Luxembourg is the eighth-largest source of posted workers into Belgium, which is remarkable given its size. The border is short, the labour market is deeply integrated, and cross-border work is entirely routine. That routineness is the problem: the more normal a Belgian job feels, the less likely anyone is to file anything.
Where Luxembourg sits in the Belgian market
Luxembourg companies filed for 7,740 posted persons into Belgium in 2023, 3.3 percent of the Limosa total, ahead of much larger countries. The number has been stable across recent years, unlike the Dutch and Lithuanian figures which fell when transport postings moved out of Limosa.
The flow is genuinely two-way. Luxembourg receives around 10 percent of A1 documents issued by Belgium under Article 12, making it one of Belgium’s main outbound destinations alongside France and the Netherlands.
The four things you must have
Work backwards from your mobilisation date, because two of these depend on third parties.
1. A1 certificates, applied for at the Centre commun de la sécurité sociale (CCSS) before departure, one per posted worker, covering the full planned period. They prove your people stay in the Luxembourgish social security system rather than becoming liable for Belgian contributions. See what Belgian inspectors check on an A1. 2. A Belgian company number (CBE) and portal access, if Check-in-at-Work will apply. Foreign employers need both to register presence at all, and this is the step most often left too late. 3. The Limosa declaration, filed at limosa.be before the first day of work. It produces a LIMOSA-1 receipt each worker must be able to show to the Belgian client. Full detail in our Limosa guide. 4. A liaison person, whose identity and contact details you give to the Belgian Labour Inspectorate, and who can actually be reached about the posting.
Does the A1 replace the Limosa declaration?
No, and this is the most expensive misunderstanding in cross-border work.
The A1 answers which country’s social security system applies. Limosa answers whether Belgium knows your worker is on its territory. They are issued by different bodies for different purposes. An employer holding a valid A1 still has to file a Limosa declaration before the work starts, and an inspector will ask for both.
Cross-border normality is not an exemption
Nothing in the Limosa rules scales with distance. A Luxembourg company sending two people 30 kilometres into Belgian territory for a day has the same obligation as one flying a crew in from Lisbon. There is no border-region exemption and no minimum duration.
The practical failure is scheduling. In an integrated cross-border labour market, Belgian jobs get slotted in by planners who treat them as domestic, and the declaration is nobody’s task. By the time anyone notices, several undeclared visits have taken place, and because fines for some infringements are multiplied by the number of workers involved, up to a maximum of 100, a pattern of small omissions compounds.
A second point: frontier workers who commute daily to a Belgian employer are in a different situation from posted workers sent by a Luxembourg employer to perform a service. If you are not sure which of the two describes your arrangement, resolve that before the work starts, because the applicable formalities differ.
When does Check-in-at-Work apply?
Registration is mandatory for works on immovable property, excluding cleaning, and for certain meat-sector activities, where the total value of the works reaches 500,000 euro excluding VAT. The threshold applies whether that value is reached at the start of the works or during them, and it attaches to the works as a whole, not to your own contract value.
Three points that catch foreign contractors:
- It is daily. Registration must happen before the person begins work, every working day, for every person on site. It is not a one-off enrolment.
- Responsibility is shared. It lies both with the party sending the worker and the party doing the work, and the two must agree who registers and verify it happened. Assuming the main contractor covers you is not a defence.
- “Works on immovable property” is broad, reaching well beyond new-build into renovation, fit-out, technical installation and groundworks.
From 1 April 2027, a Programme Law adopted on 30 May 2026 extends the obligation to registering both entry and exit, and brings the supply of ready-mixed concrete into scope. Sign-in sheets will not carry you past that date.
The differences between the systems are set out in Limosa, Dimona or Check-in-at-Work.
What it costs to get wrong
Failing to file a Limosa declaration is a level 4 offence, the heaviest category in the Belgian Social Criminal Code, carrying the possibility of imprisonment alongside financial penalties.
Fines rose on 1 February 2026, when the Act of 19 December 2025 raised the multiplier applied to social criminal fines from 8 to 10. For some infringements the amount is multiplied by the number of workers involved, up to a maximum of 100, which is what turns one administrative gap on a ten-person crew into a serious figure.
There is a commercial cost too. Your Belgian client is exposed through joint and several liability for your workers’ wages, and through the withholding obligation: where a contractor has social or tax debts, the client must withhold 35 percent of each invoice excluding VAT for social security debts and 15 percent for tax debts, checked at checkinhoudingsplicht.be. Since 1 May 2026 a further 15 percent applies for outstanding self-employed contributions. A subcontractor who creates that problem does not get invited back. Our guide for main contractors explains what main contractors in Belgium are liable for.
Keeping the paperwork and the site in agreement
The hard part of cross-border work is not understanding the rules. It is keeping the file in your home office and the reality on a Belgian site aligned, week after week, as crews rotate and programmes move.
Two habits do most of the work. Treat every crew change as a compliance event: a new person means a new A1 check, a new Limosa entry and site registration under their own identity, arranged before they travel. And capture presence digitally at the moment someone arrives, tied to the person and the project, so the site record matches your declarations by default and you can produce a specific past day on request without reconstructing it.
Suivo’s check-in-at-work is used on Belgian sites where several subcontractors work side by side, and runs on the same workforce management platform as time tracking, so the data you capture for compliance also feeds hours and project costing instead of sitting in a separate system.
See how Cegelec manages registration across Belgian sites.
A note on language
French is an official language in both countries, which makes Wallonia and Brussels straightforward. Flanders operates in Dutch, so if your work crosses the language border, plan for Dutch-capable correspondence.
Cross-border work from any country carries the same Belgian obligations, laid out in our overview of every Belgian declaration.
Book a free demo
If you are bringing Luxembourgish crews onto Belgian sites and want presence registration that satisfies an inspector without slowing your work down, we will show you how it runs day to day. Book a free demo.
