An A1 certificate proves that a worker posted to Belgium remains covered by their home country’s social security system, so neither you nor they owe Belgian social security contributions for that period. It is issued by the competent institution in the worker’s home country under EU Regulation (EC) 883/2004, not by Belgium. It is not a work permit and it does not replace the Limosa declaration. On a Belgian site, an inspector will typically ask to see both.
What does an A1 certificate actually prove?
Within the EU, EEA and Switzerland, a worker is in principle covered by the social security system of the country where they work. That would mean a Polish electrician working three months in Ghent starts paying Belgian contributions, and stops accruing rights at home.
Coordination rules exist to prevent that. Where the conditions are met, a posted worker stays affiliated to their home system, and the A1 is the document that evidences it. It names the worker, the employer, the period and the country whose legislation applies.
What it does not do is equally important. The A1 says nothing about:
- Whether Belgium has been notified that the worker is present. That is Limosa.
- Whether the worker is registered on the site today. That is Check-in-at-Work.
- Whether the worker is paid Belgian minimum wages and sector conditions. Posted workers are entitled to core Belgian working conditions regardless of which social security system they remain in.
Treating the A1 as a general “permission to work in Belgium” document is the mistake that produces most of the trouble we see.
Who issues it and how long does it last?
The A1 is issued by the competent institution in the sending country, the body that administers social security there. You apply to your own national institution before the posting begins, not to a Belgian authority.
For a standard posting, coverage in the home system can generally continue for up to 24 months. Where a worker habitually works in two or more Member States, a different rule applies and the A1 reflects that pattern instead of a single posting.
Two practical consequences:
- Apply early. Processing times vary by country and by season. A certificate that arrives after your crew has started is a certificate that did not cover the first two weeks.
- Watch the end date. Postings that overrun their declared period are a recurring finding in inspections. If the project slips, the paperwork has to move with it.
What does a Belgian inspector check?
Belgian social inspectors do not simply confirm that a document exists. They check that it corresponds to what is happening on the ground. Expect scrutiny of:
- Identity match. The person in front of them is the person named on the certificate. Crew substitutions that were never reflected in the paperwork are found quickly.
- Employer match. The employer on the A1 is the employer actually directing the work. Where a worker is in practice integrated into the Belgian company’s organisation and instruction, the posting itself can be questioned.
- Date coverage. The dates of the certificate cover today. Expired or not-yet-valid certificates are common findings.
- Place and client. What is declared corresponds to the site and the principal.
- The rest of the file. The A1 alongside the LIMOSA-1, and the presence registration for the site if Check-in-at-Work applies.
The inspectorate may also ask the posting company to supply documents from the country of origin equivalent to Belgian social records concerning remuneration, under the Posting Act of 5 March 2002. In other words, the questions do not stop at the certificate.
Why an A1 does not exempt you from Limosa
This is worth stating plainly because it is the most expensive assumption in this area.
The two documents serve different authorities and different purposes. The A1 answers “which country’s social security applies?” Limosa answers “does Belgium know this person is working here?” An employer holding a valid A1 for a posted worker still has to file a Limosa declaration before the work starts.
Failing to do so is a level 4 offence, the heaviest category in the Belgian Social Criminal Code. Fines increased on 1 February 2026 under the Act of 19 December 2025, which raised the multiplier applied to social criminal fines from 8 to 10, and for some infringements the amount is multiplied by the number of workers involved, up to a maximum of 100.
What happens if the A1 is missing or wrong?
If a worker cannot produce a valid A1, the working assumption becomes that Belgian social security applies, with contributions owed in Belgium for the period concerned. That is a direct cost, and it is usually accompanied by questions about what else was not in order.
There is a second consequence that lands on your Belgian client. Where a foreign employer cannot evidence valid A1 coverage or has outstanding social debts, the withholding obligation can be triggered: the client must withhold 35 percent of the invoice amount excluding VAT for social security debts and pay it directly to the NSSO, plus 15 percent for tax debts to FPS Finance. Clients verify this at checkinhoudingsplicht.be.
From the main contractor’s side, a subcontractor with unreliable A1 paperwork is not an administrative nuisance. It is a party whose invoices may have to be part-withheld and whose workers’ wages the contractor could become liable for under Belgium’s joint and several liability rules. That is why the good sites now ask for the file up front.
A practical checklist before your crew travels
- Apply for A1 certificates for every posted worker, including short trips and including managers who will be on site.
- Check that names on the A1, the Limosa declaration and the site registration are identical, including spelling.
- Confirm the certificate period covers the whole planned posting, with room for overrun.
- Give each worker their own copy, on their phone at minimum. A file on an office server in another country does not help at a gate at 7am.
- Name your Belgian liaison person and make sure they can actually be reached about the posting.
- Set a calendar reminder before each certificate expires, and treat crew changes as a paperwork event rather than an informal swap.
- Make sure presence registration is running from day one, not from the first invoice.
Keeping the paperwork and the site in sync
Most compliance failures in posting are not decisions to break the rules. They are drift: a crew change nobody logged, a project extended by three weeks without anyone revisiting the certificates, a site where attendance lives on a clipboard that went home in a van.
The fix is to make the record a by-product of work rather than an administrative task performed later. When arrival on site registers the worker automatically against the right project, the attendance record matches the declared crew by default, and gaps become visible while they can still be corrected. That is the principle behind Suivo’s workforce management platform: compliance data that is captured once, at the moment it happens, and then reused for hours, payroll and project costing rather than kept in a separate silo.
See how Cegelec keeps registration consistent across multiple Belgian sites.
An A1 is one document among several. For how it fits with Limosa, Dimona and site registration, see our guide for foreign employers working in Belgium.
Book a free demo
If you post workers into Belgium and want the paperwork and the site record to agree without chasing anyone, we will show you how it works in practice. Book a free demo.
