From 1 April 2027, your construction site falls under CIaO if you carry out works on immovable property or deliver ready-mixed concrete there, and the total value of the works is 500,000 euros or more, excluding VAT. What counts is not your own contract but the total of the project.
CIaO stands for Check in and out at Work, the registration system of the NSSO. For construction, the start was first planned for 1 January 2027; the NSSO postponed it to 1 April 2027. The legal basis is the Programme Law of 30 May 2026 (Title 6, Chapter 1, Articles 124 to 131).
In this article you will read which works count, how to add up the contracts, what happens when the value rises or falls, how the threshold relates to the declaration of works and who has to register. Six worked examples make it concrete. Which data you register and which fines apply is covered on our page on the CIaO obligation, so we do not repeat it here.
Which works fall under CIaO?
CIaO in construction applies to two types of assignments: works on immovable property, with the exception of cleaning, and the delivery of ready-mixed concrete.
Whether your assignment counts as works on immovable property depends on the activity itself, under Article 20, §2 of Royal Decree no. 1 on VAT. Your joint committee plays no role here. Joint committees only matter for the definition of social security debts and for the Article 30ter scheme.
In Annex 1 of its "Notice to contractors and clients", the NSSO gives a non-exhaustive list of 28 activities. Some examples:
- Demolition works (activity 03)
- Painting works (activity 14)
- Plumbing and central heating (activity 18)
- Electrical engineering works (activity 24)
- Agricultural works (activity 26): these have been outside the scope since 1 April 2019
Ready-mixed concrete is a separate case. It falls under Article 30bis (the declaration of works), but not under the definition of works on immovable property. That is why the NSSO explicitly names the delivery of ready-mixed concrete as a second category for CIaO.
How do you add up contracts for the 500,000 euro threshold?
The 500,000 euro threshold is an amount excluding VAT for the whole site. According to the NSSO FAQ on attendance registration in construction (18 April 2025), you proceed as follows:
- Add up all contracts that the declaring contractor has with the client for the same project.
- Contracts with a different purpose do not count.
- According to the FAQ, technical works or site infrastructure, such as electricity, toilets or a site hut, are not added to the works.
- For lots or public contracts, the first declaring contractor asks the client for the total project cost.
For a subcontractor, this means the value of your own contract is not decisive. An electrician with a 60,000 euro assignment falls under CIaO if the site as a whole is worth 500,000 euros or more.
Note: two NSSO sources word it differently
The FAQ on the declaration of works describes the amount as "the sum of the contracts on the workplace". The FAQ on attendance registration refers to the contracts of the declaring contractor with the client for the same project. With one main contractor who takes on all the works, both descriptions lead to the same amount.
The difference matters when a client directly hires several main contractors on the same site, for example one for the shell and another for the building services. How to add up in that case is not clear today. A cautious approach: ask the client for the total of all contracts on the workplace and prepare registration as soon as that total reaches 500,000 euros. When in doubt, ask the NSSO or your social secretariat for confirmation.
What if the value of the works rises or falls?
An estimate is not a final account. Additional work, extra contracts or a changed design can push the amount up during the works. The NSSO rules are clear on this point:
- Does the amount rise to 500,000 euros or more? Then registration starts from the moment the threshold is reached, even if the works are already under way.
- Is the cost lower than estimated? Then registration continues until provisional acceptance or the end date of the works.
How to monitor the threshold per site:
- At the start, ask the client or the declaring contractor for the total project cost excluding VAT.
- Record the estimate per site and the date you received it.
- Track additional work and new contracts and add them to the total.
- If the total reaches 500,000 euros or more, tell your teams, subcontractors and temporary workers right away that registration is mandatory.
- Keep the documents that show when the threshold was reached.
How does CIaO relate to the declaration of works?
The 500,000 euro threshold is linked to the declaration of works under Article 30bis. That declaration is made by the contractor hired by the client, and it must be filed before the works start. For temporary or mobile construction sites, it must be filed at least 15 calendar days in advance. Since 29 June 2026, only the declaring contractor can create or change a declaration.
| Value of the works (excl. VAT) | What is mandatory? | Which service from 1 April 2027? |
|---|---|---|
| From 5,000 euros, with at least one subcontractor | Declaration of works before the works start | Catena for the chain declaration |
| From 30,000 euros, also without a subcontractor | Declaration of works, even if you do everything with your own staff | Catena, through the declaring contractor |
| From 500,000 euros | Declaration of works plus IN and OUT registration of everyone who carries out construction work | Catena and Check in and out at Work |
From 1 April 2027, Catena replaces the current service for the declaration of works. For works on immovable property and ready-mixed concrete you then use two services: Catena for the chain declaration and Check in and out at Work for attendance. From that date, Checkinatwork can no longer be used for works on immovable property.
A correct declaration of works is more than a formality. Workers can register in the CIaO app via a QR code, a URL or the number of the declaration of works. Anyone who fails to file the declaration also risks a sanction of 5% of the total value excluding VAT of the undeclared works.
Who has to register on site?
On a site that falls under CIaO, employees, self-employed workers, temporary workers and interns register themselves, in real time and at the workplace. The employer may not register them in advance.
There are three ways to register: with the app on a smartphone (Android 8 or later, iOS 16 or later), through the NSSO REST web service for badge, track-and-trace or registration systems, or with a combination of both. The NSSO does not certify any supplier software; using an external solution does not release you from your liability.
Who registers under the current rules, and who does not?
The NSSO FAQ on the current construction scheme (18 April 2025) gives a good picture. Note: that FAQ predates CIaO for construction. The NSSO says more information will follow soon, so this list is not yet confirmed for CIaO.
- Must register: drivers of concrete mixers and operators of concrete pumps.
- Also required: safety coordinators and architects acting as design or supervision management.
- Only if they carry out construction work themselves: site managers.
- Not required: prevention advisers, inspectors, surveyors, EPB reporters, suppliers who only deliver, visitors and clients.
Under the current rules, a delivery without installation does not count as construction work. How to get your workers to register in practice, and what to do when someone forgets or refuses, is explained in our article on introducing CIaO and getting your employees on board
Worked examples: does this site fall under CIaO?
The examples below are based on the total value of all contracts on the site, excluding VAT. They follow the reasoning of the NSSO sources; in a different situation the outcome may differ.
| Situation | Under CIaO from 1 April 2027? | Why |
|---|---|---|
| School renovation of 350,000 euros with 3 subcontractors | No, as long as the site total stays below 500,000 euros | Provided that 350,000 euros is the total of all contracts. A declaration of works is required. If the total rises to 500,000 euros, registration starts from that moment. |
| New apartment building of 2.4 million euros | Yes, for everyone carrying out construction work there | Own employees, subcontractors, self-employed workers and temporary workers check in and out. |
| HVAC installer on a 800,000 euro site | Yes, even though his own contract is smaller | The threshold looks at the site total; central heating and air conditioning fall under activity 18. |
| Painter on a 600,000 euro project | Yes, if 600,000 euros is the project total | Painting is activity 14 on the NSSO list. |
| Delivery of ready-mixed concrete to a site of 500,000 euros or more | Yes, ready-mixed concrete is named explicitly | Today the mixer driver and pump operator register; whether this stays the same under CIaO is not yet confirmed. |
| Supplier who only drops off material, or an EPB reporter | Not under the current rules; not yet confirmed for CIaO | The current FAQ exempts them; the NSSO says more information on CIaO will follow. |
How MyVIRO helps
When a site falls under CIaO, everyone must be able to register easily, including people who are only on site for a few days. With MyVIRO, workers check in and out on site and select the right project. Registrations go directly to Check in and out at Work through the NSSO web service. Location is only recorded at the moment of checking in or out, never outside working hours.
In the back office you immediately see missing check-ins, deviating locations and open approvals before payroll preparation. Validated hours flow to your HR, ERP or payroll system, or to VIRO for pre-payroll. MyVIRO does not calculate wages itself, and your data stays in Belgium.
- See how MyVIRO works, from checking in and out to full ESS
- Read what MyVIRO does for companies with staff in the field
- Discover how GO-VIRTUAL supports planning for construction (in Dutch)
Request a demo and find out how to get your teams ready for 1 April 2027, including on sites that only reach the threshold during the works.
Read also
- Subcontractors and temporary workers under CIaO: who registers what?
- Employees' own phone and GPS for time registration: what the GDPR allows
- CIaO: the app is the easy part. How to get your people to register correctly every day
Frequently asked questions about the CIaO threshold in construction
No. The threshold applies to the total value excluding VAT. The thresholds for the declaration of works, 30,000 and 5,000 euros, are also amounts excluding VAT.
That depends on the total of the site, not on your own contract. If the project total is 500,000 euros or more, your employees also register from 1 April 2027.
Then registration starts from the moment the threshold is reached. Inform your teams and subcontractors straight away.
No. According to the NSSO FAQ, registration continues until provisional acceptance or the end date of the works.
No, for works on immovable property that is no longer possible from that date. You then use Catena for the declaration of works and Check in and out at Work for attendance.
No. What counts is the activity under Article 20, §2 of Royal Decree no. 1 on VAT. The joint committee only plays a role for social security debts and Article 30ter.
Sources
- NSSO, Check in and out at Work, portal page with alert of 30 July 2026 (in Dutch): socialsecurity.be, Check in and out at Work
- NSSO, questions on attendance registration in construction (FAQ, 18 April 2025, in Dutch): socialsecurity.be, attendance registration FAQ
- NSSO, Notice to contractors and clients, with the list of activities (in Dutch): socialsecurity.be, notice to contractors
- NSSO, declaration of works under Article 30bis (in Dutch): socialsecurity.be, 30bis declaration
- NSSO, declaration of works, frequently asked questions (in Dutch): socialsecurity.be, declaration of works FAQ
- NSSO, how to register in Check in and out at Work (in Dutch): socialsecurity.be, registration methods
Author: Jean-Philippe Delberghe at MyVIRO. Published on 23 October 2026. Last updated on 23 October 2026.
This article is an informative explanation and not legal advice. Consult your social secretariat for your specific situation.