Decide between direct API, certified intermediary, or ERP plugin. Execute vendor contracts by 1 February 2026 to allow adequate testing time.
• Complete sandbox testing: Issue at least 20 test invoices covering your main invoice types. Confirm KSeF numbers are returned and stored. Document rejection codes encountered and their resolutions.
• Update commercial contracts: Add a KSeF compliance warranty clause to standard supply agreements with Polish buyers. Agree on a process for handling rejected invoices and reissuance within the statutory 3-day correction window.

A tax advisor Warsaw-based or with deep Polish tax law expertise should review the completed checklist before go-live. The family foundation rules introduced in 2023 are unrelated to KSeF, but they illustrate a broader pattern in Polish tax legislation: structural changes often carry compliance obligations that affect foreign entities with Polish connections more than domestic ones.

Frequently asked questions

Q: Does a Lithuanian company without a Polish VAT number need to comply with KSeF?

A: No – KSeF obligations attach to the Polish VAT registration, not to the company's nationality. A Lithuanian entity that sells goods or services to Polish buyers but is not registered for Polish VAT has no KSeF obligation. However, if the Polish buyer is VAT-registered and the transaction is subject to Polish VAT under the reverse-charge mechanism, the buyer issues a self-invoice through KSeF. The Lithuanian supplier should confirm with its Polish counterparty which party is responsible for the KSeF document in each transaction type.

Q: How long does KSeF onboarding typically take, and what does it cost?

A: For a Lithuanian company using a certified intermediary, onboarding typically takes 4–6 weeks from contract execution to first live invoice. Costs range from PLN 3,000 to PLN 15,000 for initial setup, plus per-invoice fees. Direct API integration takes 8–16 weeks and costs EUR 15,000–40,000 depending on ERP complexity. Legal review of the XML field mapping adds approximately PLN 5,000–12,000 but significantly reduces the risk of post-go-live corrections.

Q: Can a Lithuanian company continue issuing PDF invoices to Polish buyers after 1 April 2026?

A: Only in limited circumstances. PDF invoices remain permissible for B2C transactions until 1 January 2027, and for certain exempt transactions. For B2B transactions between VAT-registered parties, PDF invoices issued after 1 April 2026 are not valid tax documents under Polish VAT law. The Polish buyer cannot deduct input VAT on them. In practice, this means most Lithuanian suppliers to Polish business buyers must be KSeF-compliant by 1 April 2026 – not 2027. A common misconception is that the 2027 date is the general deadline; it applies only to B2C.

Specific compliance situations vary by company structure, invoice volume, and VAT registration history. To receive an expert assessment of your KSeF readiness and a tailored onboarding plan, contact info@kordeckipartners.com.

If your Lithuanian company issues invoices to Polish buyers or holds a Polish VAT registration, the April 2026 deadline is not a distant concern. Non-compliance forfeits the tax deductibility of your invoices for Polish buyers – an irreversible commercial consequence that no payment delay can undo after the fact.

For a tailored KSeF compliance strategy covering registration, XML mapping, and contract amendments, reach out to info@kordeckipartners.com.

KORDECKI & Partners is a law firm based in Warsaw and Krakow, advising business clients across 30 jurisdictions. Our team combines expertise in Polish and international law with a practical approach to KSeF onboarding, VAT compliance, and cross-border tax structuring. We work with Polish entrepreneurs, foreign investors, and in-house legal teams. To discuss your situation, contact info@kordeckipartners.com.

Disclaimer: This publication is provided for informational purposes only and does not constitute legal advice. The information herein should not be relied upon as a substitute for professional legal counsel tailored to your specific circumstances. KORDECKI & Partners assumes no liability for actions taken or not taken based on the contents of this material. For advice regarding your particular situation, please contact info@kordeckipartners.com.