A buitenlandse aannemer (foreign contractor) working in Belgium in 2027 needs four routines. Before the contract: check the withholding status and plan permits and access. Before the first day: Limosa, the declaration of works and, where Belgian social security applies, Dimona. Every day on site: attendance registration, which moves to Check in and out at work (CIAO) on 1 April 2027, according to the RSZ. Every payment: a fresh withholding check. This calendar lists each step, its deadline and whether it is law or only announced. Each step links to a deeper guide.
Suivo supports Check in at Work on construction sites today and reports registrations directly to the RSZ. From 1 April 2027 it will do the same for Check in and out at work. Talk to us about moving your sites to CIAO before 1 April 2027.
Before you win the contract: what should you prepare?
The first mistakes happen long before anyone arrives on site. A foreign contractor usually has no Belgian eID, no Belgian company number and no access to the social security portal. Each of those takes time.
| Step | Who | Deadline | Deeper guide |
|---|---|---|---|
| Check every subcontractor you hire on checkinhoudingsplicht.be (your own client checks you too) | Client and each contractor | When the contract is signed | Inhoudingsplicht guide |
| Request secured access to the declaration of works and Checkinatwork | Foreign contractor | Now: unsecured access ends in Q4 2026 | Werfmelding guide |
| Apply for the A1 certificate for workers who stay under home social security | Foreign employer | Before the work starts | A1 in Belgium, A1 via the SVB (NL) |
| Choose the route for each non-EU worker: Vander Elst exemption or permit | Employer | Flanders: permit filed 1 to 6 months before the start | Non-EU workers guide |
| Decide whether each worker falls under Limosa, Dimona or both | Employer | Before planning the crew | Dimona guide |
Three points to settle at this stage:
- Secured access takes time. The RSZ has announced that it will withdraw unsecured access to the declaration of works during Q4 2026. After that, you need secured access to file, change or even view a declaration. Getting secured access (a Belgian company number and a digital key) takes time, so start early.
- The check at signature matters. If a subcontractor has social debts when the contract is concluded, the party that hired it can become jointly liable for those debts. Your Belgian client runs the same check on you.
- Non-EU staff need the most lead time. Since 1 September 2026, Flanders charges a fee of EUR 180 per single-permit application, and the processing period only starts once it is paid. The Vander Elst exemption for staff of companies established in the EEA or Switzerland has strict conditions and needs a Limosa declaration first.
Before the first day on site: what must be filed?
Everything below must be in order before work starts. None of it can be fixed afterwards.
| Step | Who | Deadline | What a failure costs |
|---|---|---|---|
| Declaration of works (werfmelding) with every subcontractor listed | The contractor engaged by the client | Before the works start; at least 15 calendar days before on temporary or mobile sites | 5% of the undeclared works, excluding VAT |
| Written notice to the declaring contractor if you bring in your own subcontractor | Subcontractor | Before that subcontractor starts | 5% of the subcontracted works |
| Limosa declaration and L1 certificate shown to the Belgian client | Foreign employer (self-employed: in construction, meat and cleaning) | Before work starts | Level 4 offence, per worker (self-employed: level 3) |
| Check the L1, or report “missing L1” | Belgian client | Before work starts | Level 3 offence, per worker |
| Dimona IN, if Belgian social security applies | Employer | At the latest when the worker starts | Level 4 offence, per worker |
| Permit approved (work and residence), or Vander Elst conditions met | Employer | Before the worker starts | Illegal employment: regional sanction rules apply |
| A1 certificate (or proof of the application) available | Foreign employer | Before work starts | Inspection risk |
Limosa. According to Limosa, the foreign employer must declare every worker who is not under Belgian social security and comes to work temporarily in Belgium. This applies to employers worldwide, not only in the EU. Our Limosa declaration guide explains the steps.
Dimona or Limosa? As a rule, a posted worker with an A1 needs Limosa, not a Dimona from the foreign employer. A foreign employer whose worker works only in Belgium for an indefinite period files Dimona and DmfA instead. The Dimona guide has the full table.
The fines. For offences committed from 1 February 2026, a level 4 offence carries an administrative fine of EUR 3,000 to EUR 35,000, and a level 3 offence EUR 1,000 to EUR 10,000, according to FOD WASO (Federal Public Service Employment). The effective amounts stay the same from 1 September 2026. For Limosa and Dimona, the fine is multiplied by the number of workers.
Every day on site: what do you register?
On sites where the total works are worth EUR 500,000 or more, excluding VAT, every person on site must be registered each day. That includes your own crew, your subcontractors and self-employed workers.
Today (Check in at Work):
- Registration happens before the person starts work.
- Sites are identified by the site number from the declaration of works.
- According to the RSZ, posted foreign workers can register, or be registered, by scanning the QR code on their Limosa L1, without secured access. That route only registers: it cannot view or cancel.
From 1 April 2027 (Check in and out at work):
According to the RSZ, IN and OUT registration becomes mandatory for works in immovable property (excluding cleaning) and ready-mix delivery on sites of EUR 500,000 or more. From that date, construction attendance can no longer be registered in Check in at Work. What changes:
- each arrival and each departure is registered;
- the worker registers in real time, and pre-registration is no longer allowed;
- the registration includes the declaration-of-works number.
The RSZ says more details will follow, so check its page before you finalise your site process. Ready-mix suppliers should read our guide to CIAO for ready-mix and transport drivers.
Other daily duties:
- Keep the documents on site: passport, Limosa L1, A1, and for non-EU workers the residence and work documents.
- Keep Dimona current: file the IN before a new worker starts, and the OUT or UPDATE by the first working day after a contract ends early.
- File a new Limosa declaration before the declared period ends if the job runs longer.
- Add new subcontractors to the declaration of works before they start.
Every payment: what do you check?
The withholding obligation (inhoudingsplicht) applies to invoices for works in immovable property, ready-mix delivery, security and certain meat work. It covers Belgian and foreign contractors alike.
| Check | Rule |
|---|---|
| When | At contract signature and again before every payment |
| Where | checkinhoudingsplicht.be, or the BillRetainment web service for bulk checks |
| Foreign contractor | Search by EU VAT number or another foreign identification number |
| Red status | Valid only at the moment you check. Withhold the percentage shown and pay it with the invoice |
| Rates | Tax 15%; social 35%, 15% or 50%; total capped at 50% of the invoice excluding VAT |
| Missed withholding | Social part: the amount plus a surcharge of the same size. Tax part: a fine of double the amount. You can also become jointly liable |
New from 30 October 2026. Debts under the self-employed social status also count as social debts, for works in immovable property only. According to the RSVZ, this part of the withholding obligation applies from 30 October 2026. From then on, a self-employed subcontractor with unpaid contributions above the RSVZ threshold can show a red status.
According to the RSZ invoice FAQ, statuses are updated monthly. The safe routine for accounts payable: no fresh check on the payment day, no payment.
Attendance does not replace the check. Registering who was on site does not discharge the withholding obligation, and a green status does not replace attendance registration. Use them together: match each invoice to a declared subcontractor who was actually on site.
Key dates late 2026 to 2027: which are law and which are announced?
Status legend:
- In force: the rule or change already applies.
- Law, start date announced by RSZ/RSVZ: the rule is in a published law, and the start date comes from the social security institution’s own site.
- Announced (not yet law): a plan, draft or announced change that does not apply yet. It can still change.
| Date | What changes | Status | Deeper guide |
|---|---|---|---|
| Since 1 Feb and 1 Sept 2026 | Higher Social Penal Code fines (same effective amounts after 1 September) | In force | Dimona guide |
| Since 29 June 2026 | Only the declaring contractor can create or change a declaration of works; others can only view | In force | Werfmelding guide |
| Since 1 July 2026 | Courts can withdraw RSZ contribution reductions for intentional Dimona offences | In force | Dimona guide |
| Since 1 Sept 2026 | Flanders: EUR 180 fee per single-permit application | In force | Non-EU workers guide |
| 30 Oct 2026 | Self-employed (RSVZ) debts trigger withholding | Law, start date announced by RSVZ | Inhoudingsplicht guide |
| Q4 2026 (no exact day) | Unsecured access to the declaration of works ends | Announced (not yet law) | Werfmelding guide |
| 1 Jan 2027 | Flemish single-permit fee is indexed (new amount not yet published) | In force | Non-EU workers guide |
| 1 Jan 2027 | General working-time registration duty, with a transition period to 31 March 2027 in the current draft | Announced (not yet law) | See below |
| 1 Apr 2027 | CIAO (IN and OUT) replaces Check in at Work for construction and ready-mix on sites of EUR 500,000 or more | Law, start date announced by RSZ | CIAO for ready-mix |
| Until 4 Mar 2028 | EU temporary protection for people fleeing Ukraine extended (Council Implementing Decision (EU) 2026/1912 of 30 July 2026; applies from 5 March 2027) | In force | Non-EU workers guide |
| No date yet | Optional EU e-declaration for postings | Announced (not yet law) | Limosa guide |
Two items need a clear warning:
- Working-time registration. The government plans a general duty to register working time from 1 January 2027. As of September 2026, it is a draft law, not law. It is a separate rule from CIAO, which records who is present on a construction site. Do not treat a CIAO record as proof that you meet the planned general duty.
- The EU e-declaration. The EU is preparing an optional multilingual interface for posting declarations. Limosa remains mandatory for work in Belgium.
Where do the country guides go deeper?
This calendar tells you what to do and when. The country guides explain how the rules apply to your home country, including social security, posting rules and practical tips:
- Working in Belgium: the hub for foreign employers, with all country guides
- Dutch companies working in Belgium
- German companies working in Belgium
- French companies working in Belgium
For the individual obligations, use the guides linked in the tables above: Limosa, Dimona, the declaration of works, the withholding obligation, non-EU workers and CIAO for ready-mix.
Frequently asked questions
What must a foreign contractor do before working in Belgium?
Declare posted workers in Limosa and give the L1 to the Belgian client before work starts, make sure the site is in the declaration of works, and have an A1 (or equivalent) for workers who stay under foreign social security. Non-EU workers also need a valid work route, such as the Vander Elst exemption or a permit.
Does a foreign contractor need a Belgian company number?
To file a declaration of works yourself, yes: you can request one through the declaration form. For full access to Checkinatwork you also need a main access manager with a Belgian national register or BIS number. Posted workers can still register with the QR code on their Limosa L1.
When does Check in and out at work start for construction?
According to the RSZ, IN and OUT registration becomes mandatory on 1 April 2027 for works in immovable property (excluding cleaning) and ready-mix delivery on sites of EUR 500,000 or more, excluding VAT. The RSZ says more details will follow.
Is time registration mandatory in Belgium from 2027?
Not yet. The government plans a general working-time registration duty from 1 January 2027, but as of September 2026 it is a draft law, not published in the Belgian Official Gazette. Construction attendance registration (CIAO) is a separate rule.
Are foreign subcontractors subject to the Belgian withholding obligation?
Yes. Checkinhoudingsplicht.be shows the status of foreign contractors too, using their EU VAT number or another foreign ID. Check them when you sign the contract and again before every payment.
Make the daily part of the calendar simple
Most of this calendar happens once per project. Attendance registration happens every day, for every worker and subcontractor, and from April 2027 on arrival and departure. For Check in at Work, Suivo offers an app, a mobile site pole that accepts the Belgian ID card, and a vehicle black box, so your crews register at the site, not in the office afterwards. Talk to us about your Belgian sites and your plans for 2027.
