If a worker is on your Belgian payroll and covered by Belgian social security, you declare them with Dimona. If they work for a foreign employer and stay in their home country’s social security system, you notify Belgium with Limosa. If they set foot on a construction site worth 500,000 euro or more, someone also has to register their presence that day through Check-in-at-Work. These are three different obligations answering three different questions, and on a large site a foreign subcontractor commonly needs two of them at once.
Why Belgium has three systems
Each one exists to answer a distinct question for a different authority.
| System | Question it answers | Who files | When |
|---|---|---|---|
| Dimona | Who is employed under Belgian social security? | The employer subject to Belgian social security | At the start and end of the employment relationship |
| Limosa | Is a foreign employer’s worker working on Belgian soil? | The foreign employer or their representative | Before the work starts |
| Check-in-at-Work | Who was physically present on this site today? | Both the person sending the worker and the person doing the work | Before the worker begins, daily |
They are not alternatives. Satisfying one does nothing for the others.
When do you need Dimona?
Dimona is the immediate declaration of employment to the National Social Security Office. It is the baseline obligation for employment governed by Belgian social security: the employer declares the start of the employment relationship before the worker begins, and declares the end when it finishes.
You need Dimona when your worker is subject to Belgian social security. That is typically the case if you are a Belgian-established employer, or if you are a foreign employer whose worker has become subject to the Belgian system rather than remaining in their home one.
When do you need Limosa instead?
Limosa applies when the employer is established outside Belgium and the worker is posted to work in Belgium temporarily while remaining affiliated to their home country’s social security.
In practice, the A1 certificate is the deciding document. If your worker holds a valid A1 showing they remain in, say, the Polish or Romanian system, they are not paying Belgian contributions, so there is no Belgian employment relationship to declare through Dimona. What Belgium wants instead is notification that they are here: that is Limosa.
The declaration is filed at limosa.be before work starts, covers up to 24 months per notification, and produces a LIMOSA-1 receipt the worker must be able to show. Our complete Limosa guide walks through the full filing.
A common and expensive assumption: holding an A1 does not exempt you from Limosa. The A1 settles which social security system applies. Limosa settles whether Belgium knows you are there. You need both.
Where does Check-in-at-Work fit?
Check-in-at-Work sits on a different axis entirely. Dimona and Limosa are about status, filed once per worker per assignment. CIAW is about presence, filed every working day.
According to Working in Belgium, registration is mandatory for works on immovable property, excluding cleaning, and for specified activities in the meat sector, where the total value of the works reaches 500,000 euro excluding VAT. That threshold applies at the start of the works or if the value reaches it during the works.
Three details that catch foreign contractors:
- The registration must occur before the person begins work, and it must be done daily, for each working day.
- Responsibility is shared. It lies both with the party sending someone to work and with the person doing the work. The two must agree who registers and verify that it happened. “We assumed the main contractor was doing it” is not a defence.
- Foreign employers need Belgian portal access and a Belgian company number (CBE) to register at all. Arrange that well before mobilisation, not on the first morning.
“Works on immovable property” is a broad category. It covers far more than a traditional new-build: renovation, fit-out, installation fixed to a building, groundworks and technical installation work all commonly fall inside it.
So which do you need? A decision path
Work through it in this order.
1. Is the worker subject to Belgian social security? Yes, and you are the employer: file Dimona. No, they hold an A1 and remain in their home system: continue to step 2.
2. Is the employer established outside Belgium, posting a worker to Belgium? Yes: file Limosa before the work starts, unless you fall inside a specific exemption. Check the thresholds in Limosa exemptions rather than assuming.
3. Will the work take place on immovable property worth 500,000 euro or more excluding VAT, or in the covered meat-sector activities? Yes: Check-in-at-Work applies as well, every working day, for every person on site. Agree in writing with the main contractor who registers.
For a foreign contractor on a large Belgian project, the usual answer to steps 2 and 3 is yes and yes.
What changes on 1 April 2027
A Programme Law adopted on 30 May 2026 extends the presence obligation from checking in to checking in and out. It covers works on immovable property, again excluding cleaning, above the same 500,000 euro threshold, and adds the supply of ready-mixed concrete. It enters into force on 1 April 2027.
If you currently capture arrival on a sign-in sheet and nothing else, that will not be enough. The practical implication is that presence has to become a recorded event at both ends of the day, which is difficult to sustain on paper and straightforward once it is digital.
The cost of getting the mix wrong
These are not administrative preferences. 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.
There is a second layer of exposure that falls on the Belgian side of the contract, which is why your client cares. If a posted worker cannot produce a LIMOSA-1, the Belgian principal must report it. Under Belgium’s joint and several liability rules for construction-related activities, a client can be liable for wages owed to a contractor’s workers. And where a contractor has social or tax debts, the client must withhold 35 percent of the invoice excluding VAT for social security debts and 15 percent for tax debts, checked at checkinhoudingsplicht.be.
The practical consequence: a subcontractor who cannot evidence compliance is a financial risk to the main contractor, and main contractors increasingly screen for it before awarding work.
Making all three survivable
Status declarations are a one-off per assignment. Presence registration is the one that has to work every single day, on every site, including the day the foreman is off sick.
That is an operational problem, not a legal one. The teams that pass inspections cleanly tend to have three things in place: registration that happens automatically when someone arrives rather than as a separate task, one record that covers every site and every subcontractor rather than a folder per project, and the ability to produce a specific day’s attendance on request without reconstructing it.
Suivo’s check-in-at-work and time tracking run on the same connected platform, so presence data captured for compliance is the same data used for hours, payroll and project costing. You register once, and it serves several purposes, rather than running a compliance system that produces nothing else of value.
See how B&R Bouwgroep manages this across their sites.
For the full map of Belgian obligations, including a guide per sending country, see our guide to every Belgian filing a foreign employer owes.
Book a free demo
Tell us where your crews are working and which of the three systems apply, and we will show you what compliant registration looks like day to day. Book a free demo.
