Key takeaways
- A compliant invoice rests on precise mandatory details, set by the VAT Code and Royal Decree No. 1.
- The issuance deadline is the 15th day of the month following the VAT chargeable event.
- Numbering must be unique and sequential, with no gap or duplicate.
- Invoices must be kept for ten years, counted from 1 January of the year after their date.
- Since 2026, the structured Peppol BIS invoice changes the form, not these substantive rules.
Invoicing rules in Belgium, in a single framework
The invoicing rules in Belgium are not a matter of layout. They determine the tax validity of an invoice, your customer's right to deduct VAT, and your exposure in an audit. The framework rests on the VAT Code and on Royal Decree No. 1 of 29 December 1992, which sets out the formal obligations of an invoice, rules detailed by FPS Finance on VAT accounting and invoicing.
This article gathers, as a systematic reference, what a compliant invoice must contain, when it must be issued, how it must be numbered, and how long it must be kept. It also places the structured invoice, mandatory since 1 January 2026, against the obligations that do not change. For the practical traps of day-to-day work, see our overview of the most common invoicing mistakes of Belgian SMEs.
The mandatory details of an invoice
The core of a compliant invoice lies in its mandatory invoice details. According to FPS Finance, an invoice issued by a taxable person must carry a precise set of information, without which the document is not regular. These details derive from the VAT Code and have their EU counterpart in Article 226 of Directive 2006/112/EC, which defines the common content of an invoice across the Union.
The absence of a single one of these details can be enough to weaken your customer's VAT deduction and to expose the issuer to a fine. The list below reflects the details confirmed by FPS Finance.
Mandatory details confirmed by FPS Finance
Issue date
The day the invoice is drawn up; it ties the transaction to a VAT period.
Unique sequential number
A number that identifies the invoice within a continuous series.
Supplier and customer identity
Full names or business names and addresses of both parties.
VAT numbers
The VAT identification number of the supplier and, where applicable, of the customer.
Description of the transaction
Nature and quantity of the goods supplied, or extent and nature of the services.
Date of supply
The date of delivery or service where it differs from the issue date.
Taxable base per rate
The unit price excluding VAT and the taxable base, broken down by applicable rate.
VAT rate and amount
The applicable VAT rate and the amount of tax due.
The issuance deadline: the 15th of the following month
An invoice that is compliant in content can still be irregular if it is issued too late. The general rule is clear: the invoice must be drawn up no later than the 15th day of the month following the one in which VAT became chargeable, that is, the chargeable event.
In practice, for a supply of goods made on 10 February, the invoice must be issued by 15 March at the latest. This rule applies to both supplies of goods and supplies of services. Past that deadline you are in breach with FPS Finance, even if the content of the invoice is otherwise complete.
This deadline carries a practical consequence that is often underestimated: it sets the rhythm of your invoicing. A transaction at the end of the month leaves you only a few days before the following month's cut-off. Keeping to that calendar requires a process that does not hinge on one person's availability. This is exactly where a tool that generates the invoice as soon as the transaction is confirmed secures the deadline.
Sequential numbering, with no gap or duplicate
The invoice number is not a mere internal identifier. It must be unique and belong to a continuous, uninterrupted series. It is this continuity that lets the tax authority verify that no invoice has been removed or added after the fact.
Two deviations are off limits: the duplicate, which gives the same number to two invoices, and the gap, which leaves an unused number in the series. Either one is an irregularity that surfaces in an audit. In practice, numbering driven by software, rather than assigned by hand, removes this risk by design.
Nothing stops you from structuring the series, for instance by year or by sales journal, as long as each invoice stays uniquely identifiable and the sequence remains continuous within the chosen logic. The sensitive point is not the format of the number but traceability: a cancelled invoice is not deleted, leaving a void; it is offset by a credit note that preserves the trace of the original transaction.
Retention: ten years, counted from the following year
Issuing a compliant invoice is not enough: you must also be able to produce it years later. The legal retention period is ten years, counted from 1 January of the year following the invoice date. This period is set by Article 60 of the VAT Code; it rose from seven to ten years for taxes that became chargeable from 1 January 2023.
Retention covers substance as much as form. You must guarantee the authenticity of origin, the integrity of content, and the legibility of the invoice throughout that period, whether the invoice is on paper or electronic. A well-archived structured invoice meets these three requirements naturally, provided you keep the original file.
Compliant invoices, without policing every detail
YouInv applies the mandatory details, sequential numbering and Peppol BIS format to your Belgian invoices.
Where the structured invoice fits since 2026
Since 1 January 2026, national B2B transactions between taxable persons established in Belgium use a structured electronic invoice in the Peppol BIS format, under the Law of 6 February 2024. This obligation changes the container of the invoice: a structured file (UBL) readable by software replaces the PDF or paper.
It does not change the rules reviewed above. The mandatory details, the issuance deadline, sequential numbering, and the retention period remain those of the VAT Code. The structured invoice simply carries the same information in standardised fields, compliant with the semantic model of the EN 16931 standard. In other words, moving to the structured format waives none of these obligations: it makes them easier to meet end to end.
| Still governed by the VAT Code | Changes with the 2026 reform | |
|---|---|---|
| Mandatory details | ||
| Issuance deadline (15th of following month) | ||
| Sequential numbering | ||
| Ten-year retention | ||
| Invoice format (PDF or structured) | ||
| Transmission channel (Peppol) |
Further reading
- The most common invoicing mistakes of Belgian SMEs
- VAT and e-invoicing in Belgium: what changes
- Peppol in Belgium: the 2026 e-invoicing mandate
What are the mandatory details on an invoice in Belgium?
According to FPS Finance, an invoice must show the issue date, a unique sequential number, the names and addresses of supplier and customer, their VAT numbers, the description and quantity of the goods or services, the date of supply if different, the taxable base per rate, and the applicable VAT rate and amount due. These details derive from the VAT Code and from Article 5 of Royal Decree No. 1.
What is the deadline to issue an invoice in Belgium?
The invoice must be issued no later than the 15th day of the month following the one in which VAT became chargeable (the chargeable event). For a delivery on 10 February, for example, the invoice must be issued by 15 March at the latest.
Must invoice numbering be sequential?
Yes. Each invoice must carry a unique number that belongs to a continuous, uninterrupted series. A gap in the numbering or a duplicate is an irregularity the tax authority can flag during an audit.
How long must invoices be kept in Belgium?
The legal retention period is ten years, counted from 1 January of the year following the invoice date. This period, set by Article 60 of the VAT Code, rose from seven to ten years for taxes chargeable from 1 January 2023.
Does the structured invoice change the mandatory details?
No. Since 1 January 2026, national B2B transactions between taxable persons established in Belgium use a structured Peppol BIS invoice. The format changes, but the mandatory details, the issuance deadline, the numbering and the retention rules remain those of the VAT Code.
