Key takeaways
- A single missing mandatory mention can make an invoice irregular for VAT purposes.
- Numbering must be sequential, unique and continuous, with no gaps or duplicates.
- Directive 2011/7/EU frames B2B payment terms: 30 days by default, with interest and a fixed sum on late payment.
- Since 1 January 2026, the structured Peppol BIS e-invoice is the B2B norm in Belgium.
The most common SME invoicing mistakes are costly
A badly drafted invoice is not an administrative detail. It is a tax document that governs VAT deduction, the payment deadline and, more recently, compliance with the e-invoicing obligation. The SME invoicing mistakes we see most often are neither rare nor complex: they come down to habits that were never corrected.
The good news is that most of them can be fixed for good by making one invoice template reliable. This article walks through the five most frequent traps, what the rule actually says, and the concrete move to avoid each one.
Trap 1: leaving out a mandatory mention
A Belgian invoice must carry a set of mentions defined by the FPS Finance VAT rules. The main ones: the issue date, a unique sequential number, the identity and address of both supplier and customer, their VAT numbers, a description of the goods or services, the taxable amount and the VAT rate applied.
The classic mistake is not ignoring these mentions but letting one slip: the customer VAT number missing on a B2B sale, a description that is too vague, a rate left unstated. A single missing mention is enough to weaken the invoice for VAT purposes.
Trap 2: numbering with gaps
Each invoice must carry a unique sequential number that identifies it unambiguously. In practice, numbering must be continuous: no jump, no duplicate, no cancelled invoice later "reused" under the same number.
The sources of error are almost always the same: several invoice books running in parallel, a spreadsheet shared between colleagues, or a deleted invoice that leaves a gap in the sequence. Inconsistent numbering complicates reconciliation and draws attention during an audit.
The defence is a single rule: one source of truth for your numbers. A tool that automatically assigns the next number removes the question entirely.
Trap 3: vague payment terms and mishandled late payments
Many SMEs state a vague due date, or none, then absorb late payments without reacting. Yet Directive 2011/7/EU on combating late payment frames the matter clearly for B2B.
| No clear term | A compliant framework | |
|---|---|---|
| Explicit payment term | ||
| Default term that applies | 30 days (statutory) | 30 days, up to 60 if agreed |
| Late-payment interest due | Often overlooked | ECB rate + 8 points minimum |
| Recovery compensation | Rarely claimed | EUR 40 minimum, by right |
Concretely: where no date is agreed, payment is due within 30 days, extendable by contract up to 60 days in B2B. On late payment, you are entitled to interest at the ECB reference rate plus at least 8 percentage points, as well as a minimum fixed sum of EUR 40 for recovery costs. Stating the due date and late-payment terms on the invoice is not aggressive: it is applying the legal framework.
Trap 4: a misapplied VAT rate or a misread regime
VAT concentrates the costliest mistakes, because they repeat on every invoice. The most frequent: a reduced rate applied wrongly, an exemption claimed without meeting its conditions, or an intra-Community transaction treated as a local sale.
The useful reflex is to identify the regime before issuing the invoice, not after. For a sale to a taxable person in another member state, for example, the customer's VAT number and the proper mention determine the treatment. When in doubt about a rate or an exemption, FPS Finance documentation is the authority, not habit.
The five mistakes to eliminate for good
Missing mandatory mention
VAT number, date, description, rate: all must appear on every invoice.
Broken numbering
One unique sequential number per invoice, no gaps or duplicates.
Vague payment term
Written due date and late terms aligned with Directive 2011/7/EU.
Misapplied VAT
Regime and rate checked before issuing, not by habit.
Not ready for structured invoicing
Reliable customer data and a tool connected to the Peppol network.
Trap 5: not preparing for the structured e-invoice
This is the most structural mistake in 2026. Since 1 January 2026, B2B invoices between taxable persons established in Belgium must be structured e-invoices in Peppol BIS Billing 3.0 format, which implements the European standard EN 16931, sent over the Peppol network.
A PDF emailed across is no longer an e-invoice in the sense of the obligation: it is an image, which software cannot read automatically. A structured invoice, by contrast, is a file that the customer's software processes without re-keying. SMEs that put the topic off accumulate a double risk: non-compliance on one side, rejected or manually reprocessed invoices on the other.
Compliant invoices, without thinking about it
YouInv applies the mandatory mentions, handles numbering and sends your Peppol BIS invoices over the network in a few clicks.
Further reading
- E-invoicing in Belgium: the 2026 guide for the calendar and compliance steps.
- Understanding the Peppol network in 5 minutes to grasp the role of access points.
What are the most common SME invoicing mistakes?
The most frequent are a missing mandatory mention (VAT number, date, sequential number), broken numbering, vague payment terms and a misapplied VAT rate. Most are fixed once and for all by standardising an invoice template.
Which mentions are mandatory on a Belgian invoice?
An invoice must carry, among others, the issue date, a unique sequential number, the identity and VAT number of both supplier and customer, a description of the goods or services, the taxable amount and the VAT rate applied. The full list is set out in the FPS Finance VAT rules.
What is the legal deadline to issue an invoice?
According to FPS Finance, the invoice must be issued no later than the 15th day of the month following the month in which the chargeable event took place.
What does the law say about late B2B payments?
Directive 2011/7/EU sets a default B2B payment term of 30 days, extendable by contract up to 60 days. On late payment, the creditor is entitled to interest at the ECB reference rate plus at least 8 percentage points, plus a minimum fixed sum of EUR 40 for recovery costs.
Why does continuous numbering matter?
Each invoice must carry a unique sequential number that identifies it unambiguously. Numbering with gaps or duplicates weakens your bookkeeping and can draw attention during a VAT audit.
How do I prepare for structured e-invoicing?
Since 1 January 2026, B2B invoices between Belgian VAT taxpayers must be structured e-invoices in Peppol BIS format, sent over the Peppol network. Check your customer data and connect your invoicing tool to an access point.
